Carbamate pesticides: a general introduction (EHC 64, 1986)

INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

ENVIRONMENTAL HEALTH CRITERIA 64

CARBAMATE PESTICIDES: A GENERAL INTRODUCTION

This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.

Published under the joint sponsorship of
the United Nations Environment Programme,
the International Labour Organisation,
and the World Health Organization

World Health Orgnization
Geneva, 1986

The International Programme on Chemical Safety (IPCS) is a
joint venture of the United Nations Environment Programme, the
International Labour Organisation, and the World Health
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coordination of laboratory testing and epidemiological studies, and
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chemicals.

ISBN 92 4 154264 0

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR CARBAMATE PESTICIDES: A GENERAL
INTRODUCTION

1. SUMMARY AND RECOMMENDATIONS

1.1. Summary
1.1.1. General
1.1.2. Properties, uses, and analytical methods
1.1.3. Sources, environmental transport and distribution
1.1.4. Environmental levels and exposures
1.1.5. Effects on organisms in the environment
1.1.6. Kinetics and metabolism
1.1.7. Mechanism of toxicity
1.1.8. Effects on experimental animals and in vitro
test systems
1.1.9. Mutagenicity and related end-points
1.1.10. Carcinogenicity
1.1.11. Effects on man
1.1.12. Previous evaluations by international bodies
1.2. Recommendations

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS

2.1. Identity
2.2. Physical and chemical properties
2.3. Analytical methods

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1. Natural occurrence
3.2. Man-made sources
3.2.1. Production levels, processes, and uses

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION

4.1. Transport and distribution between media
4.1.1. Air
4.1.2. Water
4.1.3. Soil
4.1.4. Vegetation and wildlife
4.1.5. Entry into the food chain
4.2. Biotransformation
4.2.1. Microbial degradation
4.2.2. Photodegradation
4.2.3. Photodecomposition in the aquatic environment

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1. Exposure of the general population
5.1.1. Food and drinking-water

6. METABOLISM AND MODE OF ACTION

6.1. Mode of action
6.1.1. Neuropathy target esterase

6.2. Metabolism
6.2.1. Absorption
6.2.2. Distribution
6.2.3. Metabolic transformation
6.2.3.1 Biotransformation mechanisms
6.2.3.2 Oxidation
6.2.3.3 Hydrolysis
6.3.2.4 Conjugation
6.2.3.5 Examples of biotransformation
of carbamates
6.3. Elimination and excretion in expired air, faeces, and
urine
6.3.1. Man
6.3.2. Laboratory animals
6.4. Metabolism in plants

7. EFFECTS ON ORGANISMS IN THE ENVIRONMENT

7.1. Microorganisms
7.2. Aquatic Organisms
7.2.1. Field studies
7.3. Terrestrial Organisms
7.3.1. Effects on soil fauna
7.3.2. Wildlife
7.3.3. Bees
7.4. Earthworm and mite populations

8. EFFECTS ON EXPERIMENTAL ANIMALS AND IN VITRO TEST SYSTEMS

8.1. Single exposures
8.1.1. Oral
8.1.2. Dermal
8.1.3. Inhalation
8.2. Short- and long-term exposures
8.2.1. Oral
8.2.1.1 Further information on short-
and long-term toxicity
8.2.2. Dermal
8.3. Skin and eye irritation; sensitization
8.3.1. Skin irritation
8.3.2. Eye irritation
8.3.3. Skin sensitization
8.4. Inhalation
8.5. Reproduction, embryotoxicity, and teratogenicity
8.5.1. Reproduction
8.5.2. Endocrine system
8.5.3. Embryotoxicity and teratogenicity
8.6. Mutagenicity and related end-points
8.7. Carcinogenicity
8.7.1. General
8.8. Special studies

9. EFFECTS ON MAN

9.1. General population exposure
9.1.1. Acute toxicity: poisoning incidents

9.1.2. Effects of short- and long-term exposure
9.1.3. Controlled human studies
9.2. Occupational exposure
9.2.1. Acute toxicity: poisoning incidents
9.2.2. Effects of short- and long-term exposure
9.2.3. Epidemiological studies
9.3. Signs and symptoms of acute intoxication by carbamates
9.3.1. Biochemical methods for measurement of effects
9.4. Treatment of acute poisoning by carbamate insecticides
9.4.1. Minimizing the absorption
9.4.2. General supportive treatment
9.4.3. Specific pharmacological treatment
9.4.3.1 Atropine
9.4.3.2 Oxime reactivators
9.4.3.3 Diazepam

10. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES

REFERENCES

ANNEX I NAMES AND STRUCTURES AND SOME PHYSICAL AND CHEMICAL
PROPERTIES OF CARBAMATE PESTICIDES

ANNEX II SUMMARY OF THE SHORT- AND LONG-TERM TOXICITY STUDIES
THAT WERE USED TO ESTABLISH THE ACCEPTABLE DAILY INTAKES
FOR HUMAN BEINGS FOR CARBAMATE COMPOUNDS

ANNEX III CARBAMATES: JMPR REVIEWS, ACCEPTABLE DAILY INTAKES,
EVALUATION BY IARC, CLASSIFICATION BY HAZARD, FAO/WHO
DATA SHEETS, IRPTC DATA PROFILE AND LEGAL FILE

ANNEX IV ABBREVIATIONS

WHO TASK GROUP ON CARBAMATE PESTICIDES

Members

Dr D. Ecobichon, Department of Pharmacology and Therapeutics,
McGill University, Montreal, Quebec, Canada

Dr A.H. El-Sebae, Department of Pesticide Chemistry, Faculty
of Agriculture, University of Alexandria, Alexandria, Egypt

Dr L. Ivanova-Chemishanska, Institute of Hygiene and Occupational
Health, Medical Academy, Sofia, Bulgaria (Vice-Chairman)

Dr M.K. Johnson, Toxicology Unit, Medical Research Council
Laboratories, Carshalton, Surrey, United Kingdom

Dr S.K. Kashyap, National Institute of Occupational Health,
Ahmedabad, India

Dr M. Lotti, Institute of Occupational Health, Padua, Italy

Dr L. Martson, All-Union Scientific Research Institute of the
Hygiene and Toxicology of Pesticides, Polymers, and
Plastics (VNIIGINTOX), Kiev, USSR^a

Dr U.G. Oleru, College of Medicine, University of Lagos,
Lagos, Nigeria.

Dr W.O. Phoon, Department of Social Medicine and Public
Health, National University of Singapore, Outram Hill,
Republic of Singapore (Chairman)

Dr A.F. Rahde, Ministry of Public Health, Porto Alegre, Brazil

Dr E. Reiner, Institute for Medical Research and Occupational
Health, Zagreb, Yugoslavia

Dr J. Sekizawa, National Institute of Hygienic Sciences,
Tokyo, Japan

Observers

Mr R.J. Lacoste, International Group of National Associations
of Pesticide Manufacturers (GIFAP), Brussels, Belgium

Secretariat

Mrs B. Bender, United Nations Environment Programme,
International Register of Potentially Toxic Chemicals,
Geneva, Switzerland

Dr J.R.P. Cabral, Unit of Mechanisms of Carcinogenesis,
International Agency for Research on Cancer, Lyons, France

---------------------------------------------------------------------------
^a Invited, but unable to attend.

Secretariat (contd.)

Dr K.W. Jager, International Programme on Chemical Safety,
World Health Organization, Geneva, Switzerland (Secretary)

Dr G.J. Van Esch, Bilthoven, The Netherlands (Temporary
Adviser) (Rapporteur)

Dr C. Xintaras, Office of Occupational Health, World Health
Organization, Geneva, Switzerland

NOTE TO READERS OF THE CRITERIA DOCUMENTS

Every effort has been made to present information in the
criteria documents as accurately as possible without unduly
delaying their publication. In the interest of all users of the
environmental health criteria documents, readers are kindly
requested to communicate any errors that may have occurred to the
Manager of the International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland, in order that they may be
included in corrigenda, which will appear in subsequent volumes.

* * *

Detailed data profiles and legal files for most of the
carbamate pesticides can be obtained from the International
Register of Potentially Toxic Chemicals, Palais des Nations, 1211
Geneva 10, Switzerland (Telephone no. 988400 - 985850).

ENVIRONMENTAL HEALTH CRITERIA FOR CARBAMATE PESTICIDES

A WHO Task Group on Environmental Health Criteria for Carbamate
Pesticides met in Geneva from 30 September to 4 October 1985. Dr
K.W. Jager opened the meeting on behalf of the Director-General.
The Task Group reviewed and finalized the draft criteria document.

The efforts of all who helped in the preparation and
finalization of the document are gratefully acknowledged.

* * *

Partial financial support for the publication of this criteria
document was kindly provided by the United States Department of
Health and Human Services, through a contract from the National
Institute of Environmental Health Sciences, Research Triangle Park,
North Carolina, USA - a WHO Collaborating Centre for Environmental
Health Effects. The United Kingdom Department of Health and Social
Security generously supported the cost of printing.

1. SUMMARY AND RECOMMENDATIONS

1.1. Summary

1.1.1. General

The carbamates discussed in this publication are those mainly
used in agriculture, as insecticides, fungicides, herbicides,
nematocides, or sprout inhibitors. In addition, they are used as
biocides for industrial or other applications and in household
products. A potential use is in public health vector control.
Thus, these chemicals are part of the large group of synthetic
pesticides that have been developed, produced, and used on a large
scale in the last 40 years.

The general formula of the carbamates is:

O
||
R¹NH - C - OR²

where R¹ and R² are alkyl or aryl groups.

More than 50 carbamates are known, and it is clear that it is
not within the scope of this introduction to include all the
information about each compound. However, all the different
aspects of the different classes of carbamates are touched on,
making use of available publications and reports of studies and
using well known carbamates, such as carbaryl, benomyl, and a few
others, as examples.

Thiocarbamates and dithiocarbamates have not been included,
because these groups of compounds have a different mode of action
and will be dealt with in a separate publication.

1.1.2. Properties, uses, and analytical methods

Three classes of carbamate pesticides are known. The carbamate
ester derivatives, used as insecticides (and nematocides), are
generally stable and have a low vapour pressure and low water
solubility. The carbamate herbicides (and sprout inhibitors) have
the general structure R¹NHC(O)OR², in which R¹ and R² are aromatic
and/or aliphatic moieties. Carbamate fungicides contain a
benzimidazole group.

The only physical or chemical properties given in this
publication are the relative molecular mass, vapour pressure, and
water solubility.

Analytical methods for a number of important carbamates have
been tabulated.

1.1.3. Sources, environmental transport and distribution

The synthesis and commercialization of the carbamate pesticides
has been in progress since the 1950s. The benzimidazole fungicides
were introduced on the market in about 1970.

In general, the vapour pressure of the carbamates is low;
nevertheless, they will evaporate or sublimate slowly at normal
temperature, which may lead to volatilization of carbamates from
water and soil. However, distribution via air will be a minor
factor. The aqueous environment will be an important route of
transport for highly-soluble carbamates. The light absorption
characteristics of carbamates contribute to their rapid
decomposition (by photodegradation or photodecomposition) under
aqueous conditions. Thus, the hazards of long-term contamination
with carbamates seem small. Carbamate insecticides are mainly
applied on the plants, and can reach the soil, while carbamate
nematocides and herbicides are applied directly to the soil.
Several factors influence the biodegradation of carbamates in soil,
such as volatility, soil type, soil moisture, adsorption, pH,
temperature, and photodecomposition. Because the different
carbamates have different properties, it is clear that each should
be evaluated on its own merits, and no extrapolation of results can
be made from one carbamate to another. One carbamate may be easily
decomposed, while another may be strongly adsorbed on soil. Some
leach out easily and may reach groundwater. In these processes,
the soil type and water solubility are of great importance.
Furthermore, it should be recognized that this not only concerns
the parent compound but also the breakdown products or metabolites.

Environmental conditions that favour the growth and activity of
microorganisms also favour the degradation of carbamates. The
first step in the metabolic degradation of carbamates in soil is
hydrolysis. The hydrolysis products will be further metabolized in
the soil-plant system.

