HEXACHLOROBENZENE (HCB) JMPR 1973
Explanation
Hexachlorobenzene was first evaluated by the 1969 Joint Meeting
(FAO/WHO 1970b) and recommendations were made for practical residue
limits in a range of food commodities. The Meeting listed certain
further work and information required.
Since that time considerable interest has developed in HCB
residues. There have been numerous problems in international trade as
a result of the discovery of HCB residues in animal products and
animal feeds. It is now recognized that HCB originates from industrial
pollution as well as from agricultural practices.
The realization that HCB is not biodegraded to any significant
degree coupled with improved analytical methods has promoted extensive
investigations which have revealed widespread contamination of animal
feeds, animal products, people and other important components of the
environment.
EVALUATION FOR ACCEPTABLE DAILY INTAKE
The 1969 Joint Meeting. on the basis of the results of a 13-week
feeding study in rats, in which no adverse effects were detected at a
daily dosage of 1.25 mg/kg, estimated a "tentative negligible daily
intake" of 0-0.0006 mg/kg bw. The 1969 Meeting indicated that before
an ADI could be established several toxicological studies would be
required.
The present Meeting decided that the term "tentative negligible
daily intake" should not be used.
Since the 1969 Joint Meeting no further work relevant to
estimating an ADI has become available. It is known, however, that
several laboratories are currently engaged in research that might
provide a reasonable basis for the safety evaluation of HCB, and the
Meeting urged that these data should be obtained for consideration by
a future Joint Meeting.
It was recognized that HCB residues in foods also arise from
sources other than its proper use as a fungicide (e.g., industrial
wastes from chlorination processes, and contamination of other
chlorinated pesticides). Because of this, it will not be sufficient to
attempt to control exposure merely by controlling the use of HCB as a
fungicide. Notwithstanding the fact that residues of HCB stem less
from its use as a pesticide than from other sources, the Meeting
reaffirmed the recommendation made in 1969 that a suitable substitute
for HCB as a seed fungicide should be sought. In addition the Meeting
recommended that efforts should be made to reduce the level of HCB as
an impurity in other pesticides. A specific recommendation to this
effect was made in the case of quintozene.
On the assumption that there is a level of HCB below which no
significant toxicological effects can be expected, it was felt
possible to give some provisional guidance. A daily intake of 0.0006
mg/kg is considerably below any dosage rate known to be harmful, and
the Meeting recommended that this value should be used as a guide for
setting upper limits for residues until it was possible to establish
an ADI based on the results of comprehensive toxicological studies.
The Meeting considered the further work that was required for the
establishment of an ADI and its recommendations are listed in the
relevant section of this monograph addendum.
RESIDUES IN FOOD AND THEIR EVALUATION
Source of residues
In the previous monograph (FAO/WHO 1970b) it was assumed that the
only important source of HCB residues was contamination resulting from
HCB seed dressings used to protect wheat from Bunt due to Tilletia
foetide and T. caries against which HCB is spectacularly
effective.
Since then it has been shown that HCB is present in significant
amounts as a contaminant of the fungicides quintozene
(pentachloronitrobenzene) and technazine (tetrachloronitrobenzene) and
in the herbicide chlorthal-methyl (Daethal), (See also the monograph
on quintozene). Analysis of 10 samples of commercial quintozene have
shown HCB present in all 10, ranging from 0.1% to 10.8%. Of 12 samples
of quintozene of European manufacture analysed in the Netherlands nine
contained HCB at levels ranging from 0.8-5.3%. Three samples contained
less than 0.1% HCR but these contained 3.6% pentachlorobenzene which
is somewhat less intractable than HCB.
HCB is also a by-product from the production of chlorinated
solvents and as an intermediate for the preparation of
pentachlorophenol. It has been shown that the commercial production of
perchlorethylene gives rise to the formation of HCB which can be
released unwittingly into the environment in quantities sufficient to
contaminate large amounts of food commodities.
