A leading American scientist says he believes pesticides may be a far more serious public health threat than anyone has realised.

Nicholas Ashford, Professor of Technology and Policy at Massachusetts Institute of Technology, says he thinks chemicals are the most serious environmental problem facing industrialised countries today.

Professor Ashford - who is also an advisor to the United Nations Environment Programme - is known for his work on the theory of multiple chemical sensitivity (MCS).

The theory suggests people can become sensitised by exposure to one form of contamination so that they are then liable to be affected by a whole range of other pollutants, including detergents, traffic fumes and tobacco smoke.

There is no known cure for MCS.

Evidence in the UK

It may sound a far-fetched theory. But the experience of many British sheep farmers gives it credence.

Hundreds of them have become ill after dipping their animals in organophosphate (OP) sheep dips. The symptoms range from the fairly mild sneezing and runny eyes of 'dipping 'flu' to muscle spasms, insomnia and overpowering fatigue. Some farmers say they have been driven to the brink of suicide.

OPs are highly toxic, derived from the same group of chemicals that the Nazis used in World War Two to manufacture nerve gases. Crucially, many farmers report that after exposure to OPs, they find that their symptoms can then be triggered off by exposure to a range of other chemicals.

And Professor Ashford thinks that OPs may be one of the most common initiators of MCS.

Patients become sensitised

OPs are not used exclusively in sheep dips. They are also used for fumigating public transport vehicles, in shampoos, and in flea collars. So they are widely available.

The other chemical which Professor Ashford believes may be a principal cause of MCS is a compound called permethrin, which is used as a woodworm treatment. It is sprayed in about 5,000 British homes every week.

He and his colleagues have found that some patients, once sensitised by exposure to a chemical, react to subsequent levels of exposure so low that the techniques available in most laboratories cannot detect them at all.

Doctors baffled

And the baffled doctors, he says, unable to find a clear cause for the problem, tend to assume that it is all in the mind.

Professor Ashford believes the huge rise in pesticide use over the last half century could explain many illnesses, ranging from skin rashes and breathing problems to cancers and birth defects.

Other research in the United States suggests that about one-third of the population -- sixty million people -- may be affected in some way.

Professor Ashford says: 'Pesticides are nerve poisons; they damage the brain and they are also known to be endocrine disruptors' (synthetic chemicals which interfere with naturally produced hormones).

He wants to see an immediate reduction in pesticide use until the effects are better understood, and is pressing for the formation of a European Union environment unit to study the problem.