Boron (EHC 204, 1998)

UNITED NATIONS ENVIRONMENT PROGRAMME
INTERNATIONAL LABOUR ORGANISATION
WORLD HEALTH ORGANIZATION

INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

ENVIRONMENTAL HEALTH CRITERIA 204

BORON

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.

First draft prepared by Ms C. Smallwood, US Environmental Protection
Agency, Cincinnati, Ohio, USA

Published under the joint sponsorship of the United Nations
Environment Programme, the International Labour Organisation, and the
World Health Organization, and produced within the framework of the
Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization Geneva, 1998

The International Programme on Chemical Safety (IPCS),
established in 1980, is a joint venture of the United Nations
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WHO Library Cataloguing in Publication Data

Boron.

(Environmental health criteria ; 204)

1.Boron 2.Environmental exposure
I.International Programme on Chemical Safety II.Series

ISBN 92 4 157204 3 (NLM Classification: QD 181.B1)
ISSN 0250-863X

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR BORON

PREAMBLE

ABBREVIATIONS

1. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

1.1. Summary
1.1.1. Identity, natural occurrence, and analytical methods
1.1.2. Production, uses, environmental fate, and sources of
exposure
1.1.3. Kinetics and biological monitoring
1.1.4. Effects on experimental animals and humans
1.1.5. Effects on organisms in the environment
1.2. Conclusions
1.3. Recommendations

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL
METHODS

2.1. Identity
2.2. Physical and chemical properties
2.3. Conversion factors
2.3.1. Conversion factors of ppm and mg/m³ for boron
2.3.2. Conversion factors for boron compounds to boron
2.4. Analytical methods

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1. Natural occurrence
3.2. Mining and production
3.3. Uses and release

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION

4.1. Transport and distribution between media
4.1.1. Air
4.1.2. Water and sediment
4.1.3. Soil
4.1.4. Vegetation and wildlife
4.2. Transformation
4.2.1. Biotransformation
4.2.2. Abiotic transformation
4.2.2.1 Air
4.2.2.2 Water
4.2.2.3 Soil
4.2.3. Bioaccumulation
4.2.3.1 Aquatic organisms
4.2.3.2 Terrestrial plants
4.2.3.3 Birds
4.3. Ultimate fate following use

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1. Environmental levels
5.1.1. Air
5.1.2. Water
5.1.2.1 Groundwater
5.1.2.2 Surface water
5.1.2.3 Rainfall
5.1.3. Sewage
5.1.4. Soil
5.1.5. Aquatic biota
5.1.6. Terrestrial biota
5.2. General population exposure
5.2.1. Ambient air
5.2.2. Drinking-water
5.2.3. Soil intake
5.2.4. Dietary intake
5.2.5. Consumer products
5.3. Occupational exposure

6. KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS

6.1. Absorption
6.1.1. Oral
6.1.2. Inhalation
6.1.3. Dermal
6.2. Distribution
6.2.1. Tissue levels
6.2.2. Blood levels
6.3. Metabolism
6.4. Elimination and excretion

7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS

7.1. Short-term exposure
7.1.1. Oral route
7.1.2. Inhalation route
7.2. Longer-term exposure
7.2.1. Oral route
7.2.2. Inhalation route
7.3. Dermal and ocular effects
7.4. Reproductive toxicity
7.5. Developmental toxicity
7.6. Mutagenicity and related end-points
7.7. Carcinogenicity
7.8. Toxicity effects summary
7.9. Physiological effects

8. EFFECTS ON HUMANS

8.1. General population exposure
8.1.1. Short-term toxicity and poisoning incidents
8.1.2. Reproductive effects
8.2. Occupational exposure
8.2.1. Short-term irritative effects
8.2.2. Male reproductive and other long-term health effects
8.3. Carcinogenicity
8.4. Physiological effects

9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD
9.1. Laboratory experiments
9.1.1. Microorganisms
9.1.1.1 Water
9.1.1.2 Soil
9.1.2. Aquatic organisms
9.1.2.1 Plants
9.1.2.2 Invertebrates
9.1.2.3 Vertebrates
9.1.3. Terrestrial organisms
9.1.3.1 Plants
9.1.3.2 Invertebrates
9.1.3.3 Vertebrates
9.2. Field observations
9.2.1. Aquatic
9.2.2. Terrestrial

10. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT

10.1. Evaluation of human health exposures
10.2. Choice of critical effect and application of uncertainty
factors
10.3. Derivation of the tolerable intake
10.4. Derivation of guidance values
10.5. Evaluation of effects on the environment
10.5.1. Exposure
10.5.2. Effects
10.5.3. Risk evaluation

11. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH
AND THE ENVIRONMENT

11.1. Conclusions
11.2. Recommendations

12. FURTHER RESEARCH

13. EVALUATIONS BY INTERNATIONAL BODIES

REFERENCES

APPENDIX

RÉSUMÉ, CONCLUSIONS ET RECOMMANDATIONS

RESUMEN, CONCLUSIONES Y RECOMENDACIONES

NOTE TO READERS OF THE CRITERIA MONOGRAPHS

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Environmental Health Criteria

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* Kinetics and metabolism in laboratory animals and humans
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* Effects on other organisms in the laboratory and field
* Evaluation of human health risks and effects on the environment
* Conclusions and recommendations for protection of human health
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* Further research
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JMPR

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WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR BORON

Members

Dr G.M. Buck, Department of Social and Preventive Medicine, State
University of New York at Buffalo, Buffalo, New York, USA

Dr R. Chapin, Department of Health and Human Services, National
Institutes of Health, National Institute of Environmental Health
Sciences, Research Triangle Park, North Carolina, USA

Dr M.L. Dourson, Toxicology Excellence for Risk Assessment,
Cincinnati, Ohio, USA

Dr P. Foster, Chemical Industry Institute of Toxicology, Research
Triangle Park, North Carolina, USA

Dr R.A. Goyer, 6405 Huntingridge Road, Chapel Hill, North
Carolina, USA (Chairman)

Mr P. Howe, Institute of Terrestrial Ecology, Monks Wood, Abbots
Ripton, Cambs., United Kingdom (Co-Rapporteur)

Dr R. Luoto, National Public Health Institute, Department of
Epidemiology and Health Promotion, Helsinki, Finland

Dr F.H. Nielsen, US Department of Agriculture, Agricultural
Research Service, Grand Forks Human Nutrition Research Center,
Grand Forks, North Dakota, USA

Dr C.J. Price, Center for Life Sciences and Toxicology, Research
Triangle Institute, Research Triangle Park, North Carolina, USA

Dr W.G. Woods, Office of Environmental Health and Safety,
University of California, Riverside, California, USA

Observers

Dr B.D. Culver, University of California, Department of Medicine,
Irvine, California, USA (representing International Commission on
Occupational Health)

Dr S. Dyer, Procter & Gamble, Ecosystems Research Station,
Environmental Science Department, Cincinnati, Ohio, USA
(representing European Centre for Ecotoxicology, Toxicology of
Chemicals)

Dr J.A. Moore, Institute for Evaluating Health Risks, Washington,
DC, USA (representing the American Industrial Health Council)

Dr F.J. Murray, 6611 Northridge Drive, San Jose, California, USA
(representing International Life Sciences Institute)

Mrs M. Richold, Unilever Research ESL, Sharnbrook, Bedford,
United Kingdom (representing International Life Sciences
Institute)

Secretariat

Dr B.H. Chen, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland (Secretary)

Dr L. Galvao, Pan American Health Organization, World Health
Organization, Geneva, Switzerland

Dr H. Otterstetter, Pan American Health Organization, World
Health Organization, Geneva, Switzerland

Ms C. Smallwood, US Environmental Protection Agency, National
Center for Environmental Assessment, Cincinnati, Ohio, USA
(Co-Rapporteur)

IPCS TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR BORON

A WHO Task Group on Environmental Health Criteria for Boron met
in Washington, DC, USA, from 18 to 22 November 1996. The meeting was
organized by the WHO Regional Office for the Americas (AMRO) on behalf
of the IPCS. Dr H. Otterstetter, WHO AMRO, opened the meeting and
welcomed the participants. Dr B.H. Chen, IPCS, welcomed the
participants on behalf of the Director of IPCS and the three IPCS
cooperating organizations (UNEP/ILO/WHO). The Task Group reviewed and
revised the draft criteria monograph and made an evaluation of the
risks for human health and the environment from exposure to boron.

The first draft of this monograph was prepared by Ms C. Smallwood
of the US EPA in Cincinnati. The second draft was also prepared by Ms
Smallwood, incorporating comments received following the circulation
of the first draft to the IPCS Contact Points for Environmental Health
Criteria monographs. Dr R. Goyer, Chairman of the Task Group,
contributed significantly to the final text of the EHC for Boron.

Dr B.H. Chen, member of the IPCS Central Unit, and Ms M. Sheffer,
Scientific Editor, Ottawa, Canada, were responsible for the overall
scientific content and linguistic editing, respectively.

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

Financial support for this Task Group meeting was provided by the
US EPA.

ABBREVIATIONS

BMD Benchmark dose
CAS Chemical Abstracts Service
CL Confidence limit
CNS Central nervous system
EPA Environmental Protection Agency (USA)
FDA Food and Drug Administration (USA)
FSH Follicle stimulating hormone
GLP Good Laboratory Practices
HSDB Hazardous Substances Data Bank
ICP Inductively coupled plasma
ICP-AES Inductively coupled plasma atomic emission spectroscopy
ICP-MS Inductively coupled plasma mass spectroscopy
LH Luteinizing hormone
LOAEL Lowest-observed-adverse-effect level
(human and animal toxicity)
LOEC Lowest-observed-effect concentration
(environmental effects)
MATC Maximum acceptable toxicant concentration
(environmental effects)
MMAD Median mass aerodynamic diameter
NADPH Reduced nicotinamide adenine dinucleotide phosphate
NIOSH National Institute for Occupational Safety and Health
NOAEL No-observed-adverse-effect level
(human and animal toxicity)
NOEC No-observed-effect concentration
(environmental effects)
NOHS National Occupational Hazard Survey
RR Rate ratio (or Relative risk)
RTECS Registry of Toxic Effects of Chemical Substances
SBR Standardized birth ratio
SGOT Serum glutamic-oxaloacetic transaminase
SGPT Serum glutamic-pyruvic transaminase
TI Tolerable intake
TLV Threshold limit value
TRI Toxic Release Inventory (US EPA)

1. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

1.1 Summary

1.1.1 Identity, natural occurrence, and analytical methods

Boron is a naturally occurring element that is found in the form
of borates in the oceans, sedimentary rocks, coal, shale, and some
soils. It is widely distributed in nature, with concentrations of
about 10 mg/kg in the Earth's crust (range: 5 mg/kg in basalts to 100
mg/kg in shales) and about 4.5 mg/litre in the ocean.

