Methyl Bromide Exposure Raises Prostate Cancer Risk
BETHESDA, Maryland, May 2, 2003 (ENS) - Exposure to the pesticide methyl bromide and six other pesticides have been linked with an increased risk of prostate cancer among pesticide applicators in North Carolina and Iowa, U.S. government scientists reported Thursday. Methyl bromide is a fumigant gas used to protect crops from pests in the soil and to fumigate grain bins and other agricultural storage areas. Prostate cancer risks were two to four times higher among pesticide applicators than among men who were not exposed to methyl bromide.
The connection emerged from large scale, long term government study of farmers and their spouses known as the Agricultural Health Study that is investigating the causes of cancer and other diseases in the farming community.
"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 Dr. Michael Alavanja, a public health specialist from the National Cancer Institute Division of Cancer Epidemiology and Genetics, and the study's principal investigator.
The current study included 55,332 men who are classified as either private pesticide applicators, 92 percent, or commercial pesticide applicators. 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.
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. The risk of prostate cancer rose as the total number of days of methyl bromide use increased. Elevated risks were seen only at the two highest levels of exposure, out of five possible levels.
Based on animal studies, the National Institute for Occupational Safety and Health lists methyl bromide as a potential occupational carcinogen.
The U.S. Agricultural Department is conducting research at 15 Agricultural Research Service locations to develop alternatives to soil fumigation with methyl bromide to control pathogens and weeds. Although methyl bromide is used to some extent on more than 100 crops, nearly 80 percent of preplant methyl bromide soil fumigation goes to strawberries, tomatoes, ornamentals and nursery crops, and peppers.
Alternatives to soil fumigation with methyl bromide include host plant resistance, biological control, alternative chemicals, and different cultural practices, either alone or in combination.
Dr. Aaron Blair of the National Cancer Institute, an author of the study, said the investigators cannot rule out the possibility that their observation "occurred by chance alone."
"Clearly, these findings need to be replicated," he said. "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 only, exposure to six pesticides - chlorpyrifos, coumaphos, fonofos, phorate, permethrin, and butylate - was associated with an increased risk of prostate cancer.
The most consistent risk factors for 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.
Farming has been linked to prostate cancer risk in several previous occupational studies. But the variety of environmental exposures in the farming community - pesticides, engine exhausts, solvents, dusts, animal viruses, fertilizers, fuels, and 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, nearly 90,000 participants from North Carolina and Iowa, 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.
The AHS, which began in 1993, is a collaborative effort involving the National Cancer Institute, the National Institute of Environmental Health Sciences, and the Environmental Protection Agency. The prostate cancer study appears in the May 1 issue of the "American Journal of Epidemiology."