Ag chemicals may cause prostate cancer. (Farm Harm).

On-the-job exposure to certain agricultural chemicals may be responsible for farmers' high rates of prostate cancer, suggest data from a large, ongoing study in two states. Farmers with relatives who have had prostate cancer may also face an elevated risk from additional chemicals that don't seem to cause problems in the larger group, says Michael Alavanja of the National Cancer Institute in Rockville, Md.

"There is a wealth of evidence that farmers tend to be at high risk of prostate cancer," says Marie-Elise Parent, a cancer epidemiologist at the University of Quebec in Laval, who isn't involved in the new study. Researchers haven't yet determined what puts farmers at greater risk than other people, but occupational exposure to pesticides, gasoline, and solvents may play a role, she says. Some pesticides appear to mimic the actions of hormones and thus cause cancer of such organs as the prostate and breast (SN: 1/23/99, p. 5G).

To investigate the possible influence of 50 different pesticides, Alavanja and his colleagues surveyed 55,332 men who had worked as farmers or professional pesticide applicators in either Iowa or North Carolina. The researchers asked the men about past pesticide exposure, family medical history, age, diet, and behaviors such as smoking and using protective gear while spreading chemicals. When the volunteers joined the study between 1993 and 1997, none had prostate cancer. By 1999, however, 566 of them had been diagnosed with the cancer. That number is 14 percent higher than would be predicted from the general rates of the cancer among men in Iowa and North Carolina.

After taking the known risk factors of age and family history into account, Alavanja and his colleagues found an association between prostate cancer and occupational exposure to five insecticides. Those chemicals include DDT and two related compounds that are no longer applied in the United States, as well as permethrin and carbofuran. Prostate cancer risk also appears to be elevated among the workers with exposure to high doses of methyl bromide, which is used to fumigate soil and stored grains, the researchers report in the May American Journal of Epidemiology.

The researchers linked five additional chemicals to increased risk of prostate cancer only among volunteers who had a family history of prostate cancer. These included four related insecticides--chlorpyrifos, coumaphos, fonofos, and phorate--that may increase prostate cancer risk through a common biological pathway, Alavanja says. Permethrin exposure increased cancer risk even more among the men with a family history of prostate cancer than it did in the group as a whole.

The new data strengthen the link between farm chemicals and prostate cancer, says Parent. She lauds the study for having more details about volunteers' exposures to chemicals than past studies have had and for taking into account such factors as the use of protective gear. Nevertheless, she says, it's too early to convincingly pin blame for prostate cancers on specific chemicals or to exonerate other chemicals.

Alavanja says his team next plans to analyze data on volunteers who've developed cancer since 1999.