The most important commercial borate products and minerals are
borax pentahydrate, borax, sodium perborate, boric acid, colemanite,
and ulexite. At the low concentrations and near-neutral pH found in
most biological fluids, monomeric B(OH)₃ will be the predominant
species present (with some B(OH)₄^œ), regardless of whether the
boron source is boric acid or one of the borates. This is because
boric acid is a very weak acid (p K_a 9.15). Sodium perborate
hydrolyses to give hydrogen peroxide plus metaborate; consequently, it
may exhibit chemical and toxicological properties that are somewhat
different from those of the other borates.

Inductively coupled plasma (ICP) methods are preferred for the
analysis of the low levels of boron found in biological and
environmental samples; colorimetric methods must be used with caution.

1.1.2 Production, uses, environmental fate, and sources of exposure

Economic borate deposits are rare, occurring in arid regions of
Turkey, the USA, Argentina, Chile, Russia, China, and Peru. Total
world production of boron minerals -- mainly colemanite, ulexite,
tincal, and kernite -- was approximately 2 750 000 tonnes in 1994.
About 800 000 tonnes of commercial borate products, expressed as
B₂O₃, were manufactured from the boron minerals.

Major end uses for borate include insulation- and textile-grade
fibreglass, laundry bleach (sodium perborate), borosilicate glass,
fire retardants, agricultural fertilizers and herbicides (as a trace
element), and enamels, frits, and ceramic glazes, as well as a myriad
of miscellaneous applications.

Boron enters the environment mainly through the weathering of
rocks, boric acid volatilization from seawater, and volcanic activity.
Boron is also released from anthropogenic sources to a lesser extent.
Anthropogenic sources include agricultural, refuse, and fuel wood
burning, power generation using coal and oil, glass product
manufacture, use of borates/perborates in the home and industry,
borate mining/processing, leaching of treated wood/paper, and
sewage/sludge disposal. Many of these sources are difficult to
quantify.

Atmospheric emissions of borates and boric acid in particulate
and vapour form occur as a result of volatilization from the sea,
volcanic activity, and, to a lesser extent, mining operations, glass
and ceramics manufacturing, the application of agricultural chemicals,
and coal-fired power plants. Boron is not present in the atmosphere at
significant levels; however, the total amount present in the
atmosphere at any one time is significant owing to the huge volume of
the atmosphere. Based on their water solubility, borates would not be
expected to persist to a significant degree in the atmosphere.

Boron can be released into water and soil water through
weathering processes and, to a much smaller extent, through
anthropogenic discharges such as sewage outfalls.
Adsorption-desorption reactions are expected to be the only
significant mechanism influencing the fate of boron in water. The
extent of boron adsorption depends on the pH of the water and the
concentration of boron in solution.

Boron is adsorbed onto soil particles, with the degree of
adsorption depending on the type of soil, pH, salinity, organic matter
content, iron and aluminium oxide content, iron- and aluminium-hydroxy
content, and clay content. Boron adsorption can vary from being fully
reversible to irreversible, depending on the soil type and condition.

Borate ions present in aqueous solution are essentially in their
fully oxidized state. No aerobic processes are likely to affect their
speciation, and no biotransformation processes are reported.
Therefore, there are unlikely to be any differences in boron species
due to biotransformation.

The octanol/water partition coefficient of boric acid has been
measured as 0.175, indicating a low bioaccumulation potential.
Laboratory experiments with aquatic organisms have confirmed this
potential. Plants accumulate boron; however, uptake is affected by the
pH of the soil solution, temperature, light intensity, and the
concentration of other elements (e.g. calcium and potassium). The
results of studies of boron accumulation in plants, insects, and fish
have shown that boron bioaccumulates in plants but does not biomagnify
in aquatic food-chains.

Boron occurs in soils at concentrations ranging from 10 to 300
mg/kg (average 30 mg/kg), depending on the type of soil, amount of
organic matter, and amount of rainfall. Concentrations of boron in
surface water are dependent on such factors as the geochemical nature
of the drainage area, proximity to marine coastal regions, and inputs
from industrial and municipal effluent discharges. Concentrations of
boron in surface water range widely, from 0.001 to as much as 360
mg/litre. However, mean boron concentrations for waters of Europe,
Pakistan, Russia, and Turkey are typically well below 0.6 mg/litre.
Concentrations of boron in water in Japan, South Africa, and South
America are generally below 0.3 mg/litre. Typical boron concentrations
in North American waters are below 0.1 mg/litre, with about 90% at or
below 0.4 mg/litre.

Boron accumulates in aquatic and terrestrial plants but does not
magnify through the food-chain. Concentrations of boron have been
shown to range between 26 and 382 mg/kg in submerged aquatic
freshwater plants, from 11.3 to 57 mg/kg in freshwater emergent
vegetation, and from 2.3 to 94.7 mg/kg dry weight in terrestrial
plants. Based on wet weights, boron concentrations in marine
invertebrates and fish are similar to the levels found in the exposure
media, between 0.5 and 4 mg/kg. The bioconcentration factor for two
freshwater fish species was found to be 0.3.

Boron concentrations in ambient air range from <0.5 to
approximately 80 ng/m³, with an average over the continents of
20 ng/m³.

Close similarity of boron concentrations in groundwater, fresh
surface water, and drinking-water indicates that boron is not removed
in the treatment of groundwater and fresh surface water used for
drinking-water.

Intakes of boron for humans are expected to be 0.44 µg/day from
ambient air, 0.2-0.6 mg/day from drinking-water, and 1.2 mg/day from
the diet. Average boron intake from the soil is considered to be 0.5
µg/day. A reasonable estimate of boron exposure from consumer products
is 0.1 mg/day.

1.1.3 Kinetics and biological monitoring

The pharmacokinetics of boron appear to be quite similar across
species in the following respects:

a) Absorption of borates is essentially complete (approximately
95% in humans and rats), and boron appears rapidly in the blood
and body tissues of several mammalian species following
ingestion.

b) Distribution of boron in mammals appears to occur by passive
diffusion throughout the body fluids. In contrast to soft tissues
and blood, bone shows selective uptake of boron (>4 times
higher than serum) and significantly longer retention times.

c) Metabolism of boric acid is thermodynamically unfavourable in
biological systems. Thus, the ionic species in systemic
circulation are expected to be equivalent across mammals. This
eliminates a major source of potential uncertainty for risk
extrapolation, as interspecies differences in enzymatic pathways
and/or metabolic rates do not need to be taken into
consideration.

d) Elimination kinetics (especially route of elimination and
terminal half-life) also appear to be similar for humans and
rats.

The similarities in pharmacokinetic parameters between humans and
rats, the species defining the no-observed-adverse-effect level
(NOAEL) for laboratory studies, reduce the uncertainty for risk
extrapolation between these two species.

1.1.4 Effects on experimental animals and humans

The data regarding developmental and reproductive toxicity show
that lower fetal body weight in rats is the critical effect. The NOAEL
for lower fetal body weight is 9.6 mg boron/kg body weight per day.
The lowest-observed-adverse-effect level (LOAEL), at which rats show
slight (approx. 5%) fetal body weight differences and rib anomalies,
is about 13 mg boron/kg body weight per day. As dose level increases,
the effects that are seen (and the doses at which they are seen) are:

a) further rib effects and testicular pathology in the rat (approx.
25 mg boron/kg body weight per day);

b) decreased fetal body weight and increased fetal cardiovascular
malformations in the rabbit, and severe testicular pathology in
the rat (approx. 40 mg boron/kg body weight per day);

c) testicular atrophy and sterility in the rat (approx. 55 mg
boron/kg body weight per day); and

d) reduced fetal body weight in the mouse (approx. 80 mg boron/kg
body weight per day).

Animal studies on mice and rats showed no evidence of
carcinogenicity of boric acid. Based on the lack of human data and the
limited animal data, boron is not classifiable as to its human
carcinogenicity.

Only a few human studies have been conducted to assess health
effects associated with exposure to boron compounds. The available
data show that exposure is associated with short-term irritant effects
on the upper respiratory tract, nasopharynx, and eye. These effects,
however, appear to be short-term and reversible. The sole long-term
(7-year) follow-up study failed to identify any long-term health
effects, although a healthy worker effect cannot be entirely ruled out
given the rate of attrition (47%). Two descriptive studies assessed
fertility and secondary sex ratios in relation to exposure. Neither
study reported a detrimental effect on demonstrated fertility for its
select sample. Although an excess percentage of female births has been
suggested, the absence of statistical significance and attention to
other co-variates known to affect sex ratios warrants careful
interpretation of this finding. No studies have been identified that
assess the spectrum of reproductive outcomes, such as
time-to-pregnancy, conception delays, spontaneous abortions, and sperm
analyses in males. The role of other lifestyle or behavioural factors
in relation to health and fertility requires further study to identify
potentially sensitive populations and to evaluate reproductive effects
more fully.

1.1.5 Effects on organisms in the environment

Bacteria are relatively tolerant towards boron. Acute and chronic
effect concentrations range between 8 and 340 mg boron/litre, with
most values greater than 18 mg boron/litre. More sensitive are
protozoa. Tests with Entosiphon and Paramecium yielded 72-h
no-observed-effect concentrations (NOECs) and EC₃ values between
0.3 and 18 mg boron/litre.

