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Carbaryl re-assessed
Carbaryl is widely used as an insecticide with acute toxic effects which
are well known. Recent data on its carcinogenic potential has indicated the need
for further research, and has prompted a revision of safety controls governing
its use.
Description
Carbaryl, a carbamate insecticide, is a
cholinesterase inhibitor, and can also act as a plant growth regulator. It is
used to kill a range of chewing and sucking insects on over 120 agricultural
crops. In UK agriculture, it is mostly used against caterpillar pests on apples.
Internationally it is used on citrus fruit, mangoes, bananas, strawberries,
nuts, vines, olives, okra, cucurbits, peanuts, soya beans, cotton, rice,
tobacco, cereals, beet, maize, sorghum, alfalfa, potatoes, ornamentals and
forestry. Carbaryl has also been used against earthworms in turf and amenity
grass. Carbaryl is used against ectoparasites of humans and animals, including
against head lice on children(1,2).
Carbaryl was introduced by Union Carbide (whose pesticide
interests were taken over by now Rhône-Poulenc after the Bhopal gas disaster)
in the early 1960s. Principal producers are: Rhône-Poulenc; Drexel Chemical
Company; Jin Hung; and Makhteshim-Agan. Carbaryl is processed by more than 290
formulators in over 1,500 different products(3,4).
In 1992 in the UK, it was the seventh
most used active ingredient by weight on top fruit and was used on over 15,000
treated hectares on apples, pears and plums(5).
Health effects
Acute toxicity
Carbaryl is classified by the World Health
Organisation (WHO) as 'moderately hazardous' (Class II)(6). The acute toxicity
varies considerably according to species and formulation. Estimates for the oral
LD50 of the rat range from 200 to 850 mg/kg. Cats are sensitive to carbaryl with
an LD50 of 150 mg/kg, whilst pigs and monkeys are less susceptible having an
LD50 greater than 1,000 mg/kg(7).
Carbaryl can produce adverse effects in
humans by skin contact, inhalation or ingestion. Its main mode of action
involves the inhibition of the nerve enzyme cholinesterase and consequential
disruption of the nervous system. The symptoms of acute toxicity are similar to
other carbamates. Direct contact with the skin or eyes with moderate levels can
cause burns. Inhalation or ingestion at high doses can be toxic to the nervous
and respiratory systems resulting in nausea, stomach cramps, diarrhoea and
excessive salivation, sweating, blurring of vision, lack of co-ordination and
convulsions(8).
The US Environmental Protection Agency
carried out a review of carbaryl-related poisoning between 1966 and 1980. During
this period, 193 cases involving solely carbaryl and 144 cases which included
carbaryl as one of the active ingredients, were assessed(9).
Workers have the greatest potential for
exposure through inhalation or dermal absorption. The highest risk of exposure
for the general public is through residues in food(10,11).
Manufacturing
On 3 December 1984 a gas leak containing methyl
isocyanate (MIC) escaped from a Union Carbide factory in Bhopal, India. The MIC,
an intermediate product used to make carbaryl, killed between 2,500 and 5,000
and injured about 200,000 (see PN26). A similar but smaller incident happened a
year later at Union Carbide's plant in West Virginia which also produced
carbaryl and aldicarb. In this case 135 people were injured(12).
Carcinogenicity
In 1987 carbaryl assessment by the International
Agency for Research on Cancer (IARC) concluded that there were no data on cancer
in humans and that the evidence of carcinogenicity in experimental animals was
inadequate. This was reinforced in 1994 by a WHO Task Group report which
concluded that most of the numerous cancer studies involving rats and mice were
old and did not meet contemporary standards. The Group was aware that a
pesticide company was carrying out new studies. Although they had not seen all
the results, the Group was informed that these studies indicated significant
increases in tumours at the highest dose in both rat and mice species. One of
the main recommendations from the WHO report included a request that
carcinogenicity studies meeting modern standards should be conducted(13).
In the UK, in November 1995, new
company data indicated carbaryl could cause cancer in humans, although the
research is not yet in the public domain. The studies were assessed by the
Committee on Carcinogenicity and the Advisory Committee on Pesticides, which, in
reporting to the government, concluded that it would be"prudent to consider
carbaryl as a potential human carcinogen"(14).
The principal government response concerned the use of carbaryl against head
lice in children. As a result, medicinal uses will only be available on
prescription (see PN30 p.4).
Mutagenicity
Studies indicate that carbaryl is slightly
mutagenic(15,16). A WHO assessment concludes from available data that carbaryl
does not pose a threat of inducing genetic changes in humans(17). However,
carbaryl can react with nitrite under certain conditions to form
N-nitrosocarbaryl. This chemical is highly mutagenic at low levels in laboratory
test systems. This may be of concern because nitrite can be found in food
additives and human saliva which can react with carbaryl in the stomach to form
N-nitrosocarbaryl(18).
