Herbicides are a heterogeneous class of chemicals used in agriculture, forestry, and urban settings to kill weeds, shrubs, and broad-leaved trees. The role of herbicides in the etiology of cancer is controversial. Several studies have noted large increases in risk of soft-tissue sarcomas with phenoxy herbicide exposure. In contrast, others have failed to observe increased risks, and evidence of an exposure-risk relationship is lacking. Although there have been too few appropriate studies for adequate assessment of risk of cancer at other sites, some findings have linked herbicide exposure with cancers of the colon, lung, nose, prostate, and ovary as well as to leukemia and multiple myeloma.

Exposure to a common herbicide significantly increases the risk of non-Hodgkin's lymphoma (NHL), according to a National Cancer Institute report. The broadleaf plant killer, a phenoxyherbicide called 2,4-dichlorophenoxyacetic acid (2,4-D), is one of the active ingredients in Agent Orange, used during the Vietnam war. Swedish researchers reported in 1979 and 1981 associations between phenoxyherbicide use and not only NHL but also two other cancers, soft-tissue sarcoma and Hodgkin's disease.

Hoar and her colleagues tracked down all cases of the three cancers diagnosed between 1976 and 1982 in white Kansas men, and questioned the patients or their survivors about occupation and exposure to herbicides. To further check the herbicide information, the researchers interviewed local pesticide suppliers. When the researchers compared the herbicide exposure among the cancer patients and among age-matched white men from the general Kansas population, they found that the incidence of NHL, but not of the other two cancers, increased with increased exposure to 2,4-D. The more exposure to that particular herbicide, they found, the greater the risk -- the incidence of NHL among farmers exposed 20 days or more per year was six times that of the general population, while those who used it six to 10 days per year were at 1.6 times the risk. The greatest danger was to people who had handled the chemical extensively -- people who mixed or applied it were at eight times the risk.

Future studies must better identify and quantify the nature of herbicide exposures. In the interim, it seems only prudent to monitor and promote safety practices among persons occupationally exposed to phenoxy herbicides, particularly farmers and professional sprayers.

References
Morrison, H. I., et al. 1992. Herbicides and Cancer. Journal of the National Cancer Institute 84(24):1866-1874.

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