Low-level exposure to organophosphates (OPs) in pesticides can cause lasting harm to the brain, scientists have now confirmed.
A review of 14 separate studies has shown the chemicals, which are frequently used by sheep farmers, can reduce memory and the ability to process information quickly.
The findings, by researchers at the University of Central London and Open University, are the most comprehensive evidence yet that organophosphates can harm human health at low levels.
High doses of the chemicals, which are also used to kill or repel insects in grain stores, have long been recognised by doctors as toxic.
Dr Sarah McKenzie Ross, a clinical psychologist and honorary senior lecturer at University College London, called for tighter safety rules for people exposed to the chemicals during their jobs.
She said: "The studies we looked at were in people who were exposed occupationally on a regular basis but were not getting ill from that exposure.
"The weight of evidence is that low-level exposure is harmful.
"It targets memory, information processing speed, and the ability to plan and have abstract thoughts."
Farmers are among those who are regularly exposed to organophosphates. One farmer in South Devon, who asked not to be named, suffered a number of symptoms for several years after using an organophosphate product on his grain.
"Following my problems with exposure with full safety gear, I removed all OPs from this farm.
"I discussed my problems with a senior toxicologist at the Department of Health, regarding heart disturbances.
"I feel it is my duty to point out to anyone using these chemicals the risk to their health.
"I am concerned that higher standards of treatment in grain stores will persuade farmers to use increasing amounts leading to low-level exposure and chronic ill health."
Elizabeth Sigmund, co-ordinator of the OP Information Network, said it was "a very important study" which should change Government policy.
She said: "For some time we have been gathering information from the large numbers of people damaged by organo-phosphates.
"Our database of more than 800 people is just the tip of the iceberg, and only represents those well enough to get in touch.
"People should not expose themselves to this dangerous chemical on any level. It can have permanent, life-changing consequences."