Methyl parathion

Methyl parathion is an organophosphate (OP) insecticide that has caused many health problems - particularly in developing countries - since its introduction onto the market in the early 1950s. This fact sheet provides information on the hazards associated with methyl parathion, focusing also on recent restrictions on its application in the US, implemented because of misuse of the product.

What is methyl parathion?
Methyl parathion was originally developed by the German pesticide company Bayer. It is a non-systemic pesticide that kills pests by acting as a stomach poison.
It is used to control chewing and sucking insects in a wide range of crops, including cereals, fruit, vines, vegetables, ornamentals, cotton and field crops(1). Methyl parathion is generally applied as a spray, mainly as an emulsifiable concentrate formulation. The recommended application rates are 15-25g of active ingredient per 100 litres(2).

The basic manufacturers of methyl parathion are All India Medical Co (India), Bayer India, Bayer Mexico, Cheminova (Denmark), Rallis India and Sundat (Singapore)(3). In 1993, other production facilities existed in Brazil, the former East Germany, China and the former USSR. Although not used in the UK, methyl parathion is widely used throughout the world, and is registered in at least 38 countries(4).
Information on global sales and production data are not widely available. For the financial year 1995-96, India produced an estimated 2,200 tonnes of technical grade methyl parathion(5).
Cheminova, a major producer, sells US$ 15 million per year in the US, one of its key markets for this product(6). Overall the company recorded a 10% rise in sales in 1996. Its forecasts for future growth in methyl parathion have however been affected by an agreement to withdraw certain formulations in the US (see below)(7).

Acute toxicity
The World Health Organisation classifies methyl parathion as a class Ia 'extremely hazardous' pesticide(8). It is highly toxic by inhalation and ingestion, and moderately toxic by dermal adsorption (it is also readily adsorbed through the skin). The oral LD50 in rats is 2.9 mg/kg, in mice is 33.1-119.5 mg/kg, in rabbits is 19-420 mg/kg and dogs is 50 mg/kg(9). The dermal rat LD50 is 44-67 mg/kg.
Like other organophosphate insecticides, methyl parathion is a cholinesterase inhibitor (see the Organophosphates fact sheet PN34 pp20-21). When inhaled, the first adverse effects are a bloody or runny nose, coughing, chest discomfort and difficulty breathing. Skin contact may cause localised sweating and involuntary muscle contractions. Following exposure by any route, other systemic effects may begin within a few minutes, or be delayed for up to 12 hours. These may include pallor, nausea, vomiting, diarrhoea, abdominal cramps, headache, dizziness, eye pain, blurred vision, constriction or dilation of the pupils, tears, salivation, sweating and confusion. In severe cases, poisoning will affect the central nervous system, producing in-coordination, slurred speech, loss of reflexes, weakness, fatigue, and eventual paralysis of the body extremities and respiratory muscles. Death may be caused by respiratory failure or cardiac arrest(10).

Chronic effects
Effects reported in workers repeatedly exposed to methyl parathion include impaired memory and concentration, disorientation, severe depressions, irritability, confusion, headache, speech difficulties, delayed reaction times, nightmares, sleepwalking, drowsiness and insomnia(11).
There are no epidemiological studies on effects related only to methyl parathion exposure(12).

The International Agency for Research on Cancer evaluated methyl parathion in 1983, and concluded that the available data do not provide evidence that methyl parathion is carcinogenic to experimental animals. No data on humans were available(13).

Mutagenicity tests have been both positive and negative. The results of most of the in vitro studies with both bacterial and mammalian cells were positive(14).

The WHO recommended that more definitive studies should be conducted on residues of methyl parathion in fresh foods(15). Residues are regularly detected in a range of fruit and vegetables. In the UK during 1995, researchers found residues in imported celery, dessert grapes and oranges, all below maximum residue limits (16).

Fate in the environment
Methyl parathion has a half-life in aqueous solution of 175 days(17), and 10 days to two months in soils(18). The rate of degradation increases with temperature and with exposure to sunlight. When large concentrations of methyl parathion reach the soil, as in an accidental spill, degradation will occur only after many years(19).
The US Environmental Protection Agency (EPA) may have detected 4-nitrophenol, a methyl parathion breakdown product, at very low levels in drinking water wells. The EPA is uncertain and cannot quantify the amount or frequency of 4-nitrophenol in drinking water because the analytical technique is not reliable(20).
Methyl parathion is unlikely to bioaccumulate.

Methyl parathion is highly toxic for aquatic invertebrates with most LC50s ranging from <1 µg/litre to about 40µg/l(21). In 1992, a massive bird kill occurred in Costa Rica after it was applied by plane in a cotton field(22). Methyl parathion has been implicated in the deaths of waterfowl in Spain and the acute poisoning of fish, birds, cattle and wild animals in the Sudan(23).

Hazards in developing countries
Conditions in developing countries make it extremely difficult to associate a particular active ingredient with a poisoning incident.
In the early 1950s the manufacturers introduced a powder formulation of methyl parathion which caused problems because of the poor conditions of use in developing countries. Methyl parathion became the mainstay of pest control in cotton, and very quickly there were hundreds of poisonings from this single product, and reportedly dozens of fatalities(24).
In Oarana State, Brazil, pesticide incidents compiled by the Toxicological Information Centre and Health Clinics noted 1,243 incidents involving methyl parathion between 1982-1991(25).
There is evidence that methyl parathion is not used safely in Central America. Research carried out in 1996 shows that methyl parathion caused a number of documented poisonings among agricultural labourers involved in Nicaraguan cotton production. In some cases they have ended up in hospital with classic OP poisoning(26).
Cheminova, the Danish manufacturers of methyl parathion, says it only sells to developing countries if they carry out 'safe farming'. However, researchers on the ground in countries like Guatemala and Nicaragua say methyl parathion is rarely used safely.