Boron is an essential micronutrient for cyanobacteria and
diatoms. Standard chronic tests with freshwater green algae resulted
in no-effect concentrations between 10 and 24 mg boron/litre.
Blue-green algae appear to be similar in sensitivity, with an 8-day
EC₃ of 20 mg boron/litre.

Based on acute toxicity values, invertebrates are less sensitive
to boron than microorganisms. For several species, 24- to 48-h EC₅₀
values ranged from 95 to 1376 mg boron/litre, with most values in the
100-200 mg boron/litre range. Chronic toxicity studies with
Daphnia magna gave NOECs ranging between 6 and 10 mg boron/litre.
Slightly lower NOEC values were obtained from laboratory and field
biocenosis studies. The 28-day laboratory study consisting of six
trophic stages yielded a NOEC of 2.5 mg boron/litre. Long-term outdoor
pond and field studies (not including fish) yielded NOECs up to 1.52
mg boron/litre.

Acute tests with several fish species yielded toxicity values
ranging from about 10 to nearly 300 mg boron/litre. Rainbow trout
(Oncorhynchus mykiss) and zebra fish (Brachydanio rerio) were the
most sensitive, providing values around 10 mg boron/litre.

The toxicity of boron to early life stages of fish has been
documented for several species in reconstituted water. Embryonic and
early larval stages of rainbow trout, largemouth bass (Micropterus
salmoides), channel catfish (Ictalurus punctatus), and goldfish
(Carassius auratus) were exposed to boron, as boric acid or borax,
from fertilization up to 8 days post-hatch in soft or hard water.
Neither water hardness nor the form of boron consistently affected
embryo-larval survival of fish. Rainbow trout was the most sensitive
species. The NOECs for rainbow trout ranged from 0.009 to 0.103 mg
boron/litre.

The effect of natural dilution water on boron toxicity was
determined by using surface waters collected from three locations,
with boron concentrations of 0.023, 0.091, and 0.75 mg/litre. No
adverse effects were determined up to 0.75 mg boron/litre.
Lowest-observed-effect concentrations (LOECs) ranged from 1.1 to 1.73
mg boron/litre. One test using deep (600 m) well-water, typically used
for aquatic toxicity tests, from a contract laboratory located in
Wareham, Massachusetts, USA, yielded a NOEC of >18.0 mg boron/litre.
Hence, reconstituted water exposures appeared to overestimate the
toxicity determined in natural waters, possibly as a result of
nutrient deficiency in the former.

Boron has been known since the 1920s to be an essential
micronutrient for higher plants, with interspecies differences in the
levels required for optimum growth. Boron plays a role in cell
division, metabolism, and membrane structure and function. Boron in
the form of borates occurs naturally in fruits, nuts, and vegetables.
There is a small range between deficiency and excess uptake (toxicity)
in plants. Boron deficiencies in terrestrial plants have been reported
in many countries. Boron deficiency is more likely to occur in
light-textured, acid soils in humid regions because of boron's
susceptibility to leaching. Boron excesses usually occur in soil
solutions from geologically young deposits, arid soils, soils derived
from marine sediments, and soils contaminated by pollutant sources,
such as releases from coal-fired power plants and mining operations.
Irrigation water is one of the main sources of high boron levels
resulting in toxicity in the field.

Mallard (Anas platyrhynchos) duckling growth was adversely
affected at dietary levels of 30 and 300 mg boron/kg, and survival was
reduced at 1000 mg/kg.

1.2 Conclusions

Boron is a naturally occurring element that is found in nature in
the form of borates in the oceans, sedimentary rocks, coal, shale, and
some soils. Natural sources of borates released into the environment
are the oceans, geothermal steam, and natural weathering of clay-rich
sedimentary rocks. Boron is also released from anthropogenic sources
to a lesser extent.

Boron is an essential micronutrient for higher plants, with
interspecies differences in the levels required for optimum growth.
Boron deficiency in terrestrial plants has been observed in many
countries throughout the world. There is a small range between
deficiency and toxicity in some plants.

Comparison of the environmental no-effect concentration
(1 mg/litre) with the general ambient environmental levels of boron
indicates that the risk of adverse effects of boron on the aquatic
ecosystem is low. In a few boron-rich environments, natural levels
will be higher. It is reasonable to assume that aquatic organisms in
such habitats may be adapted to the local conditions.

For humans, boron exposure occurs primarily through the diet and
drinking-water. The mean global boron concentration in drinking-water
was considered to be between 0.1 and 0.3 mg boron/litre.

For the general population, the greatest boron exposure comes
from the oral intake of food. The mean daily intake of boron in the
diet is about 1.2 mg.

In humans and animals, boric acid and borate are absorbed from
the gastrointestinal and respiratory tracts. More than 90% of
administered doses of these compounds are absorbed, as evidenced by
excretion in the urine, which is rapid, occurring over a few to
several days.

Animal experiments have shown that boron in the form of boric
acid and borate demonstrates reproductive and developmental toxicity
at levels that are approximately 100- to 1000-fold greater than normal
exposure levels. There is a lack of sufficient toxicity data on
humans. The tolerable intake (TI) of boron was set as 0.4 mg/kg body
weight per day. The allocation of the TI in various media should be
based on the exposure data of individual countries.

1.3 Recommendations

a) Water and food guideline values should be based on the TI
provided by this document.

b) The TI should be applied with the understanding that boron may
provide a physiological benefit for human health.

c) It should be recognized in applying standards that boron is
essential for some constituents of the environment (e.g. boron is
an essential micronutrient for higher plants).

d) Dietary supplements that exceed the TI should be avoided.

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS

This chapter deals with the identity and physical and chemical
properties of the inorganic borates of importance in commerce, as well
as the analytical methods used to determine boron concentrations in
various media.

2.1 Identity

Elemental boron (B) is a member of Group IIIB of the periodic
table, along with aluminium, gallium, indium, and thallium. It has an
atomic number of 5 and a relative atomic mass of 10.81. Boron is never
found in the elemental form in nature. Its chemistry is complex and
resembles that of silicon (Cotton & Wilkinson, 1988). The Chemical
Abstracts Service (CAS), National Institute for Occupational Safety
and Health (NIOSH) Registry of Toxic Effects of Chemical Substances
(RTECS), and Hazardous Substances Data Bank (HSDB) numbers for boron
are 7440-42-8, ED7350000, and 4482, respectively.

The borates used most widely in commerce are listed in
approximate decreasing order of usage in Table 1, along with their
formulae and CAS numbers. Elemental boron is included, even though its
production is quite small. Throughout this document, the term "borax"
refers to disodium tetraborate decahydrate (see Table 1).

2.2 Physical and chemical properties

Elemental boron exists as a solid at room temperature, either as
black monoclinic crystals or as a yellow or brown amorphous powder
when impure. The amorphous and crystalline forms of boron have
specific gravities of 2.37 and 2.34, respectively. Boron exists as a
mixture of the ¹⁰B (19.78%) and ¹¹B (80.22%) isotopes (Budavari et
al., 1989). Boron is a relatively inert metalloid except when in
contact with strong oxidizing agents. Boron dust exposed to air is
flammable and an explosion hazard. It also reacts violently when
ground with lead fluoride and silver fluoride (Lewis, 1992). Physical
and chemical properties of elemental boron and the most important
borates in commerce are provided in Table 2.

Sodium perborates are persalts that are hydrolytically unstable
because they contain characteristic boron-oxygen-oxygen bonds that
react with water to form hydrogen peroxide and stable sodium
metaborate (NaBO₂.nH₂O). This hydrolysis reaction is the basis of
the use of perborates as bleaches in detergents at high (70-100°C)
temperature. At lower washing temperatures (25-70°C), activators are
needed; these react with peroxide to give peracids, which are stronger
oxidants and give bleaching effects at lower temperatures.

Table 1. Boron compounds of commerce in approximate decreasing order
of usage^a

Substance Formula CAS No.

Borax pentahydrate (disodium Na₂[B₄O₅(OH)₄].3H₂O 3754418
tetraborate pentahydrate) (Na₂B₄O₇.5H₂O)

Borax (disodium tetraborate Na₂[B₄O₅(OH)₄].8H₂O 1303-96-4
decahydrate) (Na₂B₄O₇.10H₂O)

Ulexite NaCa[B₅O₆(OH)₆].5H₂O 1319-33-1
(Na₂O.2CaO.5B₂O₃.16H₂O)

Colemanite Ca[B₃O₄(OH)₃].H₂O 1318-33-8
(2CaO.3B₂O₃.5H₂O)

Sodium perborate tetrahydrate Na₂[B₂O₄(OH)₄].6H₂O 10486-00-7
(NaBO₃.4H₂O)

Sodium perborate monohydrate Na₂[B₂O₄(OH)₄] 10332-33-9
(NaBO₃.H₂O)

Boric acid B(OH)₃ 10043-35-3
(H₃BO₃)

Anhydrous borax Na₂B₄O₇ (amorphous) 1330-43-4
(disodium tetraborate)

Boron oxide B₂O₃ (amorphous) 1303-86-2

Boron^b B 7440-42-8

^a US EPA (1991); ATSDR (1992); Culver et al. (1994b).
^b Produced in small quantities.

Table 2. Physical and chemical properties of elemental boron and the most important borates in commerce^a

Substance Relative Colour % boron Relative Water solubility Melting point Boiling point
molecular density (°C) (°C)
mass

Borax pentahydrate 291.35 White 14.85 1.81 3.6 g/100 g @ 20 °C 742 -
Borax 381.37 Colourless 11.34 1.73 5.92 g/100 g @ 25 °C 75, decomposes -
Ulexite 810.6 White 13.33 1.62 Slightly soluble Decomposes -
Colemanite 411.1 White 15.78 2.42 Slightly soluble Decomposes -
Sodium perborate
tetrahydrate 153.9 White 7.03 1.73 23 g/litre @ 20 °C Decomposes -
Sodium perborate
monohydrate 99.8 White 10.83 - 15 g/litre @ 20 °C Decomposes -
Boric acid 61.84 Colourless 17.48 1.435 @ 15 °C 63.5 g/litre @ 30 °C 169 -
Anhydrous borax 201.22 White 21.49 2.367 2.5556 g/100 g @ 25 °C 741 1575
Boron oxide 69.62 Colourless 31.06 2.46 Slightly soluble 450 1860
Boron 10.81 Black crystal or
yellow-brown
amorphous 100 2.3 Insoluble 2300 approx. 3500

^a Muetterties (1967); Windholz et al. (1983); Weast et al. (1985); ACGIH (1991); ATSDR (1992); Lewis (1993); US NLM (1993);
Culver et al. (1994b).