In many instances the chlorination of hydrocarbons produces a
heavy tarry residue. The tar contains a variety of highly chlorinated
products, the two principal components being hexachlorobenzene and
hexachlorobutadiene. These materials are not normally contaminants of
the products of commerce produced by these reactions but if the tar is
incinerated. dumped or discharged into municipal or industrial sewers
the HCB quickly becomes dispersed in the environment. In some
industrial processes it is possible to recycle the tar and thereby
inhibit the formation of further quantities but there is no doubt a
limit to such conservation. Any release of HCB into air or water must
inevitably lead to accumulation of residues in human food. The
predisposition which HCB possesses to accumulate in fatty bodies means
that residues are detected in valuable components of the biosphere
including people, animal fat. milk, birds, fish and wildlife.
Evidence of residues in food in commerce or at consumption
Since the previous review (FAO/WHO 1970b) considerable quantities
of raw agricultural produce and food moving in trade within and
between countries have been found to contain detectable quantities of
HCB.
The United States Food and Drug Administration Import Detection
Reports issued each month indicate that substantial quantities of
cheese and other dairy products from many different countries have
been detained because of unacceptable residues in HCB.
In a survey of pesticide residues in cheese imported into
Australia in 1971, 68 of 184 samples were found to contain HCB at
levels ranging from 0.03 to 4.06 ppm. More than 10% of the samples
contained residues in excess of 0.3 ppm. Cheese from 16 countries was
involved and HCB was found in samples from every country except two
from which only single samples were examined. The incidence ranged
from 10% to 75% for the remaining 14 countries.
Results of a total diet study carried out in Australia (NHMRC
1971) indicate that 46% of 240 samples of a total diet collected in
six states over four seasons contained HCB at levels above 0.001 ppm.
Of 10 food groups the ones with the highest incidence were meat
dripping, meat and dairy produce. The highest concentration was found
in meat dripping but this food represents only 1% of the total diet. A
further survey has been conducted since the use of HCB seed dressings
has been discontinued but the results are not yet available.
There have been reports of significant residues of HCB in breast
milk from the Netherlands (Tuinstra, 1971), Australia (Siyali, 1972)
and Germany (Acker and Schulte, 1971). Swedish workers (Westoo et al.,
1972) report significant levels of HCB in the breast milk of each of
10 individual women selected at random. The level ranged from 0.07 to
0.22 ppm expressed on the fat basis. In another survey of 18 samples,
each a composite from 10-20 mothers, the level of HCB ranged from 0.06
to 0.59 ppm (mean 0.19 ppm). This was slightly less than 8% of the
level of DDT and metabolites and equal to the concentration of
dieldrin in the same samples.
An Australian study of occupationally exposed individuals showed
levels in blood ranging from 0 to 0.41 ppm. A control group with no
known exposure showed blood levels ranging from 0 to 0.095 ppm. HCB
was detected in 95% of the people tested (the minimum level of
detection being 0.00001 ppm) (Siyali, 1972).
Another Australian study of a random selection of perirenal fat
samples taken at autopsy revealed 106% of the samples positive with
levels from "trace" to 8.2 ppm; 33% of the samples contained greater
than 1 ppm. The mean of all samples was 1.25 ppm (Brady and Siyali,
1972).
A German study detected residues of 6.3 ppm in human fat and 5.3
ppm in the fat of human milk (Acker and Schulte, 1971). A recent
Japanese study has shown levels in human adipose tissue to range from
0.30 to 1.48 ppm (Curley et al., 1973).
A survey of organochlorine residues in human fat in the United
Kingdom (Abbott et al., 1972) showed that HCB residues occurred
generally in the whole population ranging from 0.01 to 0.29 ppm with a
mean of 0.05 ppm in 201 samples. Although this was the third such
study carried out by the same investigators since 1963, HCB had not
been detected or reported previously. The paper indicates some
difficulty with identification and quantitation of the HCB.
Swedish investigators (Westoo et al., 1971) report finding HCB in
samples of local and imported breakfast cereals. The concentration
ranged from 0.002 to 0.007 ppm which was considerably below the level
of lindane. Lindane occurred in all samples at levels ranging from
0.03 to 1.54 ppm.
The Netherlands authorities in the course of inspecting imported
animal foodstuffs found that certain milling fractions of raw cereals
contained HCB residues ranging up to 3 ppm. These residues were the
cause of considerable difficulties because of the magnification which
occurs in animal fat, milk and eggs.