  • The WHO has set out a number of safety remarks for methyl parathion use:
  • methyl parathion may only be used by trained personnel;
  • A field sprayed with methyl parathion may not be entered for 48 hours after application;
  • methyl parathion may not be sprayed by hand;
  • people may not be used as markers when spraying from the air(27).

There is evidence that these recommendations are broken in developing countries(28).

Problems in the US
Recently, there have been a number of important US prosecutions involving methyl parathion. Over 1,500 homes and businesses in Mississippi and Ohio were sprayed with methyl parathion by unlicensed operators. Methyl parathion is not permitted for use indoors in the US. The authorities had to relocate over 1,100 people in temporary accommodation, and clean up costs could reach US $50 million. In addition, local vets reported deaths of household pets due to methyl parathion exposure(29).
These events led the US EPA to cancel the registrations of emulsifiable concentrate formulations. These came into effect on 30 April 1997, following a voluntary agreement with the US registrants, led by Cheminova. Cheminova is to carry out a public education programme on the proper use of the insecticide(300.

Mississippi case
On 13 March 1997, Dock Eatman, Sr of Moss Point Mississippi was convicted by a jury of illegal spraying of the insecticide methyl parathion in homes and other buildings in the Pascagoula (Miss.) area in 1995 and 1996. Eatman did not have a licence for commercial pesticide application. This insecticide is only approved for outdoor agricultural use. Eatman faces a maximum of 21 years in prison and/or up to US $2.1 million in fines. This case is being investigated by the EPA's Criminal Investigation Division, the FBI and authorities from the state of Mississippi(31).

Ohio case
Lutellis Kilgore of Elyria Ohio, was also charged on 21 March 1997 with illegal use of methyl parathion. He allegedly applied the insecticide in a manner inconsistent with its label to more than 60 properties without an application certificate. The spraying led to a US $20 million publicly-funded clean up of the affected properties. Kilgore faces a maximum of one year in prison and/or a fine of up to US $100,000 for the illegal application and five years in prison and/or a fine of up to US $250,000 for making false statements to federal investigators(32).

Methyl parathion is banned in Indonesia, Sri Lanka and Tanzania, and is severely restricted in Colombia, Korea, China and Japan. It is one of five pesticides identified for inclusion in the Prior Informed Consent Procedures of the Food and Agriculture Organisation on the grounds of causing problems under conditions of use in developing countries.

As a hazardous OP pesticide, methyl parathion is regularly misused in developing countries. The measures taken recently in the US should help to reduce potential problems, but they merely highlight the difficulties of using such a product in conditions like Central America, where protective clothing and training are often lacking or ineffective. As a result, methyl parathion should be more severely restricted in developing countries.

1. Tomlin, C (Ed.), The Pesticide Manual, tenth edition, BCPC/ Royal Society of Chemistry, 1994, p771.
2. Methyl Parathion Health and Safety Guide No. 75, World Health Organisation, Geneva, 1992.
3. Methyl Parathion, Prior Informed Decision Guidance Document, FAO/UNEP Joint Database, IRPTC, Geneva, Update, 17 September 1996.
4. Ibid.
5. Agrow No. 265, 27 September 1996, p19.
6. Methyl Parathion: EPA's challenge, C&EN, 17 January 1997.
7. Agrow No. 280, 16 May 1997, p4.
8. The WHO Recommended Classification of Pesticides by Hazard, International Programme on Chemical Safety, Geneva, 1996.
9. Op. cit. 3.
10. EXTOXNET Pesticide Management Program, Cornell University, US, 1994.
11. Ibid.
12. Op. cit. 3.
13. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Lyon, France, 1983, pp144-145.
14. Op. cit 10.
15. Methyl parathion, Environmental Health Criteria No. 145, WHO, Geneva, 1993, p18.
16. Annual Report of the Working Party on Pesticide Residues: 1995, Ministry of Agriculture Fisheries and Food/Health and Safety Executive, 1996.
17. Op. cit. 15. p22.
18. Op. cit. 10.
19. Howard, P.H., (Ed.), Handbook of Environmental Fate and Exposure Data for Organic Chemicals, Vol. III: Pesticides, Lewis, 1989.
20. Op. cit. 10.
21. Op. cit 3.
22. Dinham, B., The Pesticides Hazard, The Pesticides Trust [now PAN UK], London, 1993, p105.
23. Op. cit. 3.
24. Made in Denmark (Danish video on methyl parathion), TV94, Nųrrebrogade 66 C, Copenhagen, Denmark, 1997.
25. Op. cit. 22.
26. Op. cit. 24.
27. Op. cit. 2.
28. Op. cit. 24.
29. Parathion disaster in Mississippi, PANUPS, Pesticide Action Network North America, 21 February 1997.
30. Agrow, No. 280, 16 May 1997, p16.
31. US EPA, press release, 27 March 1997.
32. US EPA press release, 4 April 1997.

[This article first appeared in Pesticides News No.36, June 1995, p20-21]