Boric acid is a very weak acid, with a p K_a of 9.15, and
therefore boric acid and the sodium borates exist predominantly as
undissociated boric acid [B(OH)₃] in dilute aqueous solution below pH
7; above pH 10, the metaborate anion B(OH)₄^œ becomes the main
species in solution. Between pH 6 and pH 11 and at high concentration
(>0.025 mol/litre), highly water soluble polyborate ions such as
B₃O₃(OH)₄^œ, B₄O₅(OH)₄^2œ, and B₅O₆(OH)₄^œ are formed.

The chemical and toxicological properties of borax pentahydrate,
borax, boric acid, and other borates are expected to be similar on a
mol boron/litre equivalent basis when dissolved in water or biological
fluids at the same pH and low concentration. Boric oxide will exhibit
properties identical to those of boric acid, as it is an anhydride
that will hydrolyse to give boric acid. Sodium perborate monohydrate
and tetrahydrate hydrolyse to give hydrogen peroxide and borate. Thus,
they are oxidants and may have chemical and toxicological properties
that are different from those of the other borates.

The chemical properties of sodium metaborate differ from those of
the other sodium borates, in that the metaborate has a much higher
solubility and alkalinity in aqueous solution. Thus, the solubility in
water at 20°C is 41.9 parts sodium metaborate octahydrate (compared
with 4.7 for borax) per hundred parts saturated solution by weight.
The pH of an aqueous solution of the metaborate at 20°C ranges from
10.5 at 0.1% w/w to 12.0 at 18% w/w (compared with pH 9.24 for borax
over a wide range of concentrations).

2.3 Conversion factors

2.3.1 Conversion factors of ppm and mg/m³ for boron

1 ppm = 0.4421 mg/m³
1 mg/m³ = 2.262 ppm

2.3.2 Conversion factors for boron compounds to boron

dose of boric acid × 0.175 = equivalent dose of boron
dose of borax × 0.113 = equivalent dose of boron
dose of anhydrous borax × 0.215 = equivalent dose of boron
dose of sodium perborate tetrahydrate × 0.070 = equivalent dose
of boron
dose of sodium perborate monohydrate × 0.108 = equivalent dose of
boron
dose of metaboric acid × 0.247 = equivalent dose of boron

2.4 Analytical methods

Analyses of environmental and biological samples for boron
content utilize a variety of preparative methods (see Table 3).

Table 3. Preparative methods for analysing boron content in
environmental and biological samples

Media Extraction method Reference

Biological Acid digestion with:
Microwave Pennington et al. (1991)
Dry ashing Wilkner (1986)
Wet ashing Kowalenko (1979)
Banuelos et al. (1992)
Low temperature, wet ashing Hunt & Shuler (1989)

Freeze drying Iyengar et al. (1990)
Smith et al. (1991)

Soil Hot water solubility Odom (1980)
Cumakov (1991)

Water Liquid-liquid extraction from
acidified solutions into
chloroform Aznarez et al. (1985)

Ion exchange column Sekerka & Lechner (1990)

The preferred method for analysis of boron in bone, plasma, and
food is inductively coupled plasma atomic emission spectroscopy
(ICP-AES) (Hunt, 1989). It is also used for tumour, blood, liver,
skin, and cell suspensions (Barth et al., 1991). It also has been used
for wastewater (Huber, 1982) and fish tissues (Hamilton & Wiedmeyer,
1990). Detection limits range from 0.005 to 0.05 mg boron/litre in the
solution analysed.

Inductively coupled plasma mass spectroscopy (ICP-MS) is used to
measure boron concentrations in plant, rat, and human samples. Isotope
ratios (¹⁰B/¹¹B) can be measured accurately (Vanderpool et al.,
1994). Using direct nebulization, ICP-MS can give a detection limit of
1 ng/g in human blood, human serum, orchard leaves, and total diet
(Smith et al., 1991).

ICP-MS is the most widely used non-spectrophotometric method for
analysis of boron, as it uses small volumes of sample, is fast, and
applies to a wide range of materials (fresh and saline water, sewage
wastewater, soils, and plant samples, as well as the biological
materials mentioned above). Interferences are minimal or can be
removed (Gregoire, 1990). ICP-MS can detect boron down to 0.15
µg/litre.

The ability to measure the boron isotope ratio accurately allows
studies starting with pure ¹⁰B compounds and following the isotopic
dilution in biological systems. This is particularly useful, as no
stable radioactive boron isotopes usable as tracers exist. A number of
boron compounds made with nearly isotopically pure ¹⁰B are available
for such studies.

When expensive ICP equipment is not available,
colorimetric/spectrophotometric methods can be utilized. However, many
of these methods are subject to interference and should be used with
caution; they should also preferably be calibrated against an ICP
method.

Azomethine-H has been used to analyse boron in environmental
water samples and is very sensitive, with a detection limit of 0.02
mg/litre (Lopez et al., 1993). The well-known curcumin method is
subject to interference by nitrate, chloride, and fluoride but is
claimed to be applicable to samples with 0.1-1 mg boron/litre (Black
et al., 1993).

A simple, sensitive spectrophotometric method for determination
of boron in soils, plant materials, and water uses Alizarin Red S but
is also subject to interference (Garcia-Campana et al., 1992). Flow
injection analysis utilizing the sorbitol/borate complex and Methyl
Orange indicator for eye lotion samples has a detection limit of
0.02 mg/litre (Nose & Zenki, 1991).

Another method of analysis of boron uses neutron activation and
mass spectrometric analysis. Mass spectrometric assay of ³He from
decay of tritium produced by thermal neutron irradiation of boron to
give ⁴He has been described by Clarke et al. (1987a). The method,
useful for trace levels of boron in blood and other biological
samples, can detect 10^œ8 g boron/g of sample (Clarke et al., 1987b).
Iyengar et al. (1990) used this method to determine boron in citrus
leaves, human erythrocytes, and food items, all with freeze-dried
samples.

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1 Natural occurrence

Boron, in the form of various inorganic borates, is widely
distributed in low concentrations throughout nature. It constitutes
about 10 mg/kg of the Earth's crust, ranging from 5 mg/kg in basalts
to 100 mg/kg in shales (Woods, 1994). The majority of the boron
resides in the ocean, at an average concentration of about 4.5
mg/litre (Weast et al., 1985). Economic deposits of borate minerals
are rare and are usually found in arid desert regions with a
geological history of volcanic and/or hydrothermal activity (Mellor,
1980). Major world deposits are found in Turkey, the USA, Argentina,
Russia, Chile, China, and Peru (Culver et al., 1994b).

The most abundant boron mineral is tourmaline, an aluminium
borosilicate that contains about 3.1% boron (Muetterties, 1967). It is
not a practicable source of usable boron, as it is widely distributed
as a minor component of rocks. Economic borate minerals include
tincal, kernite, colemanite, and ulexite.

Natural sources of borate released to air are the oceans
(largest), volcanoes, and geothermal steam (Graedel, 1978). Natural
weathering of clay-rich sedimentary rocks on land surfaces accounts
for a large proportion of the boron mobilized into soils and the
aquatic environment, amounting to some 360 000 tonnes of boron
annually (Bertine & Goldberg, 1971). Although few data are available
for quantifying boron released from industrial sources, natural
weathering and seawater evaporation are considered greater sources
than industrial emissions (see chapter 4).

3.2 Mining and production

The total world production of boron minerals in 1994 was
approximately 2 750 000 tonnes (Lyday, 1996). The main commercial
borate minerals are colemanite, kernite, ulexite, and tincal.
Approximately 800 000 tonnes of commercial borate products, expressed
as B₂O₃, were manufactured from boron minerals in 1994. The two
largest producers are the USA and Turkey. Further mining and
production facilities exist in Argentina, Bolivia, China, Chile, Peru,
and Russia (Lyday, 1996). Most US production of borates occurs in
California, where colemanite, ulexite, tincal, kernite, and brines are
processed. These minerals are also processed elsewhere in the world,
as are ascharite, hydroboracite, datolite, etc.

Disodium tetraborate (borax) containing 5 or 10 molecules of
water is produced mainly from sodium-containing borate ores. The mined
ore is crushed and ground before dissolution in a hot recycled aqueous
solution containing some borax. Insoluble gangue (clay particles)
present in the hot slurry is separated off to produce a clear
concentrated borax solution. Evaporative cooling of this solution to
selected temperatures results in crystallization of the desired

products, which are then separated from the residual liquor and dried
(personal communication from Borax US to the IPCS, 1995).

Boric acid is produced mainly from sodium- or calcium-containing
borate ores. The mined ore is crushed and ground before being reacted
with sulfuric acid in the presence of a hot aqueous recycled liquor
containing some boric acid. The resultant slurry contains insoluble
gangue and either calcium or sodium sulfate by-product. After
separation of unwanted insoluble gangue, recovery of the boric acid
product is similar to that for borax (personal communication from
Borax US to the IPCS, 1995).

3.3 Uses and release

The end uses of boron minerals and of borate products are
diverse. Estimated amounts of borate consumed in the USA for the major
end uses in 1992 are listed in Table 4 (Lyday, 1993). Partial data for
Europe are also included (ECETOC, 1997). It should be noted that
vitreous products such as fibreglass, borosilicate glass, and enamels,
frits, and glazes are not significant sources of potential human
exposure, because the boron is tied up tightly in the glassy
structure. All of the boron from the sodium perborate contained in
detergents ultimately enters the wastewater stream.