Reproductive toxicity
The US EPA has concluded that carbaryl does not pose
a teratogenic risk to humans(19). However, it is considered to have endocrine
disruptor effects(20).
Environmental effects
Carbaryl is lethal to many non-target species. The
destruction of honeybee populations in sprayed areas is sometimes a problem. The
insecticidal properties of carbaryl last for about 3-10 days. Degradation of
carbaryl in the soil is mostly due to sunlight and bacterial action. It is bound
by organic matter and can be transported by run-off. Carbaryl has a half-life of
7 days in aerobic soil and 28 days in anaerobic soil. In pond water, carbaryl
has a half life of 1 to 32 days and it has been detected in groundwater in three
separate sites in California(21).
Residues
Residues of carbaryl are regularly detected in tests
on fruit and vegetables in the UK. In the latest report covering the period
1994, residues were found in dessert and cooking apples, grapefruit, ready
prepared fruit-based baby food and lettuce. In none of these cases were the
maximum residues limits exceeded(22). Carbaryl is the tenth most commonly found
pesticide in a 1994 total diet survey carried out by the US Food and Drug
Administration(23).
Recommendations for use
Because of an announcement on 7 November by the UK
government concerning its carcinogenic affects, carbaryl will no longer be
approved for non-professional uses; will be available for medical uses only by
prescription; and professional use is subject to restrictions designed to limit
exposure. These are:
- applications must only be made using a vehicle with a
closed cab;
- a low level induction bowl or closed systems must be used
for transferring the product to the spray tank;
- coverall apron and gloves must be worn when handling the
concentrate, (previously a face shield had also to be worn);
- coverall must be worn during application;
- coverall must be worn when handling contaminated surfaces;
- the latest time of treatment of top fruit is three weeks
before harvest (previously it was seven days)(24).
Conclusion
At the end of 1995, the UK government's position on
carbaryl changed, carbaryl is now considered to be a potential human carcinogen.
Four main issues arise as a result:
- Given the health and safety and resistance concerns about
alternative pesticide active ingredients in head lice preparations
malathion, permethrin and, less frequently lindane - there should be a
review of all head lice preparations containing pesticides.
- There remains considerable risk to agricultural and
horticultural uses of carbaryl, as cited above. There should therefore be a
label warning to indicate carbaryl is, in the view of the Advisory Committee
on Pesticides, 'a potential human carcinogen'.
- Rodent carcinogenicity data, submitted by the manufacturer,
is not in the public domain. A government full evaluation document on
carbaryl should be made available promptly, particularly covering
agricultural and horticultural use.
- The stringent protective clothing requirements imposed must
call into question whether the safe use of carbaryl is possible in
developing countries.
- References
1. Tomlin, Clive (Ed), The Pesticide Manual, Tenth edition, British Crop
Protection Council (BCPC), Royal Society of Chemistry, UK, 1994, p149.
2. Whitehead, R. (Ed), The UK Pesticide Guide 1995, BCPC and CAB
International, p190.
3. Carbaryl, Environmental Health Criteria No. 153, International
Programme on Chemical Safety, UNEP, WHO, ILO, Geneva, 1993, p13.
4. Op. cit. 1.
5. Orchards and Fruit Stores in Great Britain 1992, Pesticide Usage
Survey Group 115, MAFF, 1994.
6. The WHO recommended classification of pesticides by hazard and
guidelines to classification 1994-1995.
7. Op. cit. 1.
8. Data Sheet on carbaryl, Extoxnet, Extension Toxicology Network, Oregon
State University, US.
9. Op. cit. 3 p9.
10. Baron, R., Carbamate Insecticides, In Handbook of Pesticide
Toxicology, Vol. 3, W.J. Hayyes and E.R. Lawes (Eds.), 1991, Academic Press,
US.
11. Op. cit. 3 p14.
12. Journal of Pesticide Reform, Summer 1993, Vol 13, No.1, p33.
13. Op. cit. 3. p22.
14. Carbaryl, Joint announcement of MAFF's Pesticide Safety Directorate
and Health and Safety Directorate, 7 November 1995.
15. Seibert, D., and Eisenbrand, G., Mutation Research, 1974, 80:1-64.
16. National Library of Medicine, Hazardous Substances Databank,
Carbaryl, February 4 1992.
17. Op. cit. 3 p17.
18. Op. cit. 8.
19. Ibid.
20. Chemically induced alterations in sexual and functional development:
The wildlife/human collection, eds. T. Colborn and C Clement, Princeton
Scientific Publishing, Princeton, NJ, 1992.
21. Op. cit. 8.
22. Annual report of the Working Party on Pesticide Residues 1994, HMSO,
London, UK, 154pp.
23. Pesticide Program, Residue Monitoring, 1994, Food and Drug
Administration, US, p15.
24. Op. cit. 14.
[This article first
appeared in Pesticides News No.31, March 1996, p20-21]
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