Workers exposed to the pesticide chlorpyrifos may have an elevated risk of lung cancer, U.S. government researchers reported Tuesday.

Scientists at the National Cancer Institute found that among the more than 54,000 farmers and insecticide applicators they followed for six years, those with the highest chlorpyrifos exposure had twice the risk of developing lung cancer as did those who did not work with the pesticide.

Those in the highest-exposure group had worked with the chemical an average of 224 days over their lives.

The fact that the study found an “exposure-response” relationship - meaning lung cancer risk rose in tandem with chlorpyrifos use - is probably the strongest piece of evidence that the pesticide may promote lung cancer, study co-author Dr. Aaron Blair told Reuters Health.

However, he and his colleagues are urging caution in interpreting the results, since more research is necessary to establish a definite cause-and-effect relationship.

“This is the first study to show this,” Blair said. He added that the findings should be viewed along with the “totality of the evidence available,” which includes animal research suggesting that chlorpyrifos is not a strong carcinogen.

Blair and his colleagues report the findings in the December 1st issue of the Journal of the National Cancer Institute.

Chlorpyrifos is widely used in U.S. agriculture, and until recently it was found in many home and garden insecticides. Starting in 2000, the pesticide was phased out of products used in homes, schools, parks and certain other public areas after the Environmental Protection Agency (EPA) revised its risk assessment of the chemical.

Chlorpyrifos, sold as Dursban, is one of an older class of pesticides that can harm the central nervous system, and the EPA move was designed to limit children’s exposure to the chemical.

However, there has been relatively little evidence that chlorpyrifos may raise cancer risk, though some laboratory research has suggested it’s possible.

The new study recorded the incidence of various cancers among 54,383 farmers and full-time pesticide applicators in Iowa and North Carolina, more than 22,000 of whom had worked with chlorpyrifos. All of the mostly male participants are part of the Agricultural Health Study, a project begun in 1993 to follow the health effects of farmers’ and pesticide applicators’ occupational exposures.

Blair and his colleagues found that while exposed and unexposed workers had a similar risk of developing cancer in general, lung cancer risk was higher among those who had worked with the pesticide. The link held after the researchers weighed other factors, such as age, smoking, family history of cancer, and other on-the-job exposures.

The findings, Blair said, only apply to people who have worked directly with chlorpyrifos. He explained that the main concern with this and other pesticides is not inhalation, but skin exposure, since the skin is believed to be the main route of pesticide absorption into the body.

SOURCE: Journal of the National Cancer Institute, December 1, 2004.