Table 4. Estimated amount consumed (as B2O3) for boric acid, borates,
and boron minerals in the USA in 1992^a and in Europe in 1993^b

Use Consumption (tonnes)
USA Europe

Insulation-grade fibreglass 129 000 44 600^c
Textile-grade fibreglass 78 500 27 100^c
Soaps and detergents 38 600 142 500
Borosilicate glass 34 400 12 200
Fire retardants 13 400 -^d
Agriculture 11 100 -^d
Enamels, frits, and ceramic glazes 9 300 3 500
Metallurgy 3 700 -^d
Nuclear applications 900 -^d

^a Lyday (1993).
^b ECETOC (1997).
^c Does not include minerals.
^d No data.

The average market shares for the USA, Europe, and Japan in 1992
were about 23% (fibreglass), 17% (detergents), 11% (enamels/glazes),
and 11% (glass) for major end uses (personal communication from Borax
US to the IPCS, 1995).

Other minor uses include cosmetics and pharmaceuticals (as a pH
buffer), boron neutron capture therapy (for cancer treatment), and
pesticides (personal communication from Borax US to the IPCS, 1995).
The cancer treatment application utilizes a boron compound made with
all ¹⁰B isotope, which preferentially accumulates in tumour versus
normal tissue (Barth & Soloway, 1994). Subsequent irradiation of the
patient with thermal neutrons produces ⁷Li plus alpha particles. The
latter have a destructive path length of about the diameter of a cell,
thereby selectively destroying the cancer. Research in this field is
being pursued in Japan and, to a lesser extent, in the USA.

Boron enters the environment mainly through the weathering of
rocks, volatilization from seawater, agricultural, refuse, and fuel
wood burning, power generators (coal and oil combustion), the
manufacture of glass products and other boron-containing compounds,
the industrial and household use of boron-containing products
(including soaps and detergents), borax mining and processing,
leaching from treated wood and paper, geothermal releases, chemical
plants, and sewage and sludge disposal (Versar, Inc., 1975; Larsen,
1988; ATSDR, 1992; Anderson et al., 1994a). Boron is not present in
the atmosphere at significant levels because of its low volatility,
but the total amount in the air is very significant owing to the huge
volume of the atmosphere (see chapter 4).

Boron releases to water occur from municipal sewage containing
perborates from detergents and also in runoff from areas using
boron-containing herbicides or fertilizers (Waggott, 1969; Nolte,
1988; Butterwick et al., 1989). Boron levels in sewage sludge from 23
cities in the USA ranged from 7.1 to 53.3 mg/kg (Mumma et al., 1984).
It has been estimated that 11 800 tonnes of boron are released yearly
in coal fly ash from coal combustion (Bertine & Goldberg, 1971).
Versar, Inc. (1975) estimated US boron air emissions as 10 500 tonnes
annually from mining, processing, and coal burning. Few quantitative
data on boron releases are available, because boron is not included in
the US Environmental Protection Agency (EPA) Toxic Release Inventory
(TRI) (ATSDR, 1992).

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION

4.1 Transport and distribution between media

4.1.1 Air

Boron is not present in the atmosphere at significant levels
(Sprague, 1972), but the total amount in the air is very significant
owing to the huge volume of the atmosphere. Borates exhibit low
volatility; consequently, boron would not be expected to be present to
a significant degree as a vapour in the atmosphere. Atmospheric
emissions of borates and boric acid in particulate (<1-45 µm in size)
or vapour form occur as a result of volatilization of boric acid from
the sea, volcanic activity, mining operations, glass and ceramics
manufacturing, the application of agricultural chemicals, and
coal-fired power plants. As a particulate, boron would be removed from
the atmosphere either by dry deposition or by wet deposition because
of its relatively high water solubility (Versar, Inc., 1975; Gladney
et al., 1978). Based on analogy with data on general particulate
residence times (Nriagu, 1979), the half-life of airborne boron
particles is expected to be on the order of days, depending on the
size of the particles and atmospheric conditions.

Seawater evaporation is the biggest contribution to boron in air.
The global removal of boron from marine sources has been estimated at
between 800 000 and 4 000 000 tonnes/year and compares with an
estimate of 2 000 000-7 200 000 tonnes/year for the total global
release (Anderson et al., 1994a). Anderson et al. (1994a) estimate
that the total anthropogenic release of boron to the atmosphere is
between 180 000 and 650 000 tonnes/year (9-27% of the total global
release). In spite of all these releases, the atmospheric
concentration of boron is low (mean boron concentrations range from
<0.5 to approximately 80 ng/m³).

4.1.2 Water and sediment

Waterborne boron may be adsorbed by soils and sediments.
Adsorption-desorption reactions are expected to be the only
significant mechanism influencing the fate of boron in water (Rai et
al., 1986). The extent of boron adsorption depends on the pH of the
water and the concentration of boron in solution. The greatest
adsorption is generally observed at pH 7.5-9.0 (Waggott, 1969; Keren &
Mezuman, 1981; Keren et al., 1981).

Simsiman et al. (1987) conducted a field investigation to
determine the leachability and groundwater transport of major and
minor elements, including boron, from ash disposal ponds at the
coal-fired Columbia Power Plant in Portage, Wisconsin, USA. The site
is underlain by sands interspersed with lenses of silt and clay
overlying sandstone (10-20 m below the surface). The soil pH ranged
from 7.1 to 8.8, and the organic matter content was 0.2-0.8%. Boron
plumes were identified in the groundwater at least 120 m down-gradient
of the ponds. The boron plume from the secondary fly ash pond extended

into the sandstone (26-30 m), which suggested rapid downward
infiltration of the leachate. However, attenuation of the boron
occurred at some point between the pond and the aquifer, based on
observed decreases of approximately 40% in the boron concentration.

Barber et al. (1988) monitored the extent of groundwater
contamination emanating from sewage disposal beds near Falmouth,
Massachusetts, USA, by mapping the distribution of boron. Under the pH
conditions of the aquifer (pH 5-7), dissolved boron occurred as the
neutral undissociated orthoboric acid species, which should be
transported with little sorption. The boron plume was 3500 m long,
1100 m wide, and 30 m deep during sampling in 1985.

Deverel & Millard (1988) demonstrated that boron is present in
the oxidized, alkaline, shallow groundwater of the western San Joaquin
Valley, California, USA. Boron was found to be geochemically mobile,
with concentrations significantly correlated (alpha = 0.05) with
groundwater salinity in the alluvial-fan and basin-trough geological
zones.

Corwin (1986) speculated that the adsorption of boron on
sediments provides a means by which boron may persist for long periods
of time in aquatic systems. The desorption (or leaching) of boron from
the sediments would provide a long-term source until a system
equilibrium could be reached, based on differences in the
concentrations of boron in the water column and in the sediment both
at the sediment-water interface and with increasing depth below the
interface. The primary desorption mechanism would be diffusion.

Boron levels (as admixed borate salt) as high as 1900 mg/kg have
been reported in coal fly ash. Cox et al. (1978) reported that
approximately 50% of the boron in 0.5-g samples of fly ash was leached
from the ash into water within 2 h; the leaching rate increased with
increased acidity. In a boron dissolution study, Hollis et al. (1988)
observed that 60% of the boron was removed from 6 g of ash after three
extractions at pH 9, whereas 100% was removed at pH 6 after two
extractions. In a long-term (2-year) leachability study, Dudas (1981)
observed that boron was readily leached, probably as a result of the
moderate solubility of borate salts. Consequently, the disposal of
coal fly ash in lagoons could provide a source of boron contamination
in aquatic systems.

4.1.3 Soil

Boron is adsorbed onto the surfaces of soil particles, with the
degree of adsorption depending on the type of soil, pH, salinity,
organic matter content, iron and aluminium oxide content, iron- and
aluminium-hydroxy content, and clay content (Sprague, 1972). Boron
adsorption can vary from being fully reversible to irreversible (Rai
et al., 1986; Shani et al., 1992). The lack of reversibility may be
the result of solid-phase formation on mineral surfaces (Rai et al.,
1986) and/or the slow release of boron by diffusion from the interior
of clay minerals (Griffin & Burau, 1974).

At acidic pH, boron exists in solutions in the form of
undissociated boric acid; at alkaline pH, it is present as a borate
ion, which reaches maximum adsorption at pH 8.5-9 (Sprague, 1972).
Sims & Bingham (1967) reported that hydroxy iron and aluminium
compounds, present as interlayer-contained materials, coatings on
individual particles, or impurities, resulted in increased boron
retention in layer silicates. Rhoades et al. (1970) observed that in
the silt and sand fractions of arid-zone soils, the sites of boron
adsorption are the magnesium-hydroxy clusters and coatings found on
the weathering surfaces of ferromagnesian minerals and micaceous layer
silicate minerals. Marzadoori et al. (1991) reported that the amount
of boron adsorbed by soil was considerably greater after the organic
matter had been removed from the soil. An increase in
oxalate-extractable iron and aluminium in the soil was observed after
destruction of the organic matter. It was suggested that a portion of
the iron and aluminium oxides as well as other possible adsorption
sites are generally coated or occluded by organic matter and become
active only after its removal.

Couch & Grim (1968) studied the uptake of boron in illite clays
and determined that uptake was enhanced at higher boron soil solution
concentrations in direct relationship to the salinity and temperature
of the solution. Following 30 days of treatment in soils containing
1 mol boric acid/litre at salinities of 0.1, 1.0, or 3.0 mol
CaCl₂/litre, boron levels increased by 56, 70, and 98 mg/kg,
respectively. Treatment of illites at 1 mol boric acid/litre for 30
days at 60°C yielded 55 mg boron/kg, whereas the same concentration at
215°C for 12 h yielded 180 mg boron/kg. The investigators also
observed a direct relationship between the specific surface area of
the clay types and boron uptake. Boron uptake in the illite clays was
characterized as initially rapid adsorption, followed by diffusion of
boron into the clay structure, requiring several months to reach
equilibrium.