Plants can metabolize carbamates in which arylhydroxylation and
conjugation, or hydrolytic breakdown are the main routes of
detoxification. The results of a number of studies suggest that
carbamates are exclusively distributed via the apoplastic system in
plants.

Carbamates are metabolized by microorganisms, plants, and
animals or broken down in water and soil.

1.1.4. Environmental levels and exposures

From the available data, it appears that bioaccumulation in the
different species and different food chains will only take place to
a slight extent. Certain carbamates may reach groundwater and as a
consequence may find their way into drinking-water. The few
studies available indicate that exposure of the general population
is low. However, this should be confirmed by market-basket and/or
total-diet studies.

1.1.5. Effects on organisms in the environment

Soil microorganisms are capable of metabolizing (hydrolysing)
carbamates and can easily adapt themselves to metabolize the
different types of carbamates. Nevertheless, carbamates and their
metabolites can, at high dose levels, affect the microflora and
cause changes that may be of importance in soil productivity.

Although carbamates are not very stable under aquatic conditions,
and will not persist long in this environment, some bioaccumulate
in fish, mainly because the metabolism is slow in fish. Other
carbamates are metabolized rapidly and no accumulation occurs.
Some carbamates are highly toxic for invertebrates and fish, others
much less so. In certain cases, the use of toxic carbamates may
cause a significant reduction in non-target organisms. Carbamates
are toxic for worms and other organisms living in the soil.
Although a great reduction in the earthworm population may occur
when applying carbamates to the soil, numbers will return to
normal, because of the rather rapid breakdown of these compounds.

In general, the toxicity of carbamates for wildlife is low, but
exceptions exist. This means that, in order to judge the impact of
carbamates on the organisms in the environment, the information on
the individual carbamates should be referred to. As a rule, birds
are not very sensitive to carbamates while bees are extremely
sensitive.

1.1.6. Kinetics and metabolism

The metabolic fate of carbamates is basically the same in
plants, insects, and mammals. Carbamates are usually easily
absorbed through the skin, mucous membranes, and respiratory and
gastrointestinal tracts, but there are exceptions. Generally, the
metabolites are less toxic than the parent compounds. However, in
certain cases, the metabolites are just as toxic or even more toxic
than the parent carbamate. In most mammals, the metabolites are
mainly excreted rather rapidly in the urine. The dog seems to be
different in this respect. Accumulation takes place in certain
cases, but is of minor importance because of the rapid metabolism.

The first step in the metabolism of carbamates is hydrolysis to
carbamic acid, which decomposes to carbon dioxide (CO₂) and the
corresponding amine.

The mechanism of hydrolysis is different for N -methyl and
N -dimethyl derivatives. The N -methyl carbamates pass through an
isocyanate intermediate, whereas in the hydrolysis of N -
dimethylcarbamates, an addition product with a hydroxyl ion is
formed yielding the alcohol and N -dimethyl substituted acid. The
rate of hydrolysis by esterases is faster in mammals than in plants
and insects.

Apart from hydrolysis, oxidation also takes place including:
hydroxylation of the aromatic ring, O -dealkylation, N -methyl
hydroxylation, N -dealkylation, oxidation of aliphatic side chains,
and sulfoxidation to the corresponding sulfone. Oxidation is
associated with the mixed-function oxidase (MFO) enzymes.
Conjugation leads to the formation of O - and N -glucuronides,
sulfates, and mercapturic acid derivatives in mammals. Glycosides
and phosphates are conjugation products more common in plants.

The metabolism of a number of carbamates is discussed in the
text.

Little information is available on the distribution of
carbamates in the various organs and tissues in mammals following
exposure by inhalation or the oral route. The organs in which
residues have been reported are the liver, kidneys, brain, fat, and
muscle. The half-life in the rat is of the order of 3 - 8 h. From
the limited data available, it seems that the excretion of
carbamates via urine is also rapid in man, and that the metabolic
pathways in man are the same as those in the rat.

1.1.7. Mechanism of toxicity

Carbamates are effective insecticides by virtue of their
ability to inhibit acetylcholinesterase (AChE) (EC 3.1.1.7) in the
nervous system. They can also inhibit other esterases.

The carbamylation of the enzyme is unstable, and the
regeneration of AChE is relatively rapid compared with that from a
phosphorylated enzyme. Thus, carbamate pesticides are less
dangerous with regard to human exposure than organophosphorus
pesticides. The ratio between the dose required to produce death
and the dose required to produce minimum symptoms of poisoning is
substantially larger for carbamate compounds than for
organophosphorus compounds.

Because of their chemical structure, carbamates do not cause
delayed neuropathy.

1.1.8. Effects on experimental animals and in vitro test systems

The acute toxicity of the different carbamates ranges from
highly toxic to only slightly toxic or practically non-toxic. The
LD₅₀ for the rat ranges from less than 1 mg/kg to over 5000 mg/kg
body weight. For certain methyl carbamates, the LD₅₀ is 20 or more
times the corresponding ED₅₀. This means that, in general, an
early indication of poisoning can be obtained before a lethal dose
is absorbed.

A dose-effect relationship exists between the dose, the
severity of symptoms, and the degree of cholinesterase (ChE)
inhibition. Because most carbamates have a low volatility,
inhalation studies are mainly carried out using a dust or mist. In
these studies, the toxicity is highly dependent on the size of the
particles or droplets and, therefore, difficult to evaluate.

The acute dermal toxicity of carbamates is generally low to
moderate; an exception is aldicarb, which is highly toxic. It
should be noted that data are available for only a limited number
of substances.

Carbamates produce slight to moderate skin and eye irritation,
depending on the vehicle used, duration of contact, and on whether
the substance is applied to the abraided or intact skin. From the
available data, it cannot be excluded that some of the carbamates
will have a slight to moderate sensitization potential.

Short- and long-term toxicity studies have been carried out.
Some carbamates are very toxic and others are less toxic in long-
term studies. From these studies, it is evident that, apart from
the anticholinesterase activity, the following changes can be
found: an influence on the haemopoietic system, an influence on the
functioning of, and, at higher dosages, degeneration of, the liver
and kidneys, and degeneration of testes. These abnormalities in
the different organ systems depend on the animal strain and on the
chemical structure of the carbamate. A clear influence on the
nervous system, functional as well as histological, was found,
particularly in non-laboratory animals such as pigs.

For many years, long-term toxicity data on carbamates have been
evaluated by the FAO/WHO Joint Meeting on Pesticide Residues
(JMPR), and a number of ADIs for carbamates have been established.
In Annex II and III, the no-observed-adverse-effect levels and the
ADIs are summarized.

A considerable number of reproduction and teratogenicity
studies have been carried out with different carbamates and various
animal species. Different types of abnormalities were found, i.e.,
increase in mortality, disturbance of the endocrine system, and
effects on the hypophysis and its gonadotrophic function. These
effects were mainly seen at high dose levels. Generally, the fetal
effects included an increase in mortality, decreased weight gain in
the first weeks after birth, and induction of early embryonic
death. All these effects can be summarized as embryotoxic effects.
Certain carbamates also induce teratogenic effects, mainly at high
dose levels applied by stomach tube. When the same dose level was
administered with the diet, no effects were seen.

1.1.9. Mutagenicity and related end-points

The well-known carbamates have been tested for their mutagenic
activity in different test systems. Some induce mutagenic effects,
others are negative. In general, the methyl carbamates are
negative in mammalian tests, while compounds such as carbendazim,
benomyl, and the 2 thiophanate derivatives showed a positive effect
with very high dose levels in certain systems. The benzimidazole
moiety may act as a base analogue for DNA and as a spindle poison.
They are antimitotic agents and cause mitotic arrest, mitotic
delay, and a low incidence of chromosome damage. Sometimes, the
results are contradictory or cannot be reproduced, but positive
results for point mutation and chromosome aberrations are well
documented. These benzimidazole derivatives can be considered as
weak mutagenic compounds.

1.1.10. Carcinogenicity

Ethyl carbamate (urethane) is a well-known carcinogen, and it
seems that its chemical structure is optimal for such an effect.
Any change in the molecule seems to decrease the carcinogenic
potency, particularly when the ethyl group is replaced by larger
side chains. Alkyl groups on the nitrogen also reduce this
activity.

However, no clear indications of carcinogenic effects have been
found in the available long-term carcinogenicity studies with
different carbamates.

The carcinogenicity studies with benzimidazole derivatives
showed either positive or equivocal results. Added to the fact
that certain mutagenicity studies also give positive results, it
cannot be excluded that these compounds may have carcinogenic or
promotor properties. It should be kept in mind that the dose
levels in most tests were of the order of 50 and 500 mg/kg body
weight.

Carbamate pesticides may be converted to N -nitroso compounds.
This was demonstrated in a great number of in vivo nitrosation
studies in which high levels of the carbamates were administered to
animals in combination with high levels of nitrite. These N -
nitroso compounds have to be considered as mutagenic and
carcinogenic. However, the amount of nitroso compounds that can be
expected to result from dietary intake of carbamate pesticide
residues is negligible in comparison with nitroso-precursors that
occur naturally in food and drinking-water.

1.1.11. Effects on man

Health hazards for man occur mainly from occupational over-
exposure to carbamate insecticides resulting in poisoning
characterized by cholinergic symptoms caused by inhibition of the
enzyme AChE. Various cases of intoxication have been described.
Most of them were spraymen applying insecticides inside houses in
the tropics to control mosquito vectors of malaria, or plant
protection workers. The main routes of exposure are inhalation and
skin.

From controlled human studies, it is clear that poisoning
symptoms can be seen a few minutes after exposure, and can last for
a few hours. Thereafter, recovery starts and within hours, the
symptoms disappear, and the ChE activity in erythrocytes and plasma
returns to normal, because the carbamate is rather rapidly
metabolized and the metabolites excreted. The appearance of these
metabolites in the urine may be used for biological monitoring.
Apart from the symptoms indicative of ChE poisoning, other signs
and symptoms induced by certain carbamates have been described,
such as skin and eye irritation, hyperpigmentation, and influence
on the function of testes (slight increase of sperm abnormalities).
These signs and symptoms were found in a few studies and should be
confirmed before it can be stated that they were induced by
carbamates. Epidemiological studies with persons primarily exposed
to carbamates are not available.

1.1.12. Previous evaluations by international bodies

Previous evaluations of individual carbamates by the Joint
FAO/WHO Meeting on Pesticide Residues (JMPR) and the International
Agency for Research on Cancer (IARC) are summarized in Annex III.
The WHO recommended classification of pesticides by hazard is
included, and the availability is indicated of WHO/FAO Data Sheets
and the IRPTC data profile and legal file on the substance.

1.2. Recommendations

1. More up-to-date information is necessary on the world-wide
production and uses of the different carbamates.

2. Except for the well-known carbamates, such as carbaryl,
benomyl, and carbendazim, more information is required on
environmental pathways, concentrations, and distribution.

3. More information is necessary on the occurrence and fate of the
carbamates in surface water, soil, and groundwater, and their
impact on plants, invertebrates, and mammals.

4. Further studies are necessary on the occurrence of carbamates
in the different food chains (bioaccumulation), and in food and
drinking-water (market-basket or total-diet studies), in order to
estimate the daily exposure of the human population.

5. More information is needed, for certain carbamates, on the
acute and long-term toxicity in aquatic and terrestrial organisms.

6. In general, data on the mode of action and metabolism are
available; however, more knowledge is needed on the distribution of
the carbamates in organs and tissues in mammals.

7. For many carbamates, information concerning long-term toxicity,
mutagenicity, carcinogenicity, reproduction, and teratogenicity is
still needed. This is the reason why acceptable daily intakes have
not been established for two-thirds of the carbamates.

8. Apart from a number of studies on human volunteers and a number
of accidents with well-known carbamates, information on the effects
of human exposure to carbamates is missing. There are no
epidemiological studies. More information should be collected to
evaluate the health risk in cases of human exposure to carbamates.
In particular, more information is needed to elucidate the
mechanisms of the effects of benzimidazole carbamates on the
testes.

