Dec. 28 (HealthDay News) -- Farm women who come in contact with some widely used pesticides may have an increased risk of developing allergic asthma, a new study suggests.

However, the risk of developing non-allergic asthma does not increase for women exposed to pesticides, according to the study authors.

"Women who apply pesticides on farms were 50 percent more likely to have allergic asthma, although this was not true for non-allergic asthma," said study author Jane Hoppin, a staff scientist with the National Institute of Environmental Health Sciences in Research Triangle Park, N.C. "Also, women who grew up on farms were protected against allergic asthma and that protection was evident whether or not you applied pesticides."

The findings are published in the January issue of the American Journal of Respiratory and Critical Care Medicine.

According to the Asthma and Allergy Foundation of America, non-allergic asthma is caused by factors not related to allergies. But allergic asthma -- the most common form of asthma, affecting more than 50 percent of the 20 million asthma sufferers in the United States -- is characterized by symptoms that are triggered by an allergic reaction. Some typical triggers for allergic asthma include dust mites, pet dander, pollen and mold.

Experts already knew that growing up on a farm minimizes the risk of allergic disease, that pesticides have been associated with respiratory symptoms in farmers, and that farmers are at increased risk for respiratory diseases -- including asthma -- due to exposure to grains, animals, dust and other factors.

Little research, however, has delved into respiratory risk factors for farm women.

Hoppin and her colleagues examined data on 25,814 such women in North Carolina and in Iowa who are participating in the Agricultural Health Study, a large government-funded look at the effects of environmental, occupational and other factors on the health of the agricultural population.

"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 pesticide use. I would need to see more data before coming to the same conclusions as the authors of the article did."

Because Hoppin's analysis was "cross-sectional," it can't show a cause and effect. "But given what we see in animals, it suggests that we should do a prospective analysis where we can then say pesticide use preceded asthma or asthmatic symptoms. That's really the next step," she said.

Hoppin said she and her colleagues are in the process of planning that next study.

SOURCES: Jane Hoppin, Sc.D., staff scientist, National Institute of Environmental Health Sciences, Research Triangle Park, N.C.; Len Horovitz, M.D., pulmonary specialist, Lenox Hill Hospital, New York City; January 2008 American Journal of Respiratory and Critical Care Medicine