Organophosphates

What are organophosphates?

Organophosphates (OPs) are a group of synthetic chemical compounds, composed of variable mixtures of phosphorus, carbon, and hydrogen.

Originally, organophosphates were developed as insecticides, and are used as such in thousands of licensed pesticides today. They act as cholinesterase inhibitors (chemicals that disrupt neuromuscular transmission), and as such were also developed as neurotoxins during the Second World War. Sarin - the poison gas released by Aum Shinrikyo cultists in 1995 on to the Tokyo underground - is the most well-known organophosphate-based chemical weapon.

Despite their positive effects on agricultural productivity, many believe organophosphates have negative effects on the environment and human and animal health. However, there is increasing interest in organophosphates' cholinesterase inhibiting properties in therapeutic environments.

The Department for Environment, Food and Rural Affairs (Defra) is principally responsible for regulating the agricultural use of organophosphates in the UK. Other bodies with an interest in regulating their use and research include the Health and Safety Executive, the Department of Health, the Veterinary Products Committee, the Advisory Committee on Pesticides, the Committee on Safety of Medicines and the Committee on Toxicity.

Background

The first organophosphate, physostigmine,was synthesised in 1850 as a treatment for glaucoma. Their use as a pesticide began in the 1930s and took off in the 1960s, when they were promoted as a safer alternative to organochlorines. As well as being used in agricultural products, they are used in head lice treatments, pet shampoos and other household products.

Organophosphates' little-understood non-specific toxicity lies at the root of their success as both pesticides and chemical weapons agents. In 2003, researchers at the Salk Institute, California, claimed to have explained this toxicity, arguing that organophosphates target a key enzyme in the brain, neuropathy target esterase. The research found that mice genetically engineered to have low levels of the enzyme were particularly susceptible to organophosphate poisoning. Previous research, indeed, had already pointed to susceptibility being genetic, with some more predisposed than others.

In the UK, the dangers of organophosphate poisoning have mainly been raised in relation to two issues: their use in sheep dips and Gulf War syndrome.

The 1951 Zuckerman report recommended that agricultural organophosphate pesticides should be labelled as 'deadly poison', but it was not until 1976 that containers were required to be labelled as potentially hazardous. Even then, no recommendations about protective clothing or other precautions were provided. While an HSE guidance sheet, known as MS17, was produced in the early 1980s, it was never circulated to farmers, doctors, vets, or indeed the Ministry of Defence.

In the 1980s and 1990s, hundreds of farm workers began to report symptoms including fatigue, memory loss, weakness, joint and muscle pain and depression, which they put down to low-level exposure to organophosphates over long periods of time. The government's position was to deny that there was a clear link, but in 2000 and 2001, it funded more research into the effects of organophosphate exposure and poisoning. The results of some of these studies provided support for the poisoning hypothesis, but the outbreak of a Foot and Mouth Disease epidemic in 2001 led to the postponement of the completion of Government-funded studies until 2006/07.

Also in the early 1990s, large numbers of soldiers returning from the Gulf War were claiming to be suffering from similar symptoms, which was subsequently dubbed 'Gulf War Syndrome'. Organophosphates featured prominently among the many theories for the cause of Gulf War Syndrome, with claims that troops had been exposed to some sort of organophosphate pesticide.

In December 1996, defence minister Nicholas Soames confirmed that this was indeed the case. However, the Ministry of Defence continues to refute the existence of Gulf War Syndrome as a genuine and distinct condition.

Controversies

Scientific understanding of the health effects of organophosphates and safe exposure levels is limited. Those who are convinced of the dangers continue to campaign for a ban on organophosphate pesticides, putting them into conflict with the agro-chemical industry and long-established agricultural practice.

Over the years, campaigners have alleged that the expert committees advising governments on pesticide safety have been too close to and dominated by the chemicals industry.

The matter is of intense political significance. There have been a number of high-profile suits brought worldwide over agricultural organophosphate poisoning claims: in 1998, a former employee of Lancashire Agricultural College, Robert Shepherd, received £80,000 in an out-of-court settlement over ill-health claims. The alleged failings of successive governments to take note of warnings about the risks and the imposition of a requirement on sheep farmers to dip their flocks using organophosphates as a precaution against sheep scab from 1976 to 1992, threaten a flood of compensation claims were a definite link to be established.

Moreover, because the MoD does not recognise Gulf War Syndrome as a debilitating condition, veterans who have left the armed forces due to ill-health are not eligible to take up disability pensions. The establishment of a link between Gulf War Syndrome and organophosphate poisoning could cost the MoD and its counterparts in other Gulf War coalition member states vast sums in compensation.

If a government were to ban organophosphate products, it would have to pay potentially huge sums in compensation to the chemicals industry for withdrawing licences (Marketing Authorisations) that have already been issued. At present, the government's position remains that organophosphate pesticides are safe, if used in line with the manufacturers' instructions. A certification and registration scheme exists for ensuring that those using organophosphate products understand how to do this.

Statistics

Many previously approved products are being withdrawn from use. Simazine and atrazine were withdrawn during 2007, diuron will be phased out around the end of 2008; and isoproturon will not be available on the UK market after 2009.

Source: Pesticide Forum Indicators Report

A recent report by Cranfield University showed that without pesticides, household expenditure on food and drink could rise by as much as 40 per cent or £70 billion nationally.

Source: NFU - December 2010

Quotes

"Recent research has shown that sheep farmers exposed to organophosphates may suffer from a range of impairments to cognitive functions, for example, response speed, verbal and visual memory, and fine motor control."

Pesticide Action Network UK - 2010