Pesticide exposure is a "potential risk factor for asthma and respiratory symptoms among farmers," lead author Dr. Jane A. Hoppin, from the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, told Reuters Health.

"Because grains and animals are more common exposures in agricultural settings, pesticides may be overlooked," Hoppin warned, adding: "Better education and training of farmers and pesticide handlers may help to reduce asthma risk."

Of the 19,704 farmers included in the study, 127 had self-reported (doctor diagnosed) allergic asthma and 314 had non-allergic asthma.

The main finding was that a history of high pesticide exposure was associated with a doubling of asthma risk, Hoppin noted. The link remained statistically significant after adjusting for a variety of potentially confounding factors including age, smoking, body weight, and state of residence.

Overall, 16 of the pesticides studied were associated with asthma: 12 with the allergic variety of asthma and 4 with the non-allergic type. Coumaphos, EPTC, lindane, parathion, heptachlor, and 2,4,5-TP were most strongly linked to allergic asthma. For non-allergic asthma, DDT, malathion, and phorate had the strongest effect.