Any chemical company will argue that if a product is used safely and at the recommended doses, then there will be no harmful side-effects. But that ignores a new discovery made by scientists in the United States which Dr Jamal suggests could change thinking about the toxic effects of OP chemicals.
At Duke University in North Carolina, Mohammed Abou-Donia, professor of neuro-biology and neuro-toxicology, and his team looked at the effects of a combination of three chemicals on laboratory hens. One of these chemicals – the OP chlorpyrifos – is central to the case involving Dow. The scientists established levels at which the chemicals had no effect on the nervous system of hens when used on their own.
Toxicity levels increase
They also established the lethal dose for chlorpyrifos. Their next finding was extraordinary: when they combined three compounds which had proved to be safe used at the correct levels on their own, the toxicity shot up. ‘When they combined three compounds at safe level for chlorpyrifos and the others, they produced an effect on the laboratory animals which was equivalent to the lethal dose of chlorpyrifos,’ said Dr Jamal, who is at the West London Regional Neuro-Science Centre, part of Imperial College’s School of Medicine. ‘In other words, this combination effect multiplied the toxicity of the available chlorpyrifos by hundreds of times.’
In their laboratory, Professor Abou-Donia’s team had demonstrated an effect for people which groups such as Friends of the Earth have always said happens in the environment: combinations of toxic pesticides are far more damaging than any compound acting on its own. Apart from chlorpyrifos, Professor Abou-Donia’s team used permethrin, a synthetic pyrethroid (SP), and DEET, an insect repellant which Dr Jamal said was not toxic by itself. All three compounds were used by UK and US troops in the Gulf War.
Every-day exposure
‘In real life, arable and livestock farmers in the UK use this sort of combination of products – OPs, SPs and organochlorines – during the year,’ said Dr Jamal. ‘We found this when we visited farms during the Institute of Occupational Medicine study on OPs. Unfortunately, no-one is telling farmers about this combination effect because it has never been tested as part of establishing the chemicals’ safety and there are no hazard warning labels on the chemical containers about this possible effect.’ OPs also contained solvent combinations which are toxic by themselves but their combination effect with the active ingredients is unknown.
When chemicals combine in this way, said Dr Jamal, they have three effects on people: some chemicals bind to enzymes which de-toxify the body and make them unavailable to do their work on other chemicals expressed simultaneously. ‘This is rather like releasing 200 criminals in London and taking away the police officers who are usually on duty,’ said Dr Jamal. ‘There is bound to be some damage.’
Israeli research findings
In 1998, a group of Israeli scientists demonstrated the second effect across what is called the blood-brain barrier, the brain’s protective shield. They showed that subjecting laboratory animals to the kind of stress endured from a combination of chemicals undermined the protective role of the blood brain barrier and allowed toxic substances freer access to the central nervous system. ‘The gates of the blood brain barrier are almost fully opened and toxic substances in the blood have been shown to cross the protective shield more than one hundred fold higher, thereby gaining direct access to the nervous tissue,’ said Dr Jamal. Thirdly, body tissue exposed to the combination becomes more and more sensitive.
In 1997, Dr Jamal published a paper in which he tried to distinguish the long term effects of OPs from the acute and delayed effects already accepted by company and Government
scientists(1). He described a neurotoxic effect called organophosphate induced delayed neuropathy (OPIDN) which usually happens two or three weeks after exposure. As an example, he used a famous human epidemic of OPIDN in the United States in 1930 when thousands of people drank a Jamaican headache remedy called Ginger Jake which was contaminated with an OP called TOCP-triorthocresyl phosphate.
OP incidents in aircraft
TOCP is not a veterinary product or a medicine, but has an industrial use today as a lubricant, and has been implicated in poisoning incidents on aircraft when it apparently leaked from hydraulic pipes. ‘OPIDN has been well described from the exposure right through to the effects on the peripheral nerves, spinal cord and brain stem,’ said Dr Jamal. ‘Chlorpyrifos – sold as Dursban – is one of the OPs capable of producing this effect. The manufacturer says it is not capable of producing OPIDN at usual doses. The trouble with that is that those doses are measured in the laboratory on experimental animals: that takes no account of real scenarios in which Dursban is used and applied in combination with other chemicals.’
To try to clarify the discussion more, Dr Jamal outlined another phenomenon called COPIND – or chronic OP-induced neuropsychiatric disorder. COPIND has a different pathology from OPIDN, and all OPs produce the disorder by their impact on the cortical nerve cells in the higher brain. ‘My contention is that because we are talking about issues such as abstract thought and memory, COPIND cannot be tested on laboratory animals. So far, the chemical industry has ignored COPIND completely despite the presence of many studies in the literature.’
Dr Jamal is reluctant to discuss the court case, but points out that there are several scientific accounts of chlorpyrifos causing long-term damage to the central and peripheral nervous system. A few years ago, Dow was fined $750,000 for concealing cases of chlorpyrifos causing side effects in people.
Uses for chlorpyrifos
Chlorpyrifos is sold in the UK and United States, where it is used on 112 food and feed crops, as well as in homes, schools, daycare centres, restaurants, hospitals, food-manufacturing plants, vehicles, livestock ear tags and pet collars. The US environmental protection agency’s health statistician estimated that 7,000 chlorpyrifos poisonings were reported during 1996, the most recent year for which data is available.
In December, the Northwest Coalition for Alternatives to Pesticides asked groups around the world to influence the EPA’s preliminary risk assessment of chlorpyrifos. The EPA has asked for comments on the risk assessment, which is required by the 1996 Food Quality Protection Act.
References
1. Neurological Syndromes of Organophosphorus Compounds, Goran A. Jamal, Adverse Drug React. Toxicol. Rev. 1997, 16(3) pages 133–170, Oxford University Press.
[This article first appeared in Pesticides News No. 47, March 2000, page 10]