Carcinogenicity - Process of induction of malignant neoplasms, and thus cancer, by chemical, physical or biological agents.

Mutagenicity - Ability of a physical, chemical, or biological agent to induce (or generate) heritable changes (mutations) in the genotype in a cell as a consequence of alterations or loss of genes or chromosomes (or parts thereof).


Extensive research and detailed evaluations by regulatory authorities and expert panels worldwide have not found chlorpyrifos linked to any form of cancer. Definitive regulatory guidelines studies involving lifetime feeding studies in animals have not identified cancer as an outcome associated with chlorpyrifos exposure (e.g., Yano et al., 2000). No regulatory authority in the world considers chlorpyrifos a carcinogen. While links with lung cancer and prostate cancer have been suggested by isolated studies (discussed below), the regulatory and scientific consensus after 45 years of research remains that chlorpyrifos is not carcinogenic or mutagenic.

The most detailed investigation of documented human exposures to chlorpyrifos and incidence of long-term health effects, including cancer, comes from studies of chlorpyrifos manufacturing plant workers (Brenner et al., 1989; Burns et al, 1998). In these surveys, workers routinely handling chlorpyrifos during a few to many years were not found to have elevated levels of any types of cancers.

Lung Cancer

One study (Lee et al., 2004) reported a statistical increase in lung cancer among farmers and applicators in Iowa and North Carolina reporting use of chlorpyrifos over a period of years. Actual pesticide exposure levels were never documented or confirmed, however. The applicators also used other pesticides, and all but four of them smoked tobacco, a known lung cancer risk. The study’s reported link between chlorpyrifos and lung cancer contradicted the weight of the evidence, and the study has never been replicated by other research.

Prostate Cancer

Another study (Alavanja et al. 2003) reported a link among farmers and applicators in Iowa and North Carolina between family history of prostate cancer and reported exposures to chlorpyrifos and five other pesticides. Actual exposure levels to chlorpyrifos were not measured as part of the study. There was no biological plausibility to the findings for chlorpyrifos. The findings contradicted the weight of the evidence on chlorpyrifos; they were never replicated by subsequent research.

Further Resources

Alavanja et al., “Pesticides and Lung Cancer risk in the Agricultural Health Study Cohort,” American Journal of Epidemiology, 2004,
Volume 160: 876-885.

Lee et al., “Cancer Incidence among Pesticide Applicators Exposed to Chlorpyrifos in the Agricultural Health Study,” Journal of the National Cancer Institute, 2004, Volume 96(23): 1781-1789.

Yano, B.L. et al. “Lack of Carcinogenicity of Chlorpyrifos Insecticide in a High-Dose, 2-Year Dietary Toxicity Study in Fischer 344 Rats”, Toxicological Sciences, 2000, Volume 563:135-144.

Brenner, F.E. et al. “Morbidity among employees engaged in the manufacture or formulation of chlorpyrifos.” British Journal of Industrial Medicine, 1989, Volume 46(2): 133–137.

Burns, C.J. et al. “Update of the morbidity experience of employees potentially exposed to chlorpyrifos.” Occupational and Environmental Medicine, 1998, Volume 55(1):65-70.

U.S. Department of Health & Human Services. Environmental Health & Toxicology site. IUPAC glossary: