Dichlorvos is an organophosphate (OP) insecticide widely used in developing countries. Because of its high acute toxicity and the consequent dangers to workers, there are concerns whether safe use is possible under such conditions.
Dichlorvos is an insecticide of the organophosphate (OP) group. It has been in use since about 1955 and is used in the UK both professionally and in homes and gardens in a number of areas:
It also has wide uses in some developing countries as an
insecticide on vegetables, deciduous fruits, rice and plantation crops such as
cotton, coffee, tea, cacao, banana, tobacco and spices.
Dichlorvos is an insecticide with contact, respiratory and stomach action. Like many OP insecticides it also inhibits the enzyme cholinesterase, which results in disruption to the nervous and muscular system.
In the UK, the non-agricultural uses of dichlorvos as an amateur and professional insecticide were reviewed by Health and Safety Executive in February (HSE) 1995(1). Agricultural uses together with environmental effects will be included in a later review by the Ministry of Agriculture.
On a world-wide basis no country has banned dichlorvos, although there are restrictions in Indonesia, South Korea and Vietnam. Two recent international reviews of dichlorvos have been carried out: World Health Organisation (WHO) in 1989(2); the Joint Meeting on Pesticides Residues (part of the UN Codex process) in 1994.(3)
Dichlorvos has a high acute toxicity: the oral LD50 in rats is between 56 and 108mg/kg. It is classified by the WHO as a Class IB, 'highly hazardous'(4). The dermal toxicity is similar to oral toxicity, and dermal exposure is a cause for concern.
Most human poisonings have resulted from the splashing of concentrated formulations onto the skin. Failure to remove the splash has proved fatal. Prompt removal has resulted in symptoms of intoxication but full recovery after treatment. Dichlorvos vaporises quickly. Cholinesterase inhibition has been reported from exposure by inhalation after the use of dichlorvos in non-ventilated or poorly ventilated areas(5).
Reports from the UK National Poisons Unit (NPU) show that between 1983 and 1990, 98 individual cases of poisoning involving dichlorvos were reported.
Dichlorvos is classified as 'toxic if swallowed', 'very toxic by inhalation', and 'toxic in contact with skin', by the NPU.
The HSE review found: "No fertility studies which are both adequately conducted and reported... Individually no study is considered adequate to assess the teratogenic potential of dichlorvos in any species". In spite of these data gaps, government ministers ruled that: "Overall the weight of evidence indicates that dichlorvos does not present a risk to fertility and reproduction in humans."(6)
There is evidence that dichlorvos can induce delayed neuropathy in hens at very high doses; and neurophysiological and behavioural changes in rats. But the HSE decided: "The significance of these findings for the risk assessment of dichlorvos is uncertain." (7)
The International Agency for Research on Cancer(8) places dichlorvos in Group 2B (possibly carcinogenic to humans) based on what it considers to be sufficient evidence in animals, but inadequate evidence in humans. The US Environmental Protection Agency classifies it in category 2B (possibly carcinogenic to humans) but the result of further testing is awaited and it may be reclassified. The UK ACP, following its review in respect of non-agricultural uses, takes the view that "Overall, the weight of evidence presented does not suggest that dichlorvos is a carcinogenic risk for humans."
There is evidence that dichlorvos is mutagenic in bacteria, fungi, and mammalian cells in vitro, but that there is no evidence for mutagenicity in whole animals, when it is rapidly degraded.
Dichlorvos is toxic to fish and aquatic arthropods are more sensitive than fish. It is highly toxic to birds and to honey bees.
Environmental Quality Standards have been proposed for the protection of UK freshwater and marine aquatic life. In setting the standards for dichlorvos, the Department of the Environment noted that "insufficient environmental data are available to verify the proposed standards for the protection of aquatic life."(9) However, by applying an arbitrary safety factor of 100 to the toxic dose for the most sensitive species, the Water Research Centre has recommended an annual average level of 0.001 µg/l (parts per billion) for freshwater species, and 0.04 µg/l for marine life in saline waters.
As a result of the Second North Sea Conference in 1987, a number of countries agreed to reduce discharges of certain chemicals to water. Dichlorvos is included in this UK Red List substances whose discharge is dangerous to water -and the UK government has agreed to reduce the discharge of such chemicals by 50% of their 1985 level by 1995. It is unlikely that this target will be met, as the main source of dichlorvos in water is from salmon farming which is an industry that has expanded greatly in recent years.
Because dichlorvos degrades fairly rapidly it is not generally found as a residue on food. The UK Working Party on Pesticide Residues monitors residues in food, and dichlorvos is rarely found if at all.
Dichlorvos is licensed as a veterinary medicine by the Veterinary Medicines Directorate for use against sea lice (Lepeophteirus salmonis and Caligus elongatus) that afflict salmon. The UK salmon farming industry is mainly based in the western coast and Islands of Scotland there are about 130 salmon farming companies on 280 sites. Production has increased from 600 tonnes in 1980 to 48,000 tonnes in 1993(10). There have been concerns about the discharge of dichlorvos as a result, and the possible effects of dichlorvos on wild salmon. There was an increase in the incidence of cataracts and blindness in wild salmon in the 1980s and this has been linked with exposure to dichlorvos(11).
Alternatives to dichlorvos are now being introduced including the use of sea wrasse as 'cleaner fish', and the use of hydrogen peroxide as an alternative disinfectant.
Developing country use
Because of its high acute oral and dermal toxicity, its availability in developing countries is a cause for concern. The Food and Agriculture Organisation (FAO), the World Bank, GTZ (Germany) and ODA (UK) generally discourage the procurement of such products. The accepted international guide to best practice in the procurement of pesticides is set out in the FAO's Provisional Guidelines on Tender Procedures for the Procurement of Pesticides(12) which state:
"Pesticide formulations that fall into Class IA or IB ... usually have severe restrictions in developed countries; in general they can only be used by specially trained and certified applicators. Such pesticides should not be used by small farmers or untrained and unprotected workers in developing countries".
Nevertheless research by the Pesticides Trust [now PAN UK] shows that dichlorvos is widely used in a number of countries where the conditions of use have raised concerns. Dichlorvos has caused poisonings in China, Costa Rica, Paraguay, India, Papua New Guinea and Egypt. It is also widely produced - there are facilities in India, Brazil and Mexico(13). For these reasons PAN groups have strongly urged that dichlorvos be included in the Prior Informed Consent (PIC) process from the outset(14). One manufacturer, Ciba Geigy, has agreed to withdraw dichlorvos from sale in the Colombian flower industry (see p14).
It is accepted that dichlorvos is dangerous to a number of aquatic species and that the discharge of dichlorvos to water should be reduced. Dichlorvos can inhibit cholinesterase levels in humans which may lead to short or longer term neurotoxic effects. Although it has been used for some 40 years, considerable uncertainties remain about whether or not it is implicated in cancer, and the wider environmental consequences of its use. In general and specifically in developing countries and in UK fish farming less hazardous alternatives are available.
[This article first
appeared in Pesticides News No.29, September 1995, p20-21]