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Thursday, May 1, 2003, 12:00 p.m. EDT
Exposure to certain agricultural pesticides may be associated with an increased risk of prostate cancer among pesticide applicators, according to a large study looking at the causes of cancer and other diseases in the farming community. The study, part of a long-term study of pesticide applicators and their spouses known as the Agricultural Health Study (AHS), appears in the May 1, 2003, issue of the American Journal of Epidemiology*. The AHS is a collaborative effort involving the National Cancer Institute (NCI), the National Institute of Environmental Health Sciences (http://www.niehs.nih.gov), and the Environmental Protection Agency (http://www.epa.gov).
The latest report from the AHS evaluated the role of 45 pesticides and found that only a few of them showed evidence of a possible association with prostate cancer among pesticide applicators. Methyl bromide was linked to the risk of prostate cancer in the entire group, while exposure to six other pesticides was associated with an increased risk of prostate cancer only among men with a family history of the disease.
"Associations between pesticide use and prostate cancer risk among the farm population have been seen in previous studies; farming is the most consistent occupational risk factor for prostate cancer," said Michael Alavanja, Dr.P.H., from NCI's Division of Cancer Epidemiology and Genetics in Bethesda, Md., and principal investigator of the AHS.
The AHS, which began in 1993, is following nearly 90,000 participants from North Carolina and Iowa over time to evaluate the role of a variety of agricultural and lifestyle exposures on health. The participants are either farmers, wives of farmers, or workers who use pesticides on a regular basis.
The current study included 55,332 men who are classified as either "private pesticide applicators" (92 percent) or "commercial pesticide applicators" (8 percent). Private applicators are farmers or nursery-workers. Commercial applicators work for pest control companies or for businesses such as warehouses or grain mills that use pesticides regularly. Between 1993 and 1999, 566 new prostate cancers developed among all applicators, compared to 495 that were predicted from the incidence rates in the two states. This means that the risk of developing prostate cancer was 14 percent greater for the pesticide applicators compared to the general population. The men in this study were followed for about 4.3 years.
Methyl bromide is a fumigant gas used nationally to protect crops from pests in the soil and to fumigate grain bins and other agricultural storage areas. The scientists found that among both North Carolina and Iowa pesticide applicators, the risk of prostate cancer rose with increasing frequency of use of methyl bromide and with longer lifetime exposure to this pesticide. Elevated risks were seen only at the two highest levels of exposure (out of five possible levels). Risks were two to four times higher than among men who were not exposed to methyl bromide. Based on animal studies, the National Institute for Occupational Safety and Health (NIOSH) lists methyl bromide as a potential occupational carcinogen.
"We cannot rule out the possibility that our observation occurred by chance alone," cautioned Aaron Blair, Ph.D. M.P.H., chief of the Occupational and Environmental Epidemiology Branch in NCI's Division of Cancer Epidemiology and Genetics, and an author of the current study. "Clearly, these findings need to be replicated. But, the internal consistency of our findings does not allow us to dismiss these results."
The researchers found another link between pesticides and prostate cancer: among men with a family history of prostate cancer, exposure to six pesticides -- chlorpyrifos, coumaphos, fonofos, phorate, permethrin, and butylate -- was associated with an increased risk of prostate cancer. This effect was not seen among those without a family history. This type of finding, i.e., something appearing in only a subgroup of the entire study population, is particularly difficult to interpret, since it could result from chance or from differences between subgroups other than their use of pesticides. However, four of these pesticides, chlorpyrifos, coumaphos, fonofos, and phorate, are thiophosphates and share a common chemical structure. These findings suggest that certain pesticides may interact with a particular form of one or more genes shared by men with a family history of prostate cancer, making them more susceptible to developing the disease.
The most consistent risk factors associated with prostate cancer are age, family history, and African-American ethnicity. Hormonal factors and high levels of animal fat and red meat in the diet are also suspected risk factors. Several previous occupational studies have linked farming to prostate cancer risk. However, the variety of environmental exposures in the farming community such as pesticides, engine exhausts, solvents, dusts, animal viruses, fertilizers, fuels, and specific microbes, have made it difficult for researchers in previous studies to sort out which of these factors is linked to specific diseases. Because of the large size of the AHS population, and the detailed information on specific exposures and risk factors collected by the AHS researchers, it is possible to evaluate the risks associated with a number of specific chemical exposures.
As the study continues and participants age, many new cases of cancer and other diseases will develop. With time, the researchers will be able to confirm or refute the current findings, assess additional relationships between exposures and diseases, and search for possible genetic links to the variety of environmental exposures in the farming community.
For further information, visit the AHS homepage: http://www.aghealth.org (http://www.aghealth.org).
For more information on other research results from the AHS, visit: http://cancer.gov/newscenter/pressreleases/AHSfactsheet.
About the National Institute of Environmental Health Sciences (NIEHS): NIEHS supports research to understand the effects of the environment on human health and is part of the National Institutes of Health. For more information on NIEHS or environmental health topics, visit www.niehs.nih.gov or subscribe to a news list.
About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.
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