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.


Farmers apply the fumigant gas methyl bromide to the soil under plastic sheeting. (Photo courtesy Society of Nematologists)
The latest report from the Agricultural Health Study (AHS) evaluated the role of 45 pesticides and found that several showed evidence of a possible association with prostate cancer among farmers and nursery workers. Methyl bromide was linked to the risk of prostate cancer in the entire group of more than 55,000 men in this study, 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 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.


Canisters of methyl bromide gas (Photo courtesy Pestcon)
Because methyl bromide depletes stratospheric ozone, contributing to holes in the ozone layer, developed countries are phasing the chemical out under provisions of the Montreal Protocol. In the United States there was a 25 percent reduction from 1991 levels in 1999, a 50 percent reduction in 2001, and there is supposed to be a 70 percent reduction this year. By 2005 methyl bromide will not be used at all except for what the U.S. Environmental Protection Agency calls "critical and emergency uses."

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.


Farmers exposed to pesticides are more likely to develop prostate cancer. (Photo courtesy NIEHS)
Four of these pesticides - chlorpyrifos, coumaphos, fonofos, and phorate - 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 scientists said.

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."