9. More data are needed on the use of oximes in the therapy of
poisoning, especially in light of the fact that there are
unconfirmed reports that oximes might potentiate carbamate
toxicity.

10. Further work should be done to develop more adequate
analytical methods (i.e., faster procedures and simpler equipment)
to determine carbamate residues in biological material (urine,
blood, and food).

11. Information is required concerning the changes in toxicity due
to impurities that can arise in pesticides as a consequence of
different manufacturing processes, formulating practices, and
improper storage.

12. Users should be encouraged to be aware of the necessity to
establish safe re-entry periods according to local conditions.

Recommendations for further work have also been described in
previous monographs on carbamates published in the WHO Pesticide
Residues Series and in the FAO Plant Production and Protection
Papers, as a result of the Joint FAO/WHO Meetings on Pesticide
Residues (JMPR).

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS

2.1 Identity

Carbamates are N -substituted esters of carbamic acid. Their
general formula is:

O
||
R¹NH - C - OR²

where R² is an aromatic or aliphatic moiety. Three main classes of
carbamate pesticides are known:

(a) carbamate insecticides; R¹ is a methyl group;

(b) carbamate herbicides; R¹ is an aromatic moiety; and

2.2 Physical and Chemical Properties

In general, simple esters or N -substituted derivatives of
carbamic acid are unstable compounds, especially under alkaline
conditions. Decomposition takes place, and the parent alcohol,
phenol, ammonia, amine, and carbon dioxide are formed.

The salts and esters of substituted carbamic acid are more
stable than carbamic acid. This enhanced stability is the basis
for the synthesis of many derivatives that are biologically active
pesticides.

Carbamate ester derivatives are crystalline solids of low
vapour pressure with variable, but usually low, water solubility.
They are moderately soluble in solvents such as benzene, toluene,
xylene, chloroform, dichloromethane, and 1,2-dichloroethane. In
general, they are poorly soluble in nonpolar organic solvents such
as petroleum hydrocarbons but highly soluble in polar organic
solvents such as methanol, ethanol, acetone, dimethylformamide,
etc.

The carbamate derivatives with herbicidal action (such as
pyrolan and dimetilan) are substantially more stable to alkaline
hydrolysis than the methyl carbamate derivatives (carbaryl and
propoxur), which have an insecticidal action. For example, the
half-life of carbaryl is 15 min at pH 10 compared with 10 days at
pH 7. However, pyrolan and dimetilan do not hydrolyse in the pH
range of 4 - 10 (Aly & El Dib, 1972). Instability with alkali is of
use for decontamination and clean-up. Vassilieff & Ecobichon
(1982) showed that, in acid fresh water, which is characteristic of
the lakes and streams in heavily forested Canada, aminocarb would
be rather stable and would persist long enough to be bioaccumulated
by various trophic levels of food chains.

The carbamate fungicides carbendazim, benomyl, and thiophanates
are related. Carbendazim and benomyl are derivatives of

benzimidazole. Carbendazim is slowly hydrolysed by alkali to 2-
aminobenzimidazole, but it is stable as acid-forming water-soluble
salts (Sypesteijn et al., 1977). Benomyl breaks down to methyl 2-
benzimidazole carbamate (MBC) in water. Benomyl is rather unstable
in common solvents (White et al., 1973; Chiba & Doornbos, 1974).

The names, chemical structure, and pesticidal activity of the
principal carbamates are presented in Table 1, and the CAS number,
chemical name, common names, molecular formula, relative molecular
mass, and selected chemical and physical properties are summarized
in Annex I. It should be noted that it is not within the scope of
this general introduction to describe all the physical and chemical
properties of each pesticide in detail.

2.3 Analytical Methods

Analysis for pesticide residues consists of sampling the
contaminated environmental material or matrix, extracting the
pesticide residue, removing interfering substances from the
extract, and identifying and quantifying the pesticide contaminant.
The manner in which the matrix material is sampled, stored, and
handled can affect the results; samples should be truly
representative, and their handling and storage must not further
contaminate or degrade the contaminant to be measured. Many
detection methods are available, and the one chosen depends on the
physical and chemical properties of the contaminant as well as on
the equipment available.

A detailed review of all the analytical procedures to determine
carbamates in the different matrices is beyond the scope of this
document. However, for better understanding of the validity of the
data, a brief summary of some of the analytical procedures for
different carbamates is included in Table 2.

Enzymatic methods to determine erythrocyte- and plasma-ChE
activity are used as monitors of exposure and systemic absorption
of organophosphorus compounds and carbamate pesticides. In the case
of carbamates, the inhibition may not be easily detected because of
the rapid reversibility of the carbamate-enzyme inhibition
reaction.

Conditions and time of storage must be carefully controlled
before measurement of activity. Methods must be chosen that allow
a short time for hydrolysis of the substrate (Ellman et al., 1961;
Voss & Schuler, 1967; Wilhelm & Reiner, 1973; Wilhelm et al., 1973;
Reiner et al., 1974; Abd-Elroaf et al., 1977; Izmirova, 1980).

A colorimetric screening method has been described, to estimate
Unden^(R), carbofuran, and carbaryl in the air of the manufacturing
and formulating plants, as well as in washings from body surfaces,
hands, and other contaminated surfaces (Izmirova, 1980; Izmirova &
Izmirov, unpublished report, 1985)^a.

---------------------------------------------------------------------------
^a Pharmatest-cholinesterase reactive papers for determination
of cholinesterase activity (ChEA) of serum or plasma.

Table 1. Relationship of chemical structure and pesticidal activity of carbamates
---------------------------------------------------------------------------------------------------------
Pesticidal Chemical structure Common or other names
activity
---------------------------------------------------------------------------------------------------------
Insecticide O aldoxycarb, allyxycarb, aminocarb, BPMC, bendiocarb,
|| bufencarb, butacarb, carbanolate, carbaryl, carbofuran,
CH₃-NH-C-O-aryl cloethocarb, dimetilan, dioxacarb, ethiofencarb, forme-
tanate, hoppcide, isoprocarb, trimethacarb, MPMC,
methiocarb, metolcarb, mexacarbate, pirimicarb,
promacyl, promecarb, propoxur, MTMC, XMC, xylylcarb

O aldicarb, methomyl, oxamyl, thiofanox, thiodicarb
||
CH₃-NH-C-O- N-alkyl

Herbicide O asulam, barban, carbetamide, chlorbufam, desmedipham,
|| phenmedipham, swep
aryl-NH-C-O-alkyl

O dichlormate, karbutilate, terbucarb
||
alkyl-NH-C-O-aryl

Herbicide O propham, chlorpropham
and sprout ||
inhibitors aryl-NH-C-O-alkyl

Fungicide O benomyl, carbendazim, thiophanate-methyl,
|| thiophanate-ethyl
aryl-NH-C-O-alkyl
---------------------------------------------------------------------------------------------------------

Table 2. Analytical methods for carbamate pesticide residues
---------------------------------------------------------------------------------------------------------
Chemical Sample type Extraction and clean-up Method of Detection^b Reference
detection^a limit
---------------------------------------------------------------------------------------------------------
aldicarb cotton seed, acetone-water-peracetic acid GLC/SFPD 0.01 mg/kg Romine (1973)
fruit/ extraction evaporation,
vegetables absorption on Florisil column,
elution with acetone-ether

asulam crops acetone extraction, absorption colorimetry 0.05 mg/kg Brockelsby &
on Florisil column, elution, Muggleton (1973)
reaction with chromophore

benomyl soils, fruit/ acidic methanol extraction and HPLC/UV 0.05 mg/kg Bleidner et al.
vegetables, ethyl acetate extraction, (1978)
tissues conversion to carbendazim

soils acetone-ammonium chloride UV Austin & Briggs
extraction and solvent (1976)
partition

soils, fruit/ review of methods including 0.05 - Baker & Hoodless
vegetables, those for carbendazim and 3.0 mg/kg (1974)
tissues thiophanate-methyl

carbendazim water reverse phase and adsorption HPLC, variable Austin et al.
systems wavelength (1976)
UV-detector

carbaryl urine acid hydrolysis, benzene GLC/ECD 0.02 Shafik et al.
extraction, reaction with mg/litre (1971)
chloroacetic anhydride,
absorption on silica gel column
and elution with benzene-hexane

carbaryl plant tissue extraction, hydrolysis, and colorimetry 0.1 mg/kg Stansbury &
reaction with chromophore Miskus (1964)
---------------------------------------------------------------------------------------------------------

Table 2. (contd.)
---------------------------------------------------------------------------------------------------------
Chemical Sample type Extraction and clean-up Method of Detection^b Reference
detection^a limit
---------------------------------------------------------------------------------------------------------
methiocarb fruit and acetone extraction of GLC/SFPD 0.01 mg/kg Bowman & Beroza
vegetables carbamates and free phenols, (1969)
partition on silica gel column,
hydrolysis of conjugated phenols

methomyl ethyl acetate extraction, GLC/SFPD 0.02 mg/kg Leitch & Pease
hexane extraction, chloroform (1973)
solution, hydrolysis and ethyl
acetate extraction

phenmedipham plant hydrolysis, distillation- colorimetry 0.05 mg/kg Kossmann & Jenny
tissues extraction, and reaction (1973)
with chromophore

propoxur plant and acetone-chloroform extraction, GLC/ECD 0.1 mg/kg Anderson (1973)
animal absorption on Florisil column,
tissues chloro form elution, solution
milk in benzene, and reaction with 0.01 mg/kg
trichloroacetyl chloride

thiofanox soil acetone extraction, oxidation GLC/SFPD Chin et al.
with hydrogen peroxide, (1975)
absorption on Florisil column
and elution with chloroform
and diethyl ether, solution
benzene
---------------------------------------------------------------------------------------------------------
^a GLC = gas-liquid chromatography.
ECD = electron-capture detector.
SFPD = sulfur (sensitive) flame photometric detector.
HPLC = high-performance liquid chromatography.
UV = ultraviolet.
^b Limit of detection is the sensitivity of the method; however, each method may not be able to measure
all the contaminants originally present in the sample, i.e., the recovery rates for spiked
samples < 100%.

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1 Natural Occurrence

Methyl carbamates are related to a naturally-occurring
carbamate alkaloid, physostigmine, isolated from the calabar bean
(Physostigma venenosum) in 1864.

Physostigmine (or eserine) has a pronounced cholinergic action
(Still & Herrett, 1975).

3.2 Man-Made Sources

Carbamate pesticides have been produced and in commercial use
since the 1950s.

Benomyl and carbendazim, belonging to the benzimidazole group
of fungicides, came on the market around 1970.

3.2.1 Production levels, processes, and uses

The carbamates included in this review are those mainly used in
agriculture. They are some of the many synthetic organic
pesticides that have been produced on a large scale. Additional
uses are as biocides for industrial or other commercial
applications and in household products; a potential use is in
public health vector control.

Global consumption of the carbamate insecticides and herbicides
over the period 1974-82 is summarized in Table 3. The data are not
complete but give an estimate of the magnitude of the consumption
and distribution of the compounds throughout the world. The
reported global consumption is between 20 000 and 35 000
tonnes/year. The herbicidal carbamates, an integral part of
industrialized agriculture, are used mainly in North and Central
America, and Europe, with little reported use in Africa, Asia, and
South America. However, in the period mentioned, carbamate
insecticides were substantially used in Asia (FAO, 1985).