Raw cereals and their milling fractions are not the only source
of HCB residues which find their way into animal feeds. Import
inspection in the Netherlands has revealed consistent and relatively
high levels of HCB in bran, beans, clover seed. chicory root, copra
press cake, cotton press cake, lespedasia meal, linseed, maize meal,
peas, pollard pellets, potatoes, sorghum, soybean meal, sunflower
press cake, vetch and wheat middlings. Rice was one of the few
commodities examined which was consistently free of HCB. Commodities
contaminated with HCB have been intercepted coming from Argentina,
China, Indonesia, Morocco, Madagascar, Syria, South America, Turkey
and United States of America. Although the level of HCB residues found
in imported feed commodities entering the Netherlands was lower than
that of DET residues, the frequency with which HCB residues occurred
was much higher. It was reported to FAO (Swiss Federal Health Service
(1973)) that residues of HCB had been detected in various commodities
as follows:
Meat fat 0.01-0.075 ppm
Poultry fat 0.001-0.5 ppm
Eggs (whole) 0.002-0.3 ppm
Milk fat 0.002-0.04 ppm
Milk products 0.04-0.5 ppm (1 sample
(fat basis) 0.9 ppm)
Raw cereals 0.001-0.05 ppm
Swedish workers (Westoo and Noren, 1973) report finding small
amounts of HCB in imported carrots, radishes and tomatoes, frequently
in lettuce (up to 0.25 ppm) and in parsley. This appears to be the
first occasion on which HCB had been reported from similar surveys
carried out in Sweden and it is interesting to speculate whether it
reflects improvements in methodology which enabled the detection of
residues that have always occurred but have not been detected.
Fate of residues
There continues to be a lack of any experimental evidence to
indicate that hexachlorobenzene is metabolized by plants, animals, or
soil micro-organisms or that it is degraded into other products in the
environment.
In a study resulting from accidental contamination of milk,
(Goursaud et al., 1972) it was found that the source of the
contamination appeared to be chicory roots used as cattle feed and
which had been treated, prior to forcing, with the fungicide
quintozene. The commercial 30% quintozene formulation contained 3%
HCB. When milk cows were experimentally fed diets containing 2 mg/day
of HCB and 25 mg/day of quintozene, progressive increases in the HCB
content of the milk reached 2 ppm on the seventh day. Only traces of
quintozene were found in the milk.
In a series of feeding experiments involving sheep, laying hens,
and growing chickens (Avrahami and Steele, 1972a, b, c) it was
demonstrated that feeding dietary levels of HCB ranging from 0.1 to
100 ppm resulted in rapid accumulation of HCB in omental fat (sheep),
body fat (chickens and laying pullets), and egg yolk. Sheep stored HCB
in body fat to the extent of seven to nine times the feed level at all
levels tested. The half-life of HCB in fat appeared to be 10 weeks for
sheep and 18 weeks in the case of chickens. For chickens and pullets,
the fat: feed ratio ranged from 30:1 to 20:1 with increasing feed
concentration. HCB concentration in egg yolk was about 25% of that in
the corresponding fat of hens.
Mention was made (Stijve, 1973) of some evidence for partial
degradation of hexachlorobenzene in chicken fat samples during a
survey of residues in cereals, fats, and dairy products using improved
analytical methods. However, no controlled experiments were carried
out.
In a feeding study in the Netherlands (Verschuuren et al., 1973)
young pigs were given a ration to which HCB had been added at the rate
of 0.25 ppm. The pigs were fed for 14 weeks from the age of eight
weeks. A sample of subcutaenous fat was taken by biopsy prior to the
feeding of the fortified ration. At the end of 14 weeks feeding, the
pigs were slaughtered. Vital organs were weighed. Liver and kidney
were examined microscopically and samples of fat, brain, kidneys,
liver and blood were taken for analysis. The total residue of HCB in
the body of the pigs was estimated to approximate the total HCB
administered in the feed indicating that little had been excreted. The
residue in fatty tissue (1.6 ppm) was higher than an acceptable level.
The ratio between the mounts of HCB in the diet and in the fat of the
pig, in a period of 14 weeks was 8.3:1.
Australian studies (McCray 1973) showed that sows receiving HCB
in rations for a few weeks accumulated sufficient residue to transfer
considerable quantities to the offspring at birth. They excreted
substantial amounts in the milk so that the piglets acquired a body
burden, sufficient to remain at significant levels even when they had
grown to 100 kg bw. The awe investigator found that cows receiving HCB
for a few days built up a reservoir in the fat which caused
significant residues in milk at the end of the second subsequent
lactation.