Several investigators have used either the Langmuir or the
Freundlich adsorption equation to describe the relationship between
adsorption and desorption of boron in soils. The Langmuir equation is
based on the total adsorptive capacity of the soil, the concentrations
of adsorbed boron and boron in solution, and an adsorption equilibrium
constant (K), which represents the bonding energy of the soil. Using
this equation, Hatcher & Bower (1958) determined that an equilibrium
exists between boron in solid and liquid phases. At soil pH values of
6.6-7.7, the predominant boron species in the aqueous phase is
undissociated boric acid, and the principal mechanism of retention is
by reversible, molecular adsorption, which is non-uniform based on the
energy characteristics of the bonding sites. These investigators also
showed that boron desorption was reversible; in other words, boron
that leached into the soil solution could again be adsorbed. However,
based on the Freundlich adsorption isotherms, Elrashidi & O'Connor
(1982) observed incomplete adsorption reversibility in some soils from
New Mexico, USA, at higher boron concentrations.

Biggar & Fireman (1960) determined that the fixation of boron in
soils occurs by one of three mechanisms: physical (molecular)
adsorption, in which the boron is held to the surface of the soil by
van der Waals bonds; anion exchange; or chemical precipitation.
Chemical adsorption involves ionic and covalent bonding. The
investigators speculated that the initial adsorption is probably
molecular in nature, followed by the formation of surface compounds
that result in an increase in adsorption sites, particularly at higher
boron concentrations in the soil solution. At higher concentrations,
chemical bonding of borate ions with hydroxyl ions on the soil surface
results in boron fixation to soluble aluminium, silicon, and iron.
This same mechanism (chemisorption) was observed by Couch & Grim
(1968) for the uptake of borate ions to clay mineral surfaces. The
presence of calcium ions, drying, and high pH values will tend to
increase the amount of fixed boron. Wetting and drying of the soil
will increase the maximum adsorption capacity and bonding energy of
the soil for boron.

Many of the surface boron compounds initially formed by
adsorption mechanisms may be unstable and leached by water. However,
as a result of the equilibrium that exists between adsorbed and
dissolved boron in soils, the adsorbed boron may act as a buffer,
impeding the leaching of excess boron from soils. Wierenga et al.
(1975) conducted a study to determine the downward movement of boron
through a sandstone formation in New Mexico, USA. The experimental
dispersion coefficient was calculated as 1.06 cm²/day, primarily
resulting from diffusion. Assuming an average annual rainfall of 20
cm/year and an average annual recharge of 10% of the annual
precipitation, the investigators determined that it would take 500
years for the boron front to reach a depth of 35 m into the sandstone.
As the groundwater table at this site is at 86 m, Wierenga et al.
(1975) calculated that it would take 1628 years for boron, at a
concentration one-half that of the surface concentration, to reach the
groundwater. A 10-fold increase in annual recharge from precipitation
would reduce the transit time by one-tenth.

Bingham et al. (1971) concluded that the single most important
property of soil that will influence the mobility of boron is the
abundance of amorphous aluminium oxide. Gerritse et al. (1982) showed
that the mobility of boron in sludge-amended sandy and sandy loam
soils was increased as a result of complexation with dissolved organic
compounds, high ionic strengths of the soil solutions, and other
factors.

4.1.4 Vegetation and wildlife

Hingston (1986) investigated the components of the biogeochemical
cycle for boron in two eucalypt forests. The importance of the
biological component of the cycle was indicated by the amount of boron
stored within trees (2.1 and 2.5 kg/ha for the two forests) compared
with the amount of extractable boron in the soils to a depth of 1 m (2
and 7 kg/ha), and by the highly significant correlations between
hot-water-soluble boron and organic carbon for these soils.

4.2 Transformation

4.2.1 Biotransformation

Borate ions present in aqueous solution are essentially in their
fully oxidized state. No aerobic processes are likely to affect their
speciation, and no biotransformation processes are reported in the
literature (personal communication from Borax US to the IPCS, 1995).
Therefore, there are unlikely to be any differences in boron species
due to biotransformation.

4.2.2 Abiotic transformation

Inorganic borates such as boric acid, boric oxide, and sodium
borates are stable, except for dehydration at high temperatures.
Organoboron compounds are sufficiently uncommon in nature to be
irrelevant to this document. In aqueous media, the chemical speciation
of boron-oxygen compounds is pH and concentration dependent.

4.2.2.1 Air

No information was available in the current literature concerning
the photolysis, oxidation, or hydrolysis of boron-oxygen compounds in
the atmosphere. The small amount of boron in air is assumed to be in
the form of boric acid.

4.2.2.2 Water

In natural waters, boron exists primarily as undissociated boric
acid with some borate ions. As a group, the boron-oxygen compounds are
sufficiently soluble in water to achieve the levels that have been
observed (Sprague, 1972; see chapter 5).

In seawater, inorganic boron content generally bears a linear
relationship to the amount of chloride ion present; a ratio of
0.000 24 g boron/g of total halogen expressed as chloride ion has been
calculated (Mellor, 1980). Byrne & Kester (1974) demonstrated that
weakly dissociated boric acid is the predominant species but also that
there are weakly associated ion pair neutral and positively charged
borate complexes of sodium, magnesium, and calcium. The metaborate ion
will undergo rapid hydrolysis in seawater to form the borate ion and
the weakly dissociated boric acid. Noakes & Hood (1961) concluded that
organically bound boron contributes very little, if any, to the total
boron content of seawater. Boron associated with organic matter was
found to vary with oxygen content, with the lowest concentrations
occurring in the minimum oxygen zone. Mance et al. (1988) described
boron as a significant constituent of seawater, with an average
concentration of 4.5 mg/litre.

Boric acid is a very weak acid, with a p K_a of 9.15; in fresh
water, therefore, boric acid and sodium borates exist predominantly as
undissociated boric acid below pH 7, but the metaborate anion becomes
the main species in solution above pH 10. Between these two pH bands,
there is also a characteristic presence of complex polyborate anions
in solution when the concentration is increased, leading to enhanced
solubility.

4.2.2.3 Soil

Borates as such cannot degrade, but borate complexes with organic
matter or sod mineral surfaces can be altered by water leaching or pH
change.

4.2.3 Bioaccumulation

4.2.3.1 Aquatic organisms

Highly water soluble materials are unlikely to bioaccumulate to
any significant degree, and borate species are all present essentially
as undissociated boric acid at neutral pH. The octanol/water partition
coefficient for boric acid has been measured as 0.175 (Barres, 1967),
indicating low bioaccumulation potential.

Thompson et al. (1976) studied boron uptake in two saltwater
species, juvenile Pacific oysters (Crassostrea gigas) and
underyearling sockeye salmon (Oncorhynchus nerka), in
continuous-flow systems with 95% solution replacement every 6 h.
Oysters (30/tank) were exposed to two boron levels (1 mg/litre above
background and 10 mg/litre above background) for 47 days, and salmon
(3/tank) were exposed only to the higher concentration for 21 days.
Control tanks received only seawater inflow. The background
concentration of boron in seawater in this study was approximately
3.98 mg/litre. The oysters were sampled on days 0, 8, 16, 36, and 47
of exposure. After this time, the remaining oysters were maintained in
seawater alone for another 24 days and then analysed for boron uptake.
Following the 21-day exposure period, the sockeye salmon were killed
and the boron concentration was determined in gill, liver, kidney,
muscle, and bone tissue. For both species, the tissue levels
approximated the boron concentrations in the test water, indicating
that these species take up boron in relation to its availability.

In the oyster, tissue concentrations returned to background
levels (3.67-4.13 µg/g) by the 71st day of the study, indicating a
fairly rapid clearance of boron with no evidence of long-term
retention. Boron concentrations in sockeye salmon tissues in normal
seawater ranged from 0.5 to 1.5 µg/g wet weight, with concentrations
increasing from muscle to gill and kidney, to liver, and to bone.
Boron levels were elevated in the bone and kidney tissue (5.9-17 µg/g
wet weight and 4.5-11.9 µg/g wet weight, respectively) of the exposed
salmon; however, they were not significantly different from test water
levels. Consequently, there was no evidence for active bioaccumulation
of boron in these species (Thompson et al., 1976).

Suloway et al. (1983) studied the bioaccumulation potential of
the components of coal fly ash extract in fathead minnows
(Pimephales promelas) and green sunfish (Lepomis cyanellus). Five
fish of each species were exposed for 30 days to fly ash extracts
containing boron at concentrations ranging from 1.23 to 91.7 mg/litre.
Whole-body concentrations of boron ranged from 1.16 to 4.15 µg/g in
the exposed fathead minnows and from 1.08 to 4.62 µg/g in the exposed
green sunfish. The reported bioconcentration factor was 0.3 for both
species. These results are consistent with those described above and
indicate that boron does not bioaccumulate significantly in fish.

4.2.3.2 Terrestrial plants

Eaton (1944) investigated growth reaction and boron accumulation
characteristics of plants grown in outdoor sand culture beds where
cultures were supplied with nutrient solutions containing differing
concentrations of boron. The concentrations of boron ranged from 58 to
1804 µg/g dry weight in the leaves of plants grown in 5 mg boron/litre
and from 209 to 3875 µg/g dry weight in the leaves of plants grown in
25 mg boron/litre. The boron concentrations were generally lower in
roots, stems, and fruits than in the leaves. This is consistent with
the fact that boron is absorbed from the soil solution by the roots
and passively carried in the transpiration stream to the leaves, where
the water evaporates and the boron accumulates. The absorption into
the roots usually occurs as active transport against a concentration
gradient (the concentration in the soil solution is generally lower
than in the root tissues); therefore, an expenditure of energy is
required. However, at higher boron soil solution concentrations, which
are toxic to some plant species, the mechanism of uptake is passive
diffusion (Bingham et al., 1970). Boron is relatively immobile in the
phloem; consequently, the accumulated boron does not move out of the
leaf tissues and into the fruit and other tissues (Kohl & Oertli,
1961).

Several factors affect the uptake of boron, including the pH of
the soil solution, temperature, light intensity, and the concentration
of other elements (e.g. calcium and potassium). Uptake is reduced by a
factor of four as soil pH increases from 4 to 9 (Bingham et al., 1970)
and increased by an increase in light intensity (Tanaka, 1966); the
rate of boron absorption rapidly increases at temperatures ranging
from 10 to 30°C and is sharply reduced above 35°C (Reisenauer & Cox,
1971).