Table 3. Global consumption of carbamate insecticides
(in 100 kg)^a
-----------------------------------------------------------
Region 1974-76 1981 1982 1983
-----------------------------------------------------------
Africa
Sierra Leone 25
Sudan 1590
Zimbabwe 4837

North/Central America
Bermuda
Canada 7911
Cuba 7333
El Salvador 507
Mexico 25 367 21 630 21 380 17 210
Montserrat 2 2
USA 125 000 115 000

South America
Argentina 2960 2700
Guyana 36
Suriname 369
Uruguay 63 74 70 179

Asia
Brunei 7 12 29
Cyprus 20 124 21
Hong Kong 100 125 64 102
India 33 447 32 160 23 730
Israel 1333 2160 2580 2270
Japan 27 770
Jordan 2000 16 745
Korea Republic 7618 19 270 17 716
Kuwait 2
Oman 134 6 16
Pakistan 298 3511 1136
Saudi Arabia 63
Turkey 635 666

Europe
Austria 213 184 153 86
Czechoslovakia 1490 569 712
Denmark 60 307 440
Finland 7
Greece 8767
Hungary 5854 1539 3396 11 650
Italy 28 014 28 284 23 041
Norway 5 5 10
Poland 12 353 3325 4365 4977
Portugal 512 216 200
Sweden 94 130
Switzerland 133 140 120
-----------------------------------------------------------
^a From: FAO (1985).

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION

4.1 Transport and Distribution Between Media

4.1.1 Air

In general, the vapour pressure of carbamates is rather low.
Some of them may sublimate slowly at room temperature, and this may
also explain their loss from soil surfaces (Gray, 1971).

4.1.2 Water

The aqueous environment is an important factor in transport.
Carbamates may enter surface water from industrial wastes,
accidental spillage, and dumping. However, the hazard of this is
limited by their rapid decomposition under aqueous conditions.
Thus, while long-term contamination by this type of compound is
unlikely, adverse effects on aquatic animals may result from direct
addition or from run-off shortly after application.

4.1.3 Soil

Reviews of the action of herbicidal carbamates in the soil have
been made by Gray (1971) and Ashton & Crafts (1973).

Herbicidal carbamates are often applied directly to the soil,
whereas insecticidal carbamates, generally applied to plants, reach
the soil either directly or indirectly. Several factors are
involved in the degradation of carbamates in the soil. These
include volatility, leaching, soil moisture, absorption, pH,
temperature, photodecomposition, microbial degradation, and soil
type (Ogle & Warren, 1954).

Different soil types possess different binding capabilities.
In general, carbamate insecticides are not very persistent in the
soil. However, the fungicide carbendazim is very persistent, with
a half-life of about one year.

Because of the many factors involved and the fact that many
carbamates have different properties, it is clear that results with
one soil type and one carbamate cannot be extrapolated to others.

In general, the water solubility of carbamates is rather low
and may explain the relative immobility of the carbamate herbicides
in the soil with regard to leaching and diffusion. When applied to
the soil, some chloropropham (CIPC) can be lost by vaporization or
sublimation, but otherwise it is tightly adsorbed to certain soils
(Ogle & Warren, 1954). It was found by De Rose (1951) that
chloropropham (CIPC) persisted in the soil about twice as long as
propham (IPC). The disappearance of propham from the soil took
about 24 days, while chloropropham persisted at least 48 days.
Water can displace the carbamates from adsorption sites and cause
them to be lost by volatilization (Parochetti & Warren, 1966).

The degradation of the carbamate herbicides in the environment
is summarized in Fig. 1, using chloropropham as a representative
example of the group.

Soil run-off and leaching characteristics of benomyl and its
metabolites were studied in the greenhouse and laboratory using
¹⁴C-labelled materials on soil and turf plots and on soil thin-
layer chromatographic plates (Rhodes & Long, 1974). The studies
showed that benomyl and its metabolites are immobile in soil and do
not leach or move from the site of application.

The first step in the metabolic degradation of these carbamates
in soil is hydrolysis (Sonawane & Knowles, 1971). Simple esters are
hydrolysed to the parent (unstable) acid and alcohol. The general
reaction of carbamates and the breakdown of carbamates in soils are
described in detail in the review of Still & Herrett (1975).
Chloropropham, barban, and swep are hydrolysed to their respective
anilines (Still & Herrett, 1975). In alkaline soil, phenmedipharm
converts hydrolytically to methyl-3-hydroxyphenyl-carbamate, which
in turn hydrolyses to 3-aminophenol (Sonawane & Knowles, 1971).
The substituted anilines that are formed are further metabolized by
oxidation processes (Kaufman & Blake, 1973; Still & Herrett, 1975).

Methomyl and oxamyl degrade rapidly with carbon dioxide as the
end product (Harvey & Pease, 1973; Harvey & Han, 1978b).

In soils, the predominant metabolic pathway is cleavage of the
carbamate bond to yield the alcohol and amino moieties, which may
be further metabolized by the soil-plant system.

Williams et al. (1976a,b) found that the degradation of
carbofuran in British Columbia was slower than expected. Soils

showed rather high residues of this compound (up to approximately 4
mg/kg soil), and there was some evidence of build-up where
treatments were repeated for two successive years. While
carbofuran has a half-life of up to 50 weeks in neutral or acid
soils, it degrades rapidly in alkaline soils.

4.1.4 Vegetation and wildlife

Plant metabolism of carbanilate herbicides has been shown to
involve aryl hydroxylation and conjugation besides hydro-lytic
breakdown. Consequently, they are readily transformed from
lyophilic compounds to hydrophylic metabolites, which contain the
intact carbamoyl group. These polar products are not translocated
but remain in the plant at the site of formation. All available
data indicate that the hydroxylated carbamate metabolites are
detoxification products (Still & Herrett, 1975). In studies on
chloropropham in oat shoots, Still & Rusness (1977) found that the
phenolic metabolites were converted to an S -cysteinyl conjugate,
i.e., S -cysteinyl-hydroxychloropropham.

A large number of studies on the absorption and translocation
of carbamate herbicides have been carried out to study the fate of
these compounds in plants. The results suggest that plant leaf
surfaces are a barrier to the absorption of carbamates. Roots,
however, absorb the herbicides to a much greater extent, and the
carbamate moves to all plant parts. Thus, it is suggested that
carbamates are exclusively distributed via the apoplastic system.
The carrier that is used plays an important role in these
absorption studies.

Many studies are available concerning the actual entry of
carbamates into the plant, their hydrolysis, aromatic ring
hydroxylation, hydrolytic breakdown, and oxidation of aliphatic
groups. Other types of metabolic reactions in different plant
species following different methods of application have also been
described (Baldwin et al., 1954; Abdel-Wahab et al., 1966;
Prendeville et al., 1968; Knowles & Sonawane, 1972; Still &
Mansager, 1972, 1973a,b, 1975; Wiedmann et al., 1976; Guardigli et
al., 1977).

For the metabolic pathway in wildlife, see sections 6 and 7.

4.1.5 Entry into the food chain

Carbamates are metabolized or broken down in soil, plants, and
animals. The questions of the environmental impact and the
significance of the metabolites with respect to persistence and
bioaccumulation in certain species and in the food chain have still
to be answered. For the moment, it appears that most, if not all,
of the known carbamate metabolites are biodegraded rapidly and are
less toxic for the environment than the parent molecules (Still &
Herrett, 1975).

4.2 Biotransformation

4.2.1 Microbial degradation

The carbamates are readily degraded by soil microorganisms in
most soils (Gray, 1971). Environmental conditions that favour the
growth and activity of microorganisms also favour degradation (Ogle
& Warren, 1954). The residual herbicidal activity of both barban
and propham persists longer in sterile soil than in non-sterile
soil.

Kaufman & Kearney (1965) and Kaufman & Blake (1973) isolated
(by soil-enrichment techniques) and identified a soil microorganism
capable of degrading carbamates such as chloropropham. They
suggested that hydrolysis was a major degradation pathway in soils.
Each of the soil microorganisms demonstrated a different range of
substrate specificity, but all were capable of degrading and
dehalogenating a variety of pesticides (production of 3-
chloroaniline and subsequent liberation of free chloride ion).

These studies illustrated the tremendous adaptability of the
soil microbial populations in altering the persistence and
character of the different foreign compounds. Studies carried out
by Clark & Wright (1970) confirmed that cultures of Arthrobacter
sp. and Achromobacter sp. were able to convert phenyl carbamates
to the corresponding aniline compounds.

Suzuki & Takeda (1976) studied the metabolism of dimetilan in
Aspergillus niger van Tieghem. Together with hydrolysis,
oxidation of the alkyl side chain appeared the most important
modification (detoxification) process. Williams et al. (1976b)
found that carbofuran was rapidly degraded in soils containing high
levels of actinomycetes.

4.2.2 Photodegradation

N -methyl carbamates absorb radiation available in the solar
region (lambda = 300 nm) and hence, would be expected to undergo
photo-oxidation as well as metabolic degradation. Addison et al.
(1974) studied the fate of various N -methyl carbamates when
solutions were sprayed on bean foliage and exposed to sunlight or
artificial light (lambda = 254 nm). It is not entirely clear
whether the products they observed resulted solely from a
photochemical reaction or from absorption followed by enzymatic
attack (Addison et al., 1973, 1974).

4.2.3 Photodecomposition in the aquatic environment

Carbamate insecticides in water are subject to photo-
decomposition under the effects of ultraviolet radiation (UVR).
The pH of the aqueous medium was found to be an important factor in
relationship to the rates of photolysis of carbaryl and propoxur,
which were slow at low pH values and tended to increase with
increaseing pH value. However, the decomposition of, for instance,
dimetilan, was not affected by the pH of the irradiated medium.
The primary effect of the UVR appears to be cleavage of the ester

bond resulting in the production of the phenol or heterocyclic enol
of the carbamate esters tested. The hydrolysis products produced
were further photodecomposed to other unidentified degradation
products. Carbaryl produced 5 degradation products, one of which
was identified as 1-naphthol. It is assumed that, apart from the
cleavage of the ester bond, changes at other positions in the
molecule are produced by UVR. However, the intact carbamate ester
group is retained, and consequently the ChE-inhibiting activity
(Crosby et al., 1965; Crosby 1969; Aly & El Dib, 1972).

Similar results were reported for the irradiation products of
other carbamate esters such as aminocarb, mexacarbate, and a number
of analogues (Eberle & Gunther, 1965; Abdel-Wahab F & Casida,
1967).

When the half-life for photodecomposition was studied in a
number of carbamate insecticides, the results suggested that the
light absorption characteristics of the insecticide influenced the
extent of its photodecomposition by a specific light source.
However, the extent of photodecomposition was not the same under
different conditions of irradiation (presence of solvents) and
wave-length (Crosby et al., 1965; Eberle & Gunther, 1965).

Thus, it seems reasonable to suggest that photodecomposition
may account for some loss of carbamate insecticides in clear
surface waters exposed to sunlight for a long period. However,
photolysis may be a minor factor in the decomposition of these
compounds in highly-turbid waters, where the penetration of light
will be greatly reduced (Aly & El-Dib, 1971, 1972).

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1 Exposure of the General Population

5.1.1 Food and drinking-water

From a market-basket study conducted during 1977-80 by the
Finnish National Board of Trade and Consumer Interests, it became
apparent that the residues of benomyl in food had increased.
During the period 1974-76, the average benomyl intake was about 0.2
mg/person per year from both domestic and imported foods. During
the period 1977-80, average intake was 9 mg/year. A later more
accurate study showed an intake of 14 mg/person per year (Finnish
National Board of Health, 1982).

Contamination of groundwater and drinking-water sources by
aldicarb, a carbamate that is used as an insecticide and as a
nematocide, was reported from the USA in 1979 and 1980-81; levels
even higher than 0.075 mg/litre were found (Rothschild et al.,
1982; Zaki et al., 1982). In the Federal Republic of Germany,
detectable amounts of up to 0.001 mg aldicarb/litre were found in a
few samples of drinking-water (Federal Republic of Germany,
personal communication, 1985)^a.

---------------------------------------------------------------------------
^a Letter to the IPCS from the Ministry of the Interior of
the Federal Republic of Germany, Bonn, Federal Republic of
Germany, 28 May, 1985.