Studies in the Netherlands (Vos et al., 1972) involving the
feeding of from 0.05 ppm to 0.3 ppm of HCB in the rations to broiler
chickens for seven weeks, showed that a plateau in the concentration
of HCB in the fat was reached by the twentyninth day of feeding. The
residue level in fat was directly proportional to the level of HCB in
the ration, being approximately 12 times more concentrated.
These investigators carried out parallel experiments using
lindane, DOT, dieldrin, ondrin and heptachlor. They drew particular
attention to the distinct difference between the properties and fate
of lindane and HCB. HCB and heptachlor epoxide had the strongest
tendency to accumulate of all the compounds tested. It was observed
that relatively little lindane is accumulated in broiler fat when
rations are contaminated with lindane.
Other workers in the Netherlands (Wit et al., 1973) investigating
the uptake of HCB from traces in foodstuffs set up an extensive trial
involving the feeding of groups of pigs with prepared feeds containing
varying amounts of pollard known to contain small quantities of HCB.
They found it was impossible to obtain a ration that did not give rise
to low levels of HCB in the fat of pigs fed for 15-16 weeks. Such
animals showed 0.06-0.1 ppm HCB in fat. Pigs receiving rations
containing 0.12 ppm HCB showed residues of 1.3 ppm in fat while those
receiving 0.3 ppm in their dry ration had 2.3 ppm in their fat 15-16
weeks later. The "concentrating factor" (the ratio of concentration in
fat to that in feed) was estimated to range from 8 to 11 fold. This
compared with 0.3 for lindane, 2 for dieldrin, 3 for alpha-BHC and 5
for DDT. HCB is truly cumulative when fed to pigs.
Methods of residue analysis
Several papers have appeared on improvements in analytical
techniques and these reveal that extraction and Clean-up procedures
employed in typical multi-detection procedures give relatively poor
recovery of HCB.
Taylor and Keenan (1970) point out that in the method of Mills
(1959) the exceptionally non-polar character of HCB results in it
remaining preferentially in the hexane of the hexane/acetonitrile
portion clean-up. In counter-current separations recovery is less than
10%. Other methods using direct extraction with adsorption clean-up
give acceptable recoveries from fatty substrates. (Onley and Mills
1962; Moats, 1963; Langlois et al., 1963). The recovery from grain and
cereal products where the levels of interest are much lower than
animal products is unsatisfactory unless the sample is refluxed with
hexane. The extract is then submitted to a Florisil clean-up.
Taylor and Keenan (1970) developed a technique whereby the hexane
extract was steam distilled to give recoveries of 95%-100%.
Identification using TLC at the low concentrations found in cereal
products is not sufficiently sensitive and the authors propose the use
of two separate columns. The retention times relative to dieldrin of
a range of organochlorine pesticides on five stationary phases is
given. The retention time for HCB is from 12 to 22% of the
corresponding time for dieldrin. Some columns do not adequately
separate HCB from alpha-BHC. A paper on the design of columns (Taylor,
1970) provides useful guidance.
Smythe (1972) outlines the difficulties in obtaining good
recoveries of HCB from fatty substrates and proposes means to ensure
adequate recoveries.
Example of national tolerances
Several countries have recently announced legal limits on
administrative action levels for HCB residues in various commodities.
These include:
Switzerland Meat fat 0.5 ppm
Milk products 0.3 ppm (fat basis)
Raw wheat 0.05 ppm
Milled products
from wheat 0.01 ppm
Milk (whole) 0.01 ppm
Germany Meat fat 0.5 ppm
Milk and milk
products 0.5 ppm (on fat basis)
Netherlands Meat fat 0.5 ppm
Milk and milk
products 0.3 ppm (on fat basis)
USA Milk fat 0.5 ppm
Appraisal
Since HCB was considered in 1969 information has become available
to fulfil the requirements for residue information set out in the
monograph.