4.2.3.3 Birds

Pendleton et al. (1995) exposed adult male mallard ducks to a
dietary concentration of 1600 mg boron/kg for up to 48 days.
Equilibrium levels were reached between days 2 and 15. Boron
concentrations were highest in the blood, followed by the brain and
liver. Boron was rapidly eliminated, with few detectable residues
after 1 day on a "clean diet." The presence of arsenic (300 mg/kg) in
the diet slowed the accumulation of boron.

4.3 Ultimate fate following use

No information was available in the current literature concerning
the disposal of boron or boron compounds. Information was located,
however, regarding the reclamation and revegetation of coal combustion
products (i.e. ash) that contain high concentrations of metals,
including boron. Although the chemical and physical properties of coal
ash tend to be detrimental to plant growth and establishment,
additions of fertilizer and manure provide a more suitable medium
(Schwab et al., 1991). Plant establishment on the site is only the
first phase of the reclamation process. It is also necessary to ensure
that leachate from the ash does not contaminate the surface water and
groundwater in the immediate region and that uptake of metals in the
plant materials does not result in metal concentrations that are toxic
to livestock or wildlife. In a study of the revegetation of several
ash disposal sites in Kansas, USA, Schwab et al. (1991) noted
variations in plant uptake of boron from coal ash owing to differences
in ash type, plant species, and ash treatment. Boron contained in
detergents after use releases to the municipal sewage system. It
should be noted that boron is not removed by the usual water treatment
processes. Landfill will tend to be the ultimate fate of many boron
products.

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1 Environmental levels

5.1.1 Air

Boron, as boric acid, is released into the atmosphere during
volcanic eruptions; however, most is captured by the oceans
(Muetterties, 1967). Coal-fired power plants and agricultural burning
are major sources of atmospheric boron contamination; at least 30% of
boron in coal is lost in this manner (Eisler, 1990). Nevertheless,
boron does not appear to be present in ambient air at significant
levels (Sprague, 1972), presumably because of rapid transport to other
media (see section 4.1.1). Although the concentration is low, the
atmosphere carries a significant amount of boron as boric acid vapour.

Mean boron concentrations in emissions from active volcanic sites
range from <2.5 to 31.4 µg/m³ for gaseous boron and are below
4 µg/m³ for particulate boron. Volcanic lake fumes (El Chichon,
Mexico) contained mean boron concentrations of up to 8.5 µg/m³ for
particulate boron and up to 16.1 µg/m³ for gaseous boron (Anderson et
al., 1994a).

Anderson et al. (1994a) monitored atmospheric concentrations of
boron at continental, coastal, and remote marine sites. Mean
particulate boron concentrations ranged from 1.8 to 12.2 ng/m³, from
2.4 to 3.7 ng/m³, and from <0.5 to 2.8 ng/m³ for the three types of
site, respectively. Mean gaseous boron concentrations ranged from
<0.5 to 20.7 ng/m³, from 3.5 to 82.8 ng/m³, and from 0.6 to
25 ng/m³, respectively. Anderson et al. (1994a) assumed 90% of boron
in the air is gaseous and 10% is in particulate form.

5.1.2 Water

5.1.2.1 Groundwater

Naturally occurring boron is present in groundwater primarily as
a result of leaching from rocks and soils containing borates and
borosilicates (i.e. local geology). Concentrations of boron in
groundwater throughout the world range widely, from <0.3 to
>100 mg/litre. Boron levels in European groundwaters are presented in
Table 5. In general, concentrations of boron were greatest in southern
Europe (Italy, Spain, but not Greece) and least in northern Europe
(Denmark, France, Germany, Netherlands, and the United Kingdom). For
Italy and Spain, mean boron concentrations ranged from 0.5 to
1.5 mg/litre. Concentrations ranged up to approximately 0.6 mg
boron/litre in the Netherlands and United Kingdom, and levels in
approximately 90% of samples in Denmark, France, and Germany were
found to be below 0.3, 0.3, and 0.1 mg boron/litre, respectively.

Table 5. Concentrations of boron in European groundwater^a

Country Area No. of Boron concentration
samples (mg/litre)

Denmark 525 92.2% < 0.3
7.4% > 0.3
0.4% > 1.0

France 716 99.5 < 0.3
0.5 > 0.3

Germany Baden-Wurttemberg 2574 89% < 0.1
10.7% > 0.1
0.3% > 1.0
Lower Saxony 188 96% < 1.0
4% > 1.0

Greece Patras 10 100% < 0.1
Halkidiki 3 2.3-5.4

Italy North of Rome 423 Mean = 1.0
Sicily 18 Mean = 1.5
Paglia 102 Mean = 0.75

Netherlands Inland 0.08-0.6

Spain Valencia 21 Mean = 0.64
Almeria 17 Mean = 0.98
Murcia 15 Mean = 0.51

United Kingdom London 21 0.02-0.54
Northumbria 164 Mean = 0.31
Dumfries and Galloway Mean = 0.04
Permo-Triassic
(Scotland)

^a ECETOC (1997).

Groundwater contaminated with excessive concentrations of boron
from surface water recharge has been noted beneath the Kesterson
Reservoir, California, USA. This reservoir serves as an evaporative
sink for several metalloids, including boron, and receives
agricultural drainage from farmlands within the San Joaquin River
Valley. Benson et al. (1991) reported an average boron concentration
of 15 mg/litre. Concentrations of boron elsewhere within the San
Joaquin River Valley have been shown to range from 0.14 to 120
mg/litre, with a median of 4 mg/litre (Deverel & Millard, 1988;
Butterwick et al., 1989).

5.1.2.2 Surface water

The majority of the Earth's boron occurs in the oceans, with an
average concentration of 4.5 mg/litre (Weast et al., 1985). The amount
of boron in fresh water depends on such factors as the geochemical
nature of the drainage area, proximity to marine coastal regions, and
inputs from industrial and municipal effluents (Butterwick et al.,
1989). Concentrations of boron in fresh surface water are summarized
in Table 6.

Concentrations ranged from 0.001 to 2 mg boron/litre in Europe,
with mean values typically below 0.6 mg/litre. Similar concentrations
have been reported for water bodies within Pakistan, Russia, and
Turkey; concentrations range from <0.01 to 7 mg boron/litre, with
most values below 0.5 mg/litre. Concentrations ranged up to 0.01 mg
boron/litre in Japan and up to 0.3 mg boron/litre in South African
surface waters. Samples taken in surface waters from two South
American rivers (Rio Arenales, Argentina, and Loa River, Chile)
contained boron at concentrations ranging between 4 and 26 mg/litre in
areas rich in boron-containing soils. In other areas, the Rio Arenales
contained less than 0.3 mg boron/litre. Concentrations of boron in
surface waters of North America (Canada, USA) ranged from
0.02 mg/litre to as much as 360 mg/litre, indicative of boron-rich
deposits. However, typical boron concentrations were less than
0.1 mg/litre, with a 90th-percentile boron concentration of
approximately 0.4 mg/litre.

5.1.2.3 Rainfall

The median and mean concentrations of borate in rain and snow at
six sites in western Switzerland were found to be 0.0031 and 0.0056 mg
boron/litre, respectively (Atteia et al., 1993).

5.1.3 Sewage

Concentrations of boron in sewage waters are summarized in Table
7.

The majority of the boron present in sewage occurs primarily as
undissociated boric acid; reported levels of boron in sewage in the
USA range from 0.4 to 1.5 mg/litre and up to 4.05 mg/litre because of
industrial waste discharges (Banerji, 1969). In Europe, sewage from
domestic and industrial sources typically has an average boron
concentration of 2 mg/litre, with levels up to 5 mg/litre (Butterwick
et al., 1989). Calculations by the German Government Environment
Agency attribute 50% of the boron in wastewater to the use of
detergent products (Butterwick et al., 1989). In boron mine drainage
waters in Turkey, the boron concentrations were reported to be 16-390
mg/litre (Okay et al., 1985). Boron levels in sewage sludge in 23 US
cities ranged from 7.1 to 53.3 mg/kg dry weight (Mumma et al., 1984).

Table 6. Concentrations of boron in fresh surface water

Area Boron concentration Reference
(mg/litre)

USA Median = 0.076 ECETOC (1997)
90th percentile = 0.387

Drainage basins, USA 0.019-0.289^a Kopp & Kroner (1970)

Coastal drainage waters, 15 (boron-rich deposits) Deverel & Millard (1988)
California, USA

Lakes, California, USA 157-360 (boron-rich Deverel & Millard (1988)
deposits)

Ontario, Canada 0.029-0.086 Sekerka & Lechner (1990)

Cold River drainage 0.0627 Tsui & McCart (1981)
area, western Canada

United Kingdom 0.046-0.822 Mance et al. (1988)

Italy 0.4-1.0 (range of Manfredi et al. (1975)
means)

<0.1-0.5 Tartari & Camusso (1988)

Sweden 0.013 (0.001-1.046) Ahl & Jönsson (1972)

Germany 0.02-2.0 Graffmann et al. (1974)

The Netherlands Range of medians = Unilever (1994)
0.09-0.145

Rivers, Austria <0.02-0.6 (background Schöller & Bolzer (1989)
level)

River Neva, Russia 0.01-0.02 Huber (1994)

Degh Nala, Pakistan <0.01-0.46 (near Tehseen et al. (1994)
effluent discharges)

Simav River, Turkey <0.5 (uncontaminated) Okay et al. (1985)
4 (maximum 7)
(contaminated with
boron mine waste)

Table 6. (continued)

Area Boron concentration Reference
(mg/litre)

Rio Arenales, Argentina <0.3 Bundschuh (1992)
6.9 (near borate plant)

Loa River Basin, Chile 3.99-26 (soil rich in Cáceres et al. (1992)
minerals and natural
salts; low rainfall)

Japan (River Asahi) 0.009-0.0117 Korenaga et al. (1980)

South Africa 0.02-0.33 Reid & Davies (1989)

^a Lowest concentration in the western Great Lakes Basin to highest concentration
in the western Gulf Basin.