6. METABOLISM AND MODE OF ACTION

Most carbamates are active inhibitors of AChE and they do not
require metabolic activation. However, some carbamates, such as
the benzimidazole carbamates, do not have anticholinesterase
activity. Carbamates undergo metabolism, and the metabolites are
generally less toxic than the parent compound; some exceptions
will be discussed later. The metabolism of carbamates is basically
the same in mammals, insects, and plants, and the toxic effects of
carbamates are similar in mammals and insects. Carbamates do not
accumulate in the mammalian body, but are rapidly excreted, mainly
via the urine.

6.1 Mode of Action

Carbamates are effective insecticides by virtue of their
ability to inhibit AChE in the nervous system. AChE catalyses the
hydrolysis of the neurotransmitter acetylcholine (ACh) to choline
and acetic acid.

ACh is the synaptic mediator of nerve impulses in the nervous
system of mammals and insects:

(a) as a neurotransmitter in the brain of mammals, and in
the central nervous system of insects;

(b) as a pre-ganglionic neurotransmitter in the autonomic
nervous system of mammals;

(d) at the neuromuscular junction of skeletal muscle.

Carbamates, like organophosphates, can inhibit esterases that
have serine in their catalytic centre; these are called serine-
esterases or beta-esterases. Although the inhibition of serine-
esterases other than AChE is not significant for the toxicity of
the compounds, it may have significance for the potentiation of
toxicity of other compounds after long-term low-level exposure
(Sakai & Matsumura, 1968, 1971; Aldridge & Magos, 1978).

To understand the mechanism of toxicity, it is necessary to
examine the events that take place at the neuromuscular junction.
When a muscle is innervated, as shown in Fig. 2, a nerve impulse
moving down a neuron reaches the nerve ending where ACh, which is
stored in vesicles at the nerve endings, is released into the
junction. Within 2 - 3 ms, ACh impinges on the receptor side of
the muscle.

AChE then hydrolytically converts the ACh into choline and
acetic acid, which results in a decrease in the concentration of
ACh and cessation of muscle contraction. When AChE is inhibited by
a carbamate ester, it can no longer hydrolyse the ACh. Thus, the
ACh concentration remains high in the junction giving rise to
continuous stimulation of the muscle which, in turn, leads to
exhaustion and tetany.

Thus, inhibition of AChE by carbamate esters, causes toxic
effects in animals and human beings that result in a variety of
poisoning symptoms and eventually culminate in respiratory failure
and death.

The mechanism of inhibition of AChE by a carbamate ester can be
formulated as follows (Reiner & Aldridge, 1967; Aldridge & Reiner,
1972):

O O
|| k₁ ||
AChE + RO-C-NHCH₃ ^'=========_' [AChE x RO-C-NHCH₃] (enzyme-carbamate
k_-1 | complex)
| k_c
\/
O
k_r ||
AChE + CH₃NH₂ + CO₂ <------ AChE-C-NHCH₃ + RO^-
H₂O
(carbamylated enzyme)

where: k₁ = second order rate constant for formation of
complex;
k_-1 = first order rate constant for breakdown of
complex to starting materials;
k_c = first order rate constant for carbamylation of
the enzyme;
k_r = first order rate constant for hydrolysis of the
carbamylated enzyme.

The site for carbamylation of the enzyme is the hydroxyl moiety of
the serine amino acid. The rate of regeneration of the

carbamylated enzyme to AChE (k_r) is relatively rapid compared with
that of an enzyme that has been inhibited (phosphorylated) by an
organophosphorus pesticide (Reiner & Aldridge, 1967; Reiner, 1971).
Thus, human exposure to the carbamate pesticides is less dangerous
than exposure to organophosphorus pesticides, because the ratio
between the dose required to produce mortality and the dose
required to produce minimum poisoning symptoms is, in general,
substantially larger for carbamate compounds than for
organophosphorus compounds (Goldberg et al., 1963; Vandekar, 1965;
Vandekar et al., 1971). An individual experiencing symptoms of
poisoning from a carbamate is more likely to recover on termination
of exposure and with appropriate medical treatment than an
individual poisoned by an organophosphorus compound.

The spontaneous reactivation of various carbamylated ChEs
expressed as half-life, at pH 7.0 - 7.4 and 25 °C ranged between 2
and 240 min for AChE and between 2 and 17 min for serum-ChE. This
instability of the carbamylated enzyme affects the determination of
the inhibitory power of carbamates, the recovery after poisoning,
and the determination of inhibition of blood-ChEs (Reiner, 1971).

6.1.1 Neuropathy target esterase

This esterase, formerly known as neurotoxic esterase (NTE), is
the target for the initiation of delayed neuropathy caused by some
organophosphorus esters. Both organophosphorylation of NTE and a
subsequent "aging" reaction of the inhibited enzyme are required to
initiate neuropathy. Certain N -aryl carbamates can also inhibit
NTE, but the chemistry of such carbamates is such that no "aging"
reaction is possible. These carbamate inhibitors do not initiate
delayed neuropathy in vivo but can actually prevent the neuropathic
effects of a challenge dose of an organophosphate given while the
tissue NTE is carbamylated (Johnson, 1970). No delayed neuropathic
effects have been observed in tests on carbamate pesticides
according to protocols that detect neuropathic organophosphates,
and this agrees with the mechanism described above.

6.2 Metabolism

The metabolic fate of carbamates is basically the same in
plants, insects, and mammals.

Carbamates can penetrate the skin, mucous membranes,
respiratory tract, and gastrointestinal tract of mammals.

6.2.1 Absorption

In vivo studies have shown that carbamates are almost
completely absorbed during normal transit through the gastro-
intestinal tract.

According to absorption studies on rats, the dermal absorption
of radioactive labelled benomyl is slow. After absorption, rapid
metabolism and elimination via urine took place. After 1 h, small
amounts of the major metabolites of benomyl, 5-hydroxy-2-
benzimidazole carbamate (5-HBC), and methyl 2-benzimidazole

carbamate (MBC), could be detected in urine, confirming the rapid
metabolism of benomyl. No radioactivity was found in any body
tissues sampled 24 h after application (FAO/WHO, 1985a).

6.2.2 Distribution

Ahdaya et al. (1981) studied the absorption and distribution of
carbamates. They found that, in mice, all carbamates were rapidly
distributed to the tissues and organs. The half-life values of
penetration ranged from 8 to 17 min for carbamates.

No residues of benomyl or its degradation products were
detected (level of sensitivity 0.02 mg/kg) in eggs from chickens
fed 5 mg benomyl/kg diet. Only 5-HBC (0.03 - 0.06 mg/kg) was found
in eggs from chickens fed 25 mg benomyl/kg for four weeks. No
residues of benomyl or MBC were found in milk (< 0.02 mg/litre)
from dairy cows fed benomyl for 32 days at dietary levels of 0, 2,
10, and 50 mg/kg. At the highest dietary level, the combined
residue of 5-HBC plus 5-hydroxy-2-benzimidazole carbamate (4-HBC)
in the milk was approximately 0.1 mg/litre. These metabolic
studies appear to indicate that benomyl and its metabolites do not
accumulate in animal tissues or animal products (Gardiner et al.,
1974; FAO/WHO, 1985a).

A rat was administered a diet containing 2500 mg non-labelled
benomyl/kg. Twelve days later, the animal received an intragastric
intubation of 2-¹⁴C-benomyl (7.7 mg). Urine and faeces were
collected, and organs were analysed for radioactivity. The
elimination was rather rapid and mainly via the urine. After 72 h,
0.2% of the administered dose was found in the liver, while < 0.01%
was present in all the other organs and the carcass. The same
results were found when a rat was administered 2-¹⁴C-MBC
(Gardiner et al., 1974)

A male beagle dog fed a diet containing non-labelled benomyl at
2500 mg/kg, and administered 30.8 mg 2-¹⁴C-benomyl by capsule 7
days later, showed a completely different excretion pattern. After
3 days, residues were found only in the liver (0.31% of the
administered dose); the levels in the other organs were < 0.01%
(Gardiner et al., 1974).

Rats administered an oral dose of ¹⁴C-labelled propham or
chloropropham (labelled in the chain or in the ring), showed
radioactivity in all tissues, with the highest concentration in
the kidneys. The average biological half-life of ¹⁴C from both
compounds in most organs was short, ranging from 3 to 8 h.
However, in brain, fat, and muscle, the half-life was about twice
this value (Fang et al., 1974).

6.2.3 Metabolic transformation

6.2.3.1 Biotransformation mechanisms

Carbamate pesticides are transformed metabolically by a variety
of chemical reactions into more water-soluble molecules with
increased polar properties. The initial step, usually oxidative in
nature, introduces a functional hydroxyl group that serves as a
site for secondary conjugative reactions to yield products that

can be excreted via the urine and/or faeces. In some cases, the
oxygenated metabolites, such as 5-hydroxypropoxur and 5-hydroxy-
carbaryl, are known to be toxic and to possess anticholinesterase
activity (Fig. 3) (Oonnithan & Casida, 1968; Black et al., 1973).
This undoubtedly contributes to the overall toxicity of the parent
compound (Black et al., 1973).

6.2.3.2 Oxidation

The principal route of metabolism of insecticidal carbamate
esters is oxidative and is generally associated with the mixed-
function oxidase (MFO) enzymes, which are present in several
tissues. Examples of the sites of oxidative attack on a
hypothetical methyl carbamate are given in Fig. 4.

Depending on the functional groups in the molecule, a variety
of reactions catalysed by these enzymes may occur (Fukuto, 1972),
as shown in Fig. 4. Typical oxidative reactions include: (a)
hydroxylation of aromatic rings, or epoxidation; (b) O -
dealkylation; (c) N -methyl hydroxylation; (d) N -dealkylation; (e)
hydroxylation and subsequent oxidation of aliphatic side chains;
and (f) thioether oxidation to sulfoxides and sulfones.

Because of the variety of different groups present in carbamate
insecticides, the metabolism of these compounds is often complex.
Carbaryl, for example, is a relatively simple compound, yet it is
metabolized to at least 15 different compounds in mammals through a
variety of oxidative and hydrolytic reactions (Leeling & Casida,
1966). A study showed that, as well as the organic solvent-soluble
unconjugated metabolites, the urine from carbaryl-treated rabbits
contained at least 4 or 5 other water-soluble metabolites. These
are probably conjugates of the hydroxylated products of carbaryl,
i.e., glucuronides or sulfates.

6.2.3.3 Hydrolysis

Carbamates are hydrolysed either spontaneously or by esterases
(Aldridge & Reiner, 1972), yielding, as final products, an amine,
carbon dioxide (CO₂), and an alcohol or phenol:

R¹HN-C(O)OR² + H₂O ---> R¹NH₂ + CO₂ + R²OH

The mechanism of hydrolysis is different for N -methyl and
N -dimethyl carbamates.

In general, the rates of hydrolysis of carbamates by esterases
are faster in mammals than in plants and insects, though there are
exceptions. The differences in enzymatic rates of hydrolysis
depend on the structure of the carbamate and on the particular
esterase.

Hydrolysis of the carbamates is catalysed by a group of enzymes
known as A-esterases or arylesterases (EC 3.1.1.2). It has been
established that enzymatic hydrolysis occurs both in vitro and in
vivo, but, to date, it has not been determined to what extent the
in vivo hydrolysis contributes to the detoxification of the
chemical.

The hydrolytic activity of plasma-albumin has also been
indicated (Augustinsson & Casida 1959; Casida & Augustinsson, 1959;
Reiner & Skrinjaric-Spoljar, 1968).

6.2.3.4 Conjugation

The conversion to conjugated compounds of the hydroxy products
produced by the drug-metabolizing enzyme systems is an important
reaction that leads to the formation of water-soluble compounds
such as O - and N -glucuronides, sulfates, and mercapturic acid,
which can be eliminated via the urine or faeces.

6.2.3.5 Examples of biotransformation of carbamates

A few examples of the biotransformation of carbamates are given
in Fig. 5.

Aldicarb is unusual in that it does not contain an aromatic
ring, and it is also a carbamate ester of an oxime. Knaak et al.
(1966) studied its metabolism in rats. The metabolic reactions of
aldicarb are similar to the oxidative and hydrolytic pathways found
in the thioether-containing organophosphorous esters. The
thioether moiety is rapidly oxidized to the oxime-sulfoxide; the
oxime-sulfoxide is oxidized much more slowly to the oxime-sulfone.
A large number of metabolites resulting from the hydrolysis of the
carbamate moiety of the oxidative metabolites of aldicarb have been
identified.