In addition, considerable data have been published indicating
that HCB is a widespread contaminant of many food commodities, most
particularly those of animal origin. Originally it appeared that the
only source of contamination was the HCB seed-dressing fungicide used
to disinfect wheat seed from bunt (Tilletia sp.). Steps have been
taken to prohibit the use of HCB fungicides in several countries. It
is now obvious that HCB occurs as an impurity in several other
pesticides, particularly quintozene and technazine and possibly
pentachlorophenol. More important still is the widespread occurrence
of HCB as a by-product of industrial processes and the incineration of
industrial wastes. The detection of HCB residues in many commodities
used in animal feeds give rise to concern.
Improved analytical techniques have become available recently and
it is anticipated that their use will reveal that HCB occurs more
frequently and at higher levels than is currently believed to be the
case.
The practical residue limits recommended by the 1969 Joint
Meeting have been reviewed in the light of information that has since
come to hand and the following comments are offered on the individual
proposals:
(1) Raw cereals (wheat) - 0.05 ppm
In view of the evidence that cereals other than wheat are
contaminated by HCB it would be wise to delete reference to (wheat)
and to make the limit applicable to all cereals. The level 0.05 ppm
still appears necessary to accommodate residues that appear from
unknown sources. Such a limit will, however, discourage mis-use of
treated seed. In due course it is hoped that the limit could be
reduced to 0.01 ppm but much more data are required before such an
amendment could be made.
(2) Cereal products from wheat - 0.01 ppm
It is considered that this definition is not sufficiently
specific and that it should be changed to "flour". Milled products
with a high bran content must inevitably contain higher amounts since
the contamination is generally on the outside of the individual
grains.
(3) Fat of cattle, sheep, goats, pigs and poultry - 1 ppm
It would be desirable to aim at reducing this limit to 0.5 ppm
within five years. However, it is known that breeding animals with
residues pass a large proportion of the HCB residue to their offspring
which retain the burden for many years. Pigs especially are prone to
accumulate high levels. There is no known way to reduce the residue
level once it has been built up in an animal as a result of any form
of accidental exposure. To reduce the limit to 0.5 ppm at this stage
would necessitate the destruction of highly valuable food and there is
insufficient evidence that any or many countries could comply with
such a level without severe losses.
(4) Milk products (fat basis) - 0.3 ppm
This limit can apparently be met only when the cows have adequate
access to residue-free pasture or forage and that there is strict
control over residues in feeding stuffs. The slightest contamination
of dairy cows will result in residues in excess of 0.3 ppm. It is
considered that 0.5 ppm would be a more realistic level until there is
adequate data obtained by reliable analytical procedures.
(5) Eggs (shell-free) - 1 ppm
There is insufficient evidence from more than a few countries on
which to decide whether such a limit could be reduced. More data are
required from many countries.
RECOMMENDATIONS
In the light of information considered it is recommended that
action be taken to reduce the general environmental contamination by
HCB especially from industrial sources not previously recognized as
important in the emission of this intractable contaminant.
Until such time as it is possible to reduce the contamination
significantly, practical residue limits are required as a basis for
uniform and reasonable administrative action. The recommendations made
in 1969 have been reviewed in the light of new information and the
limits have been amended as follows:
Practical residue limits
Fat of meat of cattle, sheep, pigs 1 ppm
and poultry
Eggs (shell-free) 1 ppm
Milk and milk products (fat basis) 0.5 ppm
Raw grain 0.05 ppm
Flour and similar milled cereal products 0.01 ppm
At this stage it is not possible to propose limits for HCB
residues in animal feedstuffs or compound feeds but such limits must
of necessity be low if foods of animal origin are to conform to the
above limits.
FURTHER WORK OR INFORMATION
Required (before an acceptable daily intake can be estimated).
1. Long-term studies in suitable mammalian species to provide
histologic data and biochemical data, particularly with respect to the
known porphyrogenic action, and an assessment of tumorigenic
potential.
2. Reproduction studies.
3. Studies of teratogenic potential.
4. Studies of pathways of metabolism and pharmacokinetics of HCB in
rats and preferably in other species, including studies on tissue
levels producing toxic effects.
5. Information on HCB occurring in foods moving in commerce, using
analytical procedures known to recover and determine the total of any
such residues that may be present.
6. Information on all possible sources of environmental contamination
by HCB
7. Information from many countries on residue levels in animal
foodstuffs and compound feeds.
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See Also:
Toxicological Abbreviations