Most pesticides are highly toxic chemicals, they have to be in order to perform their essential function of killing pests.
Toxicity studies, including carcinogenicity research, are required as part of the registration process for any chemical, including pesticides. However, the seriously flawed system for regulating chemicals in the U.S. means that companies get to submit their own science to the EPA to prove their products are safe.
EPA’s Office of Pesticide Programs, Health Effects Division is tasked with reviewing studies on mice and rats that evaluate the carcinogenic potential of individual pesticide active ingredients.
Also consider that many cancers take years or decades to develop. The National Cancer Institutes defines latency-period as “the time that passes between being exposed to something that can cause disease (such as radiation or a virus) and having symptoms.”
For example, mesothelioma is the cancer most commonly association with asbestos exposure. It has an average latency period of 30-45 years. Non-Hodgkin lymphoma, a blood cancer, is thought to have a latency period of around 10 years, although cancer incidence as quickly as a few months after exposure has been reported.
So, even though some pesticides have been used for decades, we may be just beginning to observe changes to cancer rates.
Roberts, J. R., Karr, C. J., & Council On Environmental, Health. (2012). Pesticide exposure in children. Pediatrics, 130(6), e1765-1788. doi:10.1542/peds.2012-2758. https://www.ncbi.nlm.nih.gov/pubmed/23184105.