Table 7. Concentrations of boron in sewage waters

Area/source Boron concentration Reference
(mg/litre)

USA
Industrial waste 0.4-1.5 Banerji (1969)
discharge (maximum 4.05)

Europe
Domestic and industrial 2 (maximum 5) Butterwick et al. (1989)

Egypt
Sewage water 0.32-0.38 El-Hassanin et al. (1993)

Sweden
Effluent 0.34-0.436 Ahl & Jönsson (1972)

Spain, Alicante
Industrial waste 1.45 Navarro et al. (1992)

Spain, Elche
Industrial waste 3 Navarro et al. (1992)

United Kingdom
Municipal 1.21-3.96 Waggott (1969)
(range of means)

5.1.4 Soil

According to Whetstone et al. (1942), boron occurs in soils at
concentrations ranging from 10 to 300 mg/kg (average 30 mg/kg),
depending on the type of soil, amount of soil organic matter, and
amount of rainfall. Background boron levels in US soils were reported
at a geometric mean concentration of 26 mg/kg, with a maximum
concentration of 300 mg/kg (Eckel & Langley, 1988).

5.1.5 Aquatic biota

Concentrations of boron in aquatic biota are summarized in
Table 8.

Little specific information was found concerning the
bioaccumulation of boron in aquatic plants. At Kesterson National
Wildlife Refuge in the San Joaquin River Valley, California, USA (an
evaporative sink that has high concentrations of boron, selenium, and
arsenic and is supplied with subsurface drainage water from
agricultural fields), studies of the aquatic food-chain contamination
have suggested that aquatic plants bioaccumulate high levels of boron,
but boron does not biomagnify in aquatic food-chains. The following
studies report observed concentrations in marine algae and freshwater
aquatic vascular species. Igelsrud et al. (1938) reported boron levels
ranging from 4.2 to 14.9 mg/kg of dried material in marine algae.
Yamamoto et al. (1973) compared the boron content in freshwater and
marine phytoplankton and observed that minor differences occurred
between forms, even though the boron content of seawater averages 460
times that of fresh water.

Adams et al. (1973) conducted a survey to determine the
concentration of 11 potentially polluting ions, including boron, in a
wide variety of aquatic plants from three major watersheds in
Pennsylvania, USA: the Delaware, Susquehanna, and Allegheny rivers.
Sources of pollution in this area are quite diverse, including
lumbering activities, coal strip-mining, recreation, agricultural use,
and urban-industrial centres. Boron constituent levels in 21 species
of submerged and floating aquatic vascular plants ranged from 26.3 to
170 µg/g, and levels in 8 species of emergent aquatic vascular plants
ranged from 11.3 to 57 µg/g.

Tsui & McCart (1981) studied the bioaccumulation of several
elements, including boron, in five freshwater fish species from the
Cold River drainage area in western Canada. Test species were selected
to represent different feeding habits and modes of life. Northern pike
(Esox lucius) and lake trout (Salvelinus namaycush) are primarily
predators; lake herring (Coregonus artedii) is a plankton feeder;
and lake whitefish (Coregonus clupeaformis) and white sucker
(Catostomus commersoni) are primarily bottom-feeders. The fish were
collected during spring and summer of 1978 from seven lakes within
this area, and the muscle tissue was analysed for the presence of
boron. The maximum average concentration of boron in the lakes was

Table 8. Concentrations of boron in aquatic biota

Species Area Tissue Boron concentration Reference
(mg/kg)^a

Marine algae 4.2-14.9 dw Igelsrud et al. (1938)

Filamentous algae Lower San Joaquin River and its 3.5-280 dw Saiki et al. (1993)
tributaries, California, USA

Plankton Lower San Joaquin River and its 1.1-46 dw Saiki et al. (1993)
tributaries, California, USA

Aquatic plants Lower San Joaquin River, 382 (270-510) dw Ohlendorf et al. (1986)
California, USA

Submerged and floating Pennsylvania, USA 26.3-170 Adams et al. (1973)
aquatic vascular plants

Emergent aquatic Pennsylvania, USA 11.3-57 Adams et al. (1973)
vascular plants

Various marine shellfish British Columbia, Canada 0.9-5.5 ww Thompson et al. (1976)

Benthic bivalve San Joaquin River and its
(Corbicula fluminea) tributaries, California, USA Soft tissue <2-2 dw Leland & Scudder (1990)

Clam (Elliptio dilitata) Precambrian Shield lake, Soft tissue 2.6 ww Wren et al. (1983)
Ontario, Canada

Chironomid larvae Lower San Joaquin River and its
tributaries, California, USA <1.8-27 dw Saiki et al. (1993)

Amphipods Lower San Joaquin River and its <2.2-23 dw Saiki et al. (1993)
tributaries, California, USA

Crayfish Lower San Joaquin River and its 1.2-23 dw Saiki et al. (1993)
tributaries, California, USA

Table 8. (continued)

Species Area Tissue Boron concentration Reference
(mg/kg)^a

Freshwater fish Cold River drainage area, Muscle 3.23-12.44 Tsui & McCart (1981)
western Canada (range of means)

Freshwater fish Precambrian Shield lake, Muscle 1.8-2.9 ww Wren et al. (1983)
Ontario, Canada

Bluegill (Lepomis San Joaquin River, California, Whole body 14 dw (3.5 ww) Saiki & May (1988)
macrochirus) USA

Lower San Joaquin River and its <1.8-7.9 dw Saiki et al. (1993)
tributaries, California, USA

Largemouth bass Lower San Joaquin River and its <1.8-2.0 dw Saiki et al. (1993)
(Micropterus salmoides) tributaries, California, USA

Common carp San Joaquin River, California, Whole body 20 dw (5 ww) Saiki & May (1988)
(Cyprinus carpio) USA

San Joaquin River, California, Whole body 0.5-6.2 dw^b Klasing & Pilch (1988)
USA

Mosquitofish Lower San Joaquin River and its <1.9-8.4 dw Saiki et al. (1993)
(Gambusia affinis) tributaries, California, USA

Volta, California, USA Whole body mean = 2.8 dw Ohlendorf et al. (1986)

Kesterson, California, USA Whole body mean = 11.1 dw Ohlendorf et al. (1986)

Tilapia spp. Mexicali Valley, Baja California, Muscle 2.9 ww Mora & Anderson (1995)
Mexico

Mugil spp. Mexicali Valley, Baja California, Muscle 1.9 ww Mora & Anderson (1995)
Mexico

Table 8. (continued)

Species Area Tissue Boron concentration Reference
(mg/kg)^a

Sockeye salmon British Columbia, Canada Gill mean = 0.6 ww Thompson et al. (1976)
(Oncorhynchus nerka) Liver mean = 0.7 ww
Bone mean = 1.5 ww

Atlantic cod (Gadus Northwest Atlantic Ocean Muscle 28 (1-93) ww Hellou et al. (1992)
morhua) Liver 9.7 (5.2-35.4) ww
Ovaries <0.8

Anchoveta Whole body 3.3-3.8 aw Jenkins (1980)
(Cetengraulis
mysticetus)

Aquatic birds Precambrian Shield lake, Muscle 2.5-3.7 ww Wren et al. (1983)
Ontario, Canada

Grassland water district, Liver 1.7-40 dw Paveglio et al. (1992)
California, USA

Double-crested Mexicali Valley, Baja California,
cormorant Mexico Liver 4.2 (2.9-8.2) ww Mora & Anderson (1995)
(Phalacrocorax auritus)

Duck species Grassland water district, Egg 3.07-6.17 dw Hothem & Welsh (1994)
California, USA

Wading bird species Grassland water district, Egg 2.2-3.45 dw Hothem & Welsh (1994)
California, USA

Aquatic mammals Precambrian Shield lake, Muscle 7.9 ww Wren et al. (1983)
Ontario, Canada

^a ww = wet weight; dw = dry weight; aw = ash weight; concentrations are given as means or ranges of means; ranges are given in parentheses.
^b Exposed to tile drainage water.

0.0627 mg/litre. The mean tissue concentrations of boron in the five
fish species ranged from 3.23 µg/g for lake whitefish to 12.44 µg/g
for white sucker.

Wren et al. (1983) reported boron concentrations in freshwater
fish and clams from a Precambrian Shield lake in Ontario, Canada. The
lake was free from direct human impact. Boron levels in the
undeveloped and protected muscle tissue of the fish were generally
lower than those observed in fish from the Cold River drainage area in
western Canada. The mean concentrations (wet weights) in the fish
ranged from 1.8 to 2.9 µg/g. The boron concentration in the soft
tissue of the clam (Elliptio dilitata) was 2.6 µg/g.

In contrast, boron levels were only slightly elevated in
whole-body samples of bluegill (Lepomis macrochirus) and common carp
(Cyprinus carpio) from the San Joaquin River and two tributaries
that receive agricultural subsurface drainage water. The highest boron
concentrations (dry weights) measured were 14 µg/g (approx. 3.5 µg/g
wet weight) in bluegills and 20 µg/g (approx. 5 µg/g wet weight) in
carp (Saiki & May, 1988). Ohlendorf et al. (1986) reported similar
values for mosquitofish (Gambusia affinis) from the San Joaquin
River Valley. However, Saiki & May (1988) reported that the elevated
boron levels may also result from natural boron deposits in adjacent
soils or from sand-and-gravel mining operations in the area.

Paveglio et al. (1992) analysed boron concentrations in livers of
aquatic birds collected from the Grassland Water District of
California, USA, during 1985-1988. The use of subsurface agricultural
drainage water for marsh management resulted in trace element
contamination of components of the food-chain in this region. During
the breeding and wintering periods, livers of birds from northern and
southern areas of the grasslands contained high concentrations of
boron (1.7-40 mg/kg dry weight).

A number of studies have investigated the accumulation of boron
in aquatic food organisms, such as plants, insects, and fish (Saiki &
May, 1988; Hothem & Ohlendorf, 1989; Smith & Anders, 1989; Paveglio et
al., 1992; Saiki et al., 1993). The results of these studies suggest
that aquatic plants bioaccumulate boron, but that boron does not
biomagnify in aquatic food-chains.

5.1.6 Terrestrial biota

Concentrations of boron in terrestrial biota are summarized in
Table 9.

The studies discussed in section 4.2.3 suggest that plants grown
in boron-rich soil often contain high levels of boron in their
tissues. Another source of boron