The oxime-sulfoxide was found to be the major hydrolytic
metabolite in plants and insects. Other research workers recovered
the nitrile sulfoxide, in greater quantities, both compounds
arising from cleavage of the carbamate moiety of aldicarb sulfoxide
(Metcalf et al., 1966; Fukuto, 1972).

Propham and chlorpropham have been studied in a number of
animal species. Both compounds are rather rapidly metabolized.
The major metabolites recovered from the animals were 4-
hydroxypropham and 4-hydroxychloropropham, respectively. Both were
excreted as the sulfate and/or glucuronide. Depending on the animal
species, hydroxylation of propham also occurred in the 2- and 3-
position, and even 3,4-dihydroxypropham was identified. Oxidation
of the 1-carbon of the isopropyl moiety of chlorpropham resulted in
the formation of the monohydroxy product, which was converted to
the 1,3-hydroxy and 1-carboxy derivative. Hydrolysis of
chloropropham also occurred to a significant extent, since the
other major metabolites found were conjugates of 3-chloro-4-
hydroxy- and 5-chloro-2-hydroxy-acetanilide. Chlorpropham is more
susceptible to hydrolytic degradation than propham, or, conversely,
propham is more susceptible to oxidative degradation than
chlorpropham (Bobik et al., 1972; Paulson et al., 1973; Fang et
al., 1974).

Phenmedipham and desmedipham were hydrolysed mainly into
methyl- N or ethyl- N -(3 hydroxyphenyl) carbamates. Both compounds
were subsequently hydrolysed to 3-aminophenol. Furthermore, the
aminophenol was N -acetylated, and the phenolic hydroxyl moiety was
conjugated with the sulfate or glucuronide (Sonawane & Knowles,
1972).

Carbaryl is rapidly hydroxylated or hydrolysed and thereafter
conjugated and eliminated from a number of animal species,
principally in the urine as glucuronides or sulfates. Depending on
the animal species, at least 8 water-soluble metabolites were
found. Dorough & Casida (1964) and Fukuto (1972) studied the
biotransformation of carbaryl in animals. It was found that
hydroxylation took place, and the following metabolites were
identified: 1-naphthyl N -hydroxymethylcarbamate (and as a result
of ring hydroxylation) 4-hydroxy-1-naphthyl- N -methylcarbamate,
5-hydroxy-1-naphthyl- N -methyl-carbamate, and 5,6-dihydroxy-1-
naphthylmethylcarbamate.

Certain metabolites appeared to be the hydrolytic products of
carbamates with modified ring structures, such as 1-hydroxy-5,6-
dihydro-naphthalene and 1-naphthol. The latter compound is found
in the urine as 1-naphthyl glucuronide and/or 1-naphthyl sulfate.
Sullivan et al. (1972) identified one of these metabolites, 5,6-
dihydroxy carbaryl glucuronide, in the urine of rats. Most
metabolites of carbaryl are eliminated in a similar pattern in
human beings, rats, guinea-pigs, and sheep, but in a different way
in dogs (FAO/WHO, 1968b).

The metabolism of benomyl has been studied in the mouse, rat,
rabbit, dog, sheep, and cow and is qualitatively the same in all
animal species studied.

The basic route of benomyl metabolism involves hydroxylation
to 5-hydroxy-2-benzimidazolecarbamate (5-HBC). This is the main
metabolite and conjugates (glucuronides or sulfates) of the
compound are usually eliminated via the urine, bile, and faeces
(Douch, 1973; Gardiner et al., 1974). In contrast, dogs treated
orally with ¹⁴C-benomyl eliminated only 16% of the radioactivity in
the urine and 83% in the faeces. The principal metabolites in the
faeces were carbendazim and 5-hydroxycarbendazim. In addition, 4-
hydroxycarbendazim was identified as a metabolite in the urine,
faeces, and milk of a cow treated with benomyl (Fig. 6).

Heybroek et al. (1984) studied the metabolism of [ring-¹⁴C]
asulam in the rat. Most of the radioactivity (61 -74%)
administered orally or intravenously was excreted in the urine in
24 h as unchanged asulam, 8 - 14% as N ⁴-acetylasulam, and up to
2.5% as N ⁴-acetylsulfanilamide. Small amounts of radioactivity
were found in faeces.

These examples illustrate the different metabolic pathways of
biotransformation. For more details, readers should refer to
Fukuto (1972) who has reviewed the metabolism of carbaryl,
mexacarbamate, aminocarb, formetanate, aldicarb, methiocarb,

carbofuran, MPMC, trimethacarb carbanolate, propoxur, and
dimetilan. Harvey & Han (1978a) have described the metabolism of
oxamyl and Harvey et al. (1973), of methomyl in the rat. The
metabolism of pirimicarb was extensively described in FAO/WHO
(1977b). Furthermore, information on the metabolism is provided in
the handbooks on carbamates, and the different monographs of the
Joint FAO/WHO Meetings on Pesticide Residues.

6.3 Elimination and Excretion in Expired Air, Faeces, and Urine

6.3.1 Man

In a study on human volunteers, 2 men were dosed orally with
carbaryl in gelatin capsules at 2.0 mg/kg body weight.
Chromatographic analyses of a 4-h urine sample revealed 1-naphthyl
glucuronide (15%), 1-naphthyl sulfate (8%), and 4-(methylcarbamoyl-
oxy)-1-naphthyl glucuronide (4%). In addition, 1-naphthyl
methylimidocarbonate O -glucuronide was identified by fluorometry
(FAO/WHO, 1968b). During the first 8 h, the urinary excretion of
these metabolites constituted 12 - 15% of the carbaryl
administered. Small amounts of urinary metabolites were detected
by fluorometric analysis on the second day, but not afterwards
(Knaak et al., 1968). In another study, 1-naphthyl glucuronide, 1-
naphthyl sulfate, and other unidentified metabolites were found in
24-h urine samples from men exposed to carbaryl dust during a
packaging operation in a factory (details not available) (Knaak et
al., 1965).

The determination of 1-naphthol or its sulfate or glucuronide
in the urine of persons occupationally exposed to carbaryl is one
method of biological monitoring for this exposure (FAO/WHO, 1982b).

Dawson et al. (1964) investigated the metabolism and excretion
of propoxur in 3 male volunteers orally administered a dose of 50
mg. Within 8 - 10 h, 27% of the propoxur appeared in the urine as
its metabolite 2-isopropoxyphenol, indicating that propoxur is
rapidly absorbed and hydrolysed in human beings.

The absorption and fate of benomyl in animals following oral
administration of the compound varies with species. According to a
study that involved one rat and one dog administered 2-¹⁴C-benomyl
by stomach tube, the rat eliminated about 86% of a single dose via
the urine and 13% via faeces. The dog eliminated only 16% of a
similar benomyl dose via the urine, while almost 83% was detected
in faeces. No data are available concerning the possible roles of
the enterohepatic circulation and biliary excretion of benomyl or
its metabolites (Gardiner et al., 1974).

6.3.2 Laboratory animals

In animals, oxidation of carbamates often, but not always,
results in detoxification. Oxidative metabolism generally leads to
products of greater polarity and water solubility, and these can be
more readily eliminated through the urine and faeces than the
parent compound.

In the rat, benomyl is mainly excreted via the urine (78.9% of
the given dose) as 5-HBC conjugates. In the dog, 99% of the dose
administered was eliminated within 72 h. The major route of
elimination was via the faeces in which benomyl and/or MBC and 5-
HBC were detected. On the basis of studies on the rat, dog, dairy
cow, and chicken, neither benomyl nor its metabolites tend to
accumulate in animal tissues (Gardiner et al., 1974).

According to FAO/WHO (1974b), feeding of ¹⁴C- or ³⁵S-labelled
thiophanate-methyl to rats, mice, and dogs resulted in 80 - 100%
recovery of the label in the faeces and urine within 96 h.
Unmetabolized thiophanate-methyl has been reported to be the major
component of faecal elimination. The minor part consisted of 4-
hydroxy-thiophanate-methyl, dimethyl-4,4- O -phenylenebisallophanate.
MBC and 5-hydroxy-MBC were also detected.

Following application of carbendazim to rats and mice by
intragastric intubation, almost all metabolites in the urine were
conjugated as sulfate esters. 5-HBC was released as the only
metabolite extractable from water. A higher proportion of polar
compounds was found in the urine of mice than in that of rats.
Polarity was caused by the phenolic hydroxyl group (FAO/WHO,
1985a).

The excretion of ¹⁴C-labelled propham and chlorpropham was
investigated in female rats after a single oral dose. The average
3-day urinary excretion of radioactivity was between 56 and 85%,
depending on the position of labelling: viz chain or ring
labelling. With chain ¹⁴C-chlorpropham, an average of 35% of the
administered radioactivity appeared in the respired air, compared
with only 5% for chain ¹⁴C-propham (Fang et al., 1974).

6.4 Metabolism in Plants

Studies on the metabolism and fate of carbamate pesticides in
plants are of great importance for assessing the human hazards from
the consumption of fruit and vegetables containing residues of the
applied pesticides and metabolites.

Generally, the metabolism of carbamates in plants follows
another route of detoxification. The hydroxylated carbamates
produced by enzymes are conjugated either with amino acids (for
instance cysteine) or as glycosides and phosphates, which are
stored as terminal metabolites (Still & Rusness, 1977; Aldridge &
Magos, 1978).

The toxicological properties of plant conjugates in mammals or
other animals are not generally known. Insecticidal methyl
carbamate esters are vulnerable to hydrolytic cleavage, which
results in detoxified products. Detoxification of carbamate esters
in plants by hydrolysis occurs to a lesser extent than
detoxification by oxidative metabolism; nevertheless, hydrolytic
cleavage of the ester moiety is a significant detoxification
mechanism. For instance, in sugar beets treated with desmedipham,
the major metabolites were ethyl-3-hydroxy phenyl carbamate and 3-
aminophenol (Knowles & Sonawane, 1972). Carbetamide is hydrolysed

to aniline (Guardigli et al., 1977). Aromatic ring hydroxylation
appears to be the predominant metabolic reaction that carbamate
herbicides undergo in plants. The major metabolites isolated from
soybean plants, after uptake of chlorpropham, were glucoside
conjugates of 2-hydroxy- and 4-hydroxypropham. Similar results
were obtained with propham (Still & Mansager, 1973a,b, 1975).

There is evidence that the ring hydroxylation varies with the
plant species. In other carbamates, for instance in the case of
barban, ring hydroxylation did not appear to take place, and polar
metabolites were produced, which were hydrolysed into
chloroanilines (Still & Mansager, 1972).

Furthermore, oxidation of aliphatic groups takes place in
carbamate herbicides (Wiedmann et al., 1976). Still & Herrett
(1975) demonstrated N -methyl hydroxylation for dichlormate, a
metabolite that is either conjugated or loses the hydroxymethyl
moiety to produce N -demethylated dichlormate.

Thiophanate-methyl is mainly metabolized into carbendazim (MBC)
and, to a less extent, into 2-aminobenzimidazole (2AB). A few other
metabolites have also been found (FAO/WHO, 1974b).

The metabolic pathway for carbaryl in plants is identical,
whether the compound is injected into the stem or applied to the
leaf surface. After entering the plant, carbaryl undergoes
biotransformation to its primary metabolites, which are similar to
the ones formed in animals. These hydroxylated metabolites, which
are less toxic than carbaryl itself, are conjugated by plants to
form water-soluble glycosides. Injection of carbaryl into the
bean plant produced water-soluble metabolites attributable to
hydroxylation of the ring or N -methyl group followed by
conjugation, mainly as glycosides (Kuhr, 1968; FAO/WHO, 1970b;
Fukuto, 1972).

