"This is the largest study of farmers and their families in the world, so it gives us an opportunity to look at diseases that haven't been well characterized," Hoppin said.
The women reported whether or not they had been diagnosed with asthma and were then divided into two groups: those with atopic (allergic) asthma and those with nonatopic (non-allergic) asthma.
More than half the women in the study had used or been exposed to pesticides. Sixty-one percent of the women were raised on a farm, which protected against allergic asthma and, to a lesser extent, non-allergic asthma. Using pesticides was associated almost solely with allergic asthma, increasing the risk almost 50 percent, the study found.
The association between pesticide use and allergic asthma was strongest among women who had grown up on a farm. But because of the protective effect of having grown up in an agricultural setting, these women still had a lower overall risk of allergic asthma than women who did not grow up on a farm.
Women who were raised on farms but did not use pesticides had the lowest overall risk of allergic asthma, compared with women who neither grew up on farms nor applied pesticides.
Some less commonly used pesticides such as parathion were associated with triple the risk of allergic asthma. Some more commonly used pesticides such as malathion were also associated with a higher risk, the researchers said.
Other experts said the study findings were far from clear-cut.
"It's a little bit confusing," said Dr. Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York City. "I would expect that there would be more allergic asthma in anybody who grew up on a farm, but that's not exactly what they're saying. Also, it's difficult to separate farm work from pe
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