Men with certain genetic variations who were exposed to some toxic pesticides which are now largely banned run an increased risk of developing Parkinson's disease, French scientists said on Monday.

Researchers found that among men exposed to pesticides such as DDT, carriers of the gene variants were three and a half times more likely to develop Parkinson's than those with the normal version of the gene. The scientists, whose work was published in the Archives of Neurology journal, think the brains of people with the gene variant fail to flush out toxins as efficiently as those with normal versions of the gene, suggesting environmental as well as genetic factors are important in the risk of Parkinson's.

DDT, which belongs to a group of pesticides known as organochlorines, is one of the "Dirty Dozen" chemicals banned by a 2001 United Nations convention after it was found to be a toxin that can suppress the immune system. It is infamous for threatening bird populations by thinning eggshells, and has also been linked to increase risks in humans of diseases such as cancer and Parkinson's — an incurable and often deadly brain disease. But exemptions to the DDT ban are allowed in many developing nations because it so effective in killing mosquitoes.

DDT's Swiss inventor Paul Hermann Muller won the 1948 Nobel Prize for Medicine — before its wider toxic effects were known. Alexis Elbaz and Fabien Dutheil, of France's National Institute for Health and Medical Research (INSERM) studied 101 men with Parkinson's and 234 without the disease to look at links between organochlorine exposure and Parkinson's disease. The study included only men, and all of them had had high levels of exposure to pesticides through their work as farmers.

The scientists found the link was around 3.5 times stronger in men who carried two copies of a gene known as ABCB1, which plays a role in helping the brain flush out dangerous chemicals.

"The gene encodes for a kind of pump in the brain, and in people who have the (two copy) variation, this pump doesn't work as well," Elbaz said in a telephone interview. "It seems therefore that people who have these variations would have higher levels of insecticides in the brain because the brain's pump is not clearing them out properly."

Parkinson's is a neurodegenerative disease that affects one to two percent of people over the age of 65. Sufferers have tremors, sluggish movement, muscle stiffness, and difficulty with balance. Although medical treatments may improve symptoms, there are none that can slow down or halt the progression of the disease.

Elbaz said his work supported a growing body of evidence that genetic factors alone were not to blame for Parkinson's, but that when they combined with factors in the environment, the risk could significantly increase.