The metabolism of aldicarb in plants is the same as that in
mammals. In the cotton plant, the thioether moiety is rapidly
oxidized to the sulfoxide, and the latter is slowly transformed to
the sulfone. The sulfoxide is rather stable and can be present in
cotton plants, even as long as 2 months after treatment. The total
metabolites present can be as high as 80% (Kuhr, 1968). Because of
their persistence in plants, and their even higher anticholinesterase
activity compared with aldicarb itself, the sulfoxide and sulfone
should be taken into account when considering residue tolerances.

The major metabolite isolated from bean plants, 28 days after
treatment with carbofuran, was a conjugate of 3-hydroxy-carbofuran,
which remained in the aqueous phase after extraction with an
organic solvent. The conjugate represented 55% of the total ¹⁴C-
labelled residues present in the plant. 3-Hydroxy-carbofuran is
highly toxic for mammals (LD₅₀ in the rat 7 mg/kg body weight).
The conjugate may also be toxic, particularly taking into account
the acid conditions of the mammalian stomach under which the
conjugate will be hydrolysed (FAO/WHO, 1977b, 1980b).

Harvey et al. (1978) studied the metabolism of oxamyl in plants
(tobacco, alfalfa, peanuts, potatoes, apples, oranges, and
tomatoes). The major route of degradation involved hydrolysis to
the corresponding oximino compound, which in turn became conjugated
with glucose. Further metabolism resulted in the loss of one of
the N' -methyl groups and/or addition of other glucose units to the
sugar moiety of the original conjugate. Total breakdown into
normal natural products has been demonstrated.

In a study by Harvey & Reiser (1973), methomyl rapidly degraded
to carbon dioxide and acetonitrile, which volatilized from the
plant tissues. The half-life for methomyl was of the order of 3 -
6 days. The remainder of the compound had been reincorporated into
natural plant components.

The metabolic pathways for a number of carbamate pesticides are
described in more detail in Fukuto (1972).

7. EFFECTS ON ORGANISMS IN THE ENVIRONMENT

7.1 Microorganisms

As described in section 4.2.1, soil microorganisms are capable
of hydrolysing carbamates. Furthermore, it seems that
microorganisms can easily adapt themselves to metabolize the
different types of carbamates (Aly & El-Dib, 1972). Nevertheless,
carbamates and their metabolites can affect the microflora and
cause changes that may have an important impact on the maintenance
of the soil productivity (Filip, 1974).

7.2 Aquatic Organisms

In general, carbamates are not very stable under aquatic
conditions (section 4). The solubility in water differs
considerably for the different carbamates (Annex I). Furthermore,
photodecomposition takes place and microorganisms are able to
degrade these compounds. By these processes, the carbamate is
broken down (oxidation, hydrolysis) into other compounds. Some
will be less toxic, others even more toxic than the parent
compound. Carbaryl will be hydrolysed into 1-naphthol, which is
just as toxic (Armstrong & Millemann, 1974a). It seems unlikely
that most of the carbamates will persist long in the environment,
but there are exceptions, such as dimetilan and carbendazim.

Many data are available on the acute toxicity of the carbamates
in several fresh- and salt-water fish and other organisms, some of
which are listed in Tables 4 and 5.

When carbamates are deliberately added to an aquatic system,
caution is necessary since, with high concentrations, a number of
species are at risk. For example, while applications of carbaryl
of 0.93 kg/ha were effective in controlling the ghost shrimp
(Callianassa californiensis), an oyster pest, they also caused a
reduction in the population of juvenile clams (Armstrong &
Millemann, 1974a).

In a similar study (Armstrong & Millemann, 1974b), carbaryl
and its hydrolytic product, 1-naphthol, were examined for other
effects on the mussel (Mytilus edulis). An age dependence was
noted; the most sensitive stage (appearance of the first polar body
shortly after fertilization) had EC₅₀s of 5.3 and 5.2 mg/litre for
carbaryl and 1-naphthol, respectively. Effects of the compounds
on development were characterized by disjunction of blastomeres and
asynchronous and unaligned cleavages. Carbaryl, at a single dose
of 0.1 mg/litre, also disrupted normal schooling behaviour in
juvenile Menidia medidia; the disruptive effects were attributed
to the 1-naphthol rather than to the parent carbamate. These
effects were reversed within 3 days of placing the fish in clean
water (Weis & Weis, 1974). Hansen (1969) studied the capacity of
sheepshead minnows (Cyprinodon variegatus) to avoid carbaryl. The
fish did not avoid carbaryl in concentrations of 0.1 - 10 mg/litre
water.

Table 4. Acute aquatic toxicity (LC₅₀ in mg/litre)^a
----------------------------------------------------------------------------------------------------
Compound Type of Name of organism Stage or Temp- Concentration
organism weight (g) erature (mg/litre)
(°C) (after 96 h)
----------------------------------------------------------------------------------------------------
Aldicarb fish Rainbow trout (Salmo gairdneri) 0.5 12 0.56
fish Bluegill (Lepomis machrochirus) 1.3 24 0.05

Aminocarb fish Rainbow trout (Salmo gairdneri) 1.5 12 13.5^b
fish Bluegill (Lepomis machrochirus) 2.0 20 3.1^b
crustacea Daphnia (Daphnia magna) first instar 21 0.01 - 0.1^c
(48-h EC₅₀)
fish Rainbow trout (Salmo gairdneri) 1.5 10 0.13^c
fish Bluegill (Lepomis machrochirus) 0.6 20 0.1^c
insect Midge (Chironomus plumosus) fourth instar 20 0.27^c

Benomyl fish Rainbow trout (Salmo gairdneri) 1.2 12 0.17^d
fish Bluegill (Lepomis machrochirus) 0.9 22 0.85^d
fish Rainbow trout (Salmo gairdneri) 1.0 12 0.31^e
fish Bluegill (Lepomis machrochirus) 0.6 22 1.2^e

Benomyl fish Rainbow trout (Salmo gairdneri) 0.2 12 0.37^f
metabolite
MBC

Bufencarb crustacea Scud (Gammarus fasciatus) mature 15 0.001
fish Goldfish (Carassius auratus) 1.0 18 0.29

Carbaryl fish Rainbow trout (Salmo gairdneri) 1.5 12 1.95^d
fish Bluegill (Lepomis machrochirus) 1.2 18 6.76^d
crustacea Daphnia (Daphnia pulex) first instar 16 0.0064^d
(48-h EC₅₀)
crustacea Scud (Gammarus fasciatus) mature 21 0.026^d
----------------------------------------------------------------------------------------------------

Table 4. (contd.)
----------------------------------------------------------------------------------------------------
Compound Type of Name of organism Stage or Temp- Concentration
organism weight (g) erature (mg/litre)
(°C) (after 96 h)
----------------------------------------------------------------------------------------------------

Carbofuran fish Rainbow trout (Salmo gairdneri) 1.5 12 0.38^d
fish Bluegill (Lepomis machrochirus) 0.8 18 0.24^e

Dichlormate fish Rainbow trout (Salmo gairdneri) 0.8 12 4.9

Methiocarb fish Rainbow trout (Salmo gairdneri) 1.3 12 0.80
fish Bluegill (Lepomis machrochirus) 1.0 24 0.21

Methomyl fish Rainbow trout (Salmo gairdneri) 1.1 12 1.60^b
fish Bluegill (Lepomis machrochirus) 0.9 20 1.05^b
crustacea Daphnia (Daphina magna) first instar 21 0.009^b
(48-h EC₅₀)

Mexacarbate fish Rainbow trout (Salmo gairdneri) 1.0 11 12.0^g
fish Bluegill (Lepomis machrochirus) 0.7 12 22.9^g
crustacea Daphina (Daphina pulex) first instar 15 0.010^g
(48-h EC₅₀)
crustacea Scud (Gammarus fasciatus) mature - 0.04^g

Trimethacarb fish Rainbow trout (Salmo gairdneri) 1.2 12 1.0
fish Bluegill (Lepomis machrochirus) 0.9 18 11.6
----------------------------------------------------------------------------------------------------
^a From: Johnson & Finley (1980).
^b Technical material, 95 - 98%.
^c Liquid formulation, 17%.
^d Technical material, 99%.
^e Wettable powder, 50%.
^f Methyl-2-benzimidazole (MBC), 99%.
^g Technical material, 90 - 95%.

Note: It should be recognized that the LC₅₀ and EC₅₀ values only give an indication of the
toxicity and that the toxicity for these organisms may be appreciably changed by variations
in temperature, pH, oxygen content, and water hardness. A 10-fold increase in the toxicity
may be found. Also, the life stage of the organisms is an important factor in estimating
the toxicity of the compound.

Table 5. Acute toxicity of carbamate pesticides for some aquatic organisms
------------------------------------------------------------------------------------------------
Pesticide^a TLMs for organisms at indicated time (mg/litre) Reference
Carp Goldifsh Killifish Guppy Water flea
(Cyprinus (Carassius ( Fundulus ( Lebistes ( Daphnia
carpio auratus) sp) reticulatus pulex
Linné) Peters) Leydig)
48 h 48 h 48 h 48 h 3 h
------------------------------------------------------------------------------------------------
Benomyl 7.5 12 11 (wettable - 14 Yoshida & Nishiuchi
powder) (1972)

BPMC 16 10 _~ 40 1.7 5.0 0.32 Yoshida & Nishiuchi
(1972); Nishiuchi
(1974)

Carbaryl 13 10 _~ 40 2.8 2.8 0.05 Yoshida & Nishiuchi
(1972)

Carbendazim > 40 - 40 - > 40 Yoshida & Nishiuchi
(1976)

CPMC 10 _~ 40 10 _~ 40 5.6 7.0 0.1 _~ 0.5 Yoshida & Nishiuchi
(emulsifiable (1972); Nishiuchi
concentrate) (1974)

Isoprocarb 10 _~ 40 10 _~ 40 5.9 3.0 0.30 Yoshida & Nishiuchi
(1972); Nishiuchi
(1974)

Mecarbam 0.70 0.68 0.35 0.055 0.03 Yoshida & Nishiuchi
granule (1972); Nishiuchi
(1974)

Methomyl 2.8 2.7 0.87 1.0 0.045 Yoshida & Nishiuchi
(1972); Nishiuchi
(1974)
------------------------------------------------------------------------------------------------

Table 5. (contd.)
------------------------------------------------------------------------------------------------
Pesticide^a TLMs for organisms at indicated time (mg/litre) Reference
Carp Goldifsh Killifish Guppy Water flea
(Cyprinus (Carassius ( Fundulus ( Lebistes ( Daphnia
carpio auratus) sp) reticulatus pulex
Linné) Peters) Leydig)
48 h 48 h 48 h 48 h 3 h
------------------------------------------------------------------------------------------------
MPMC 10 _~ 40 10 _~ 40 12 7.3 0.07 Yoshida & Nishiuchi
(1972); Nishiuchi
(1974)

MTMC 10 _~ 40 10 _~ 40 27 13 0.35 Yoshida & Nishiuchi
(1972); Nishiuchi
(1974)

Pirimicarb > 40 - 40 - 0.048 Yoshida & Nishiuchi
(1976)

Promecarb 2.7 - 3.3 - 0.020 Yoshida & Nishiuchi
(1976)

Terbam 0.92 - 2.2 - 0.025 Yoshida & Nishiuchi
(1976)

Thiophanate > 40 > 40 > 40 - > 40 Yoshida & Nishiuchi
(1972)

Thiophanate 11 > 40 11 (wettable - > 40 Yoshida & Nishiuchi
-methyl powder) (1972)

XMC > 40 > 40 33 25 0.055 Yoshida & Nishiuchi
(1972); Nishiuchi
(1974)
------------------------------------------------------------------------------------------------
^a BPMC = 1-sec-butylphenylmethyl carbamate
CPMC = 1-chlorophenylmethyl carbamate
MPMC = 3,4-xylylmethyl carbamate
MTMC = 4-tolylmethyl carbamate
Terbam = 4-tert-butylphenylmethyl carbamate
XMC = 3,5-xylylmethyl carbamate

Note: Test methods are officially recognized methods based on the Notification of the
Ministry of Agriculture, Forestry, and Fisheries of Japan.
Carbamates can have adverse effects on algae. Stadnyk et al.
(1971) reported that carbaryl at a concentration of 0.1 mg/litre
caused an increase in cell numbers and in the biomass of the green
algae Scenedesmus quadricaudata.

Motsuage fish (Motsugo pseudoras bora parva) were reported to
accumulate carbaryl and BPMC during a 30-day exposure to a
concentration of 0.6 - 1.2 mg/litre. Carbaryl uptake was greatest.
However, this compound underwent more rapid metabolism and
excretion than BPMC. Metabolism of BPMC was slow and introduced
permanent spinal curvature of the backbone in about 30% of the fish
exposed, an effect seen with the organophosphorus insecticide
diazinon (Kanazawa, 1975).

A residue study indicated that over a 28-day period at the
highest exposure level of 0.75 mg methomyl/litre, no accumulation
of the carbamate took place. During the entire exposure period,
the values ranged from 0.36 to 0.45 mg/kg. After a 3-day withdrawal
period, no methomyl (< 0.02 mg/kg) was detected in the fish
(Kaplan & Sherman, 1977).

The results summarized in Table 4 are representative of the
toxicity of the different carbamates for fish and invertebrate
species. In general, the LC₅₀s of carbamates for different fish
species range from approximately 0.1 to 10 mg/litre. Invertebrates
are usually more sensitive. The EC₅₀ values for Daphnids and
other species are mainly below 0.1 mg/litre. Some carbamates seem
to be very toxic (< 0.01 mg/litre) for these invertebrates.

7.2.1 Field studies

Little information is available concerning the fate and
persistence of carbamates in the aquatic environment. The presence
of these carbamates in surface waters may have an effect on water
organisms.

Quraishi (1972) studied the persistence of aldicarb. Field
water treated in the laboratory at 100 mg aldicarb/litre resulted
in residues of aldicarb and its metabolites of 0.4 mg/litre after
11 months. Water was stored at 16 - 20 °C and exposed for
approximately 507 h to sunlight.

Data concerning the fate of aminocarb have been reviewed by
Vassilieff & Ecobichon (1982). A half-life of 28.5 days in pond
water was found under normal environmental conditions. The
principal metabolite appeared to be the phenol, though the
methylamino and formylamino analogues were also observed.

Harvey & Pease (1973) found that, under field conditions in
Delaware, Florida, and North Carolina, methomyl broke down almost
completely within 1 month; most of it was lost from the soil by
volatilization, presumably as carbon dioxide. Small amounts
extracted from the soil consisted of methomyl, S -methyl N -
hydroxythioacetimidate, and some polar compounds. A run-off study,
under farm conditions, showed that the compound did not move into
untreated areas with run-off water.

7.3 Terrestrial Organisms

7.3.1 Effects on soil fauna

Thiophanate-methyl and carbendazim have been shown to be
equally as toxic through contact as benomyl. Earthworms (Lumbricus
terrestris) immersed for 1 min in a 0.6% aqueous suspension of
these compounds died in 14 days. Worms in pots containing soil
drenched at a rate of 0.78 g/m² died within 18 days (Wright &
Stringer, 1973).

7.3.2 Wildlife

The LD₅₀ of benomyl for mallard ducks and quail is > 5000
mg/kg body weight. The cumulative toxicity of formetanate (7 days)
is approximately 6800 mg/kg body weight for ducks, and > 4640 mg/kg
body weight for quail and for pheasant (Aldridge & Magos, 1978).

Lethal and sub-lethal doses of aldicarb, methiocarb, oxamyl,
pirimicarb, and thiofanox administered to Japanese quail produced
significant inhibition of plasma- and brain-ChE and influenced the
activity of other enzymes, such as alpha-naphthyl acetate esterase,
glutamate dehydrogenase, and glutamate oxaloacetic transaminase.
With sub-lethal dose levels, plasma-ChE activity recovered within a
few days (Westlake et al., 1981).

An 8-day dietary LC₅₀ for the Peking duck was 1890 mg/kg
feed; for bobwhite quail, the dietary LC₅₀ was 3680 mg/kg feed
(Kaplan & Sherman, 1977).

Bobwhites (Colinus virginianus) were provided diets containing
sublethal levels of carbaryl (237 or 1235 mg/kg), for 7 days, or
carbofuran (26 mg/kg), for 14 days. Food intake, body weight, and
the locomotor activity of adult bobwhites were normal.
Administration of a diet containing 131 mg carbofuran/kg resulted
in reductions in food intake, body weight, and locomotor activity
(Robel et al., 1982).

7.3.3 Bees

An undesirable effect caused by the use of carbamate
insecticides has been the mortality of honey bees, a species that
exhibits a high sensitivity to these compounds. Because of the
agricultural and ecological importance of bees, thorough
comparative toxicity studies have been carried out on a large
number of compounds to identify those with the widest margin of
safety (Atkins et al., 1973).

Abdel-Aal & Fahmy (1977) compared the effects of aldicarb,
methomyl, N -desmethylmethomyl, carbaryl, and carbofuran on the
honey bee (Apis mellifera) and found that all of the compounds
were extremely toxic.

7.4 Earthworm and Mite Populations

Studies by Stringer & Wright (1973) and Wright & Stringer
(1973) showed that earthworm populations in apple orchard plots

sprayed with either benomyl or thiophanate-methyl were reduced. In
particular, Lumbricus terrestris, an important species in the
ecology of the apple orchard, was virtually eliminated. Captive
worms would not feed on leaf material that had been sprayed at 1.75
µg/cm² with benomyl, methyl benzimidazol-2-yl carbamate (MBC), or
thiophanate-methyl, and feeding was significantly reduced at a
level of 0.87 µg/cm². All the worms were killed following contact
with benomyl as an aqueous suspension or soil drench (7.75 kg/ha).

It was suggested by the authors that the toxicity of benomyl
might be due to the anticholinesterase activity of the carbamate
moiety. Effects such as lethargic movements, muscular paralysis,
and death support this supposition.

The spraying of benomyl and thiophanate-methyl in orchards
reduced the number and biomass of all earthworm species combined
and also of each individual species (Stringer & Lyons, 1974). It
was observed that over 90% reduction in earthworm populations
occurred in pastures treated with benomyl. It was suggested that
these changes were reversible, and that the populations of
earthworms would return to normal a few years after the termination
of the treatment (Tomlin & Gore, 1974).

The carbamate insecticide most commonly used in the soil is
carbaryl; it is used mainly in woodlands and orchards where the
formation and fertility of the soil are important.

Stegeman & LeRoy (1964) applied carbaryl at 1.3, 11.2, and 56
kg ai/ha to a red pine plantation and mixed hardwood stands, and,
though the lowest dose did not have any effects on mite and
Collembola populations, the other 2 dose levels considerably
decreased the numbers of mites and Collembolla. Populations began
to recover after 2 months, but Collembola were more sensitive and
recovered more slowly. There seems to be little likelihood of
long-term effects of carbaryl on mite populations (Edwards &
Thompson, 1973).

8. EFFECTS ON EXPERIMENTAL ANIMALS AND IN VITRO TEST SYSTEMS

8.1 Single Exposures

Short- and long-term toxicity studies with carbamates have been
carried out for a period of 20 - 30 years. It is clear that the
production of these carbamates has improved during this period and
that, consequently, purer products have become available. The use
of less pure substances in earlier toxicity studies could explain
the conflicting results between these and later studies.

8.1.1. Oral

Acute oral and dermal toxicity data on animals, for most of the
carbamate pesticides, are given in Table 6. The WHO Recommended
Classification of Pesticides by Hazard is also cited. This
classification is based primarily on the acute oral and dermal
toxicity of the technical material for the rat (WHO, 1984). The
oral LD₅₀s range from less than 1 mg/kg body weight for the highly-
toxic aldicarb to over 5000 mg/kg body weight for the non-
insecticidal carbamates such as phenmedipham, carbendazim, propham,
and benomyl.

A dose-effect relationship is generally noticed between the
administered dose, the severity of effects, and the amount of ChE
inhibition. A correlation also exists between the duration of
symptoms and the in vivo persistence of the compound.

In general, the toxicological effects produced by carbaryl are
typical of those produced by methyl carbamates. The effects of
carbaryl on a number of different animal species were studied by
Carpenter et al. (1961). Rats given a single oral dose of 560
mg/kg body weight showed 42% and 30% inhibition of erythrocyte- and
brain-ChE, respectively, within 1/2 h of treatment; only about 5%
inhibition of plasma-ChE was observed. All ChE levels were almost
normal after 24 h. In studies on rats administered single oral
doses ranging from 5 to 50 mg/kg body weight, a decrease in ChE
activity occurred within 5 min. Recovery was quite rapid
(Krechniak & Foss, 1982).

In 4 dogs, marginal inhibition of erythrocyte-ChE activity was
seen after treatment with a single oral dose of 375 mg carbaryl/kg
body weight. After 30 min, signs of poisoning were observed
including salivation, lachrymation, constriction of pupils,
urination, defaecation, increase in respiratory rate, muscular
twitching, tremors, and mild convulsions. The animals appeared to
have completely recovered the following day. These signs of
poisoning are classic of overstimulation of the parasympathetic
nervous system (Carpenter et al., 1961).

Vassilieff & Ecobichon (1983) studied the influence of a single
dose of 25 mg aminocarb/kg body weight on the activity of
erythrocyte- and brain-AChE, plasma-ChE, and hepatic
carboxylesterases in rats. Significant inhibition of all the
esterases was observed, 30 min or more after the administration of
aminocarb. This severe, but transient, inhibition of the tissue
esterases had recovered after 24 h.

Table 6. Acute oral and dermal toxicity data for a number of
carbamate pesticides
---------------------------------------------------------------------
Carbamate LD₅₀ (mg/kg body weight)^a WHO Recommended
oral dermal Classification
of Pesticides
by Hazard^b
---------------------------------------------------------------------
aldicarb 0.9 > 10.0 (rabbit) IA

aldoxycarb 26.8 700 - 1400 -

allyxycarb 90 - 99 500 -

aminocarb 30 - 40 275 IB

asulam > 4000 > 1200 -

barban 1376 - 1429 > 1600 -
> 20 000 (rabbit)

BPMC 623 - 657 > 5000 -

bendiocarb 40 - 156 566 - 600 II

benomyl > 10 000 > 10 000 (rabbit) 0
> 1000 (dog)

bufencarb 87 680 (rabbit) -

butacarb NA NA -

carbanolate NA NA -

carbaryl approximately > 4000 II
500 - 600 > 2000 (rabbit)

carbendazim > 15 000 > 2000 0
> 2500 (dog) > 10 000 (rabbit)
> 10 000 (quail)

carbetamide 10 000 > 2000 -
1250 (mouse) > 500 (rabbit)
1000 (dog)

carbofuran 6 - 14 3400^c (rabbit) IB
15 - 19 (dog)

chlorbufam 2500 NA -

chlorpropham 5000 - 8000 10 200^d O
5000 (rabbit) 2000 (dog)

cloethocarb 35.4 4000 -
---------------------------------------------------------------------

Table 6. (contd.)
---------------------------------------------------------------------
Carbamate LD₅₀ (mg/kg body weight)^a WHO Recommended
oral dermal Classification
of Pesticides
by Hazard^b
---------------------------------------------------------------------
desmedipham > 10 250 2000 - 10 000 -
2025^e (rabbit)

dimetilan 64 > 2000 -
60 - 65 (mouse)

dichlormate NA NA

dioxacarb 72 approximately -
3000 1950
(rabbit)

ethiofencarb 411 - 499 > 1150 II
224 - 256 (mouse)
155 (quail)

formetanate 21, 18 (mouse) > 5600
19 (dog) > 10 200 (rabbit)

hoppicide NA NA -

isoprocarb 403 - 485 > 500
487 - 512 (mouse)
ca 500 (rabbit)

karbutilate 3000