Mythbusters: Does This Cause Cancer?

Do Pesticides Increase Cancer Risk?

What the science says: The relationship between pesticide exposure and cancer risk remains difficult to discern. Farmers and others in the agricultural workforce tend to have lower cancer rates overall, which is largely attributed to their physically active lifestyles and lower smoking rates.^[37] Still, certain cancers—primarily lymphoma, multiple myeloma, and prostate cancer—occur in higher frequencies in this population.^[38]

Some experts surmise that exposure to pesticides and other chemicals may be a strong contributor to the elevated rates of these specific cancers. Recent data from the Agricultural Health Study (AHS), the longest-running prospective cohort study exploring a potential link between pesticides and cancer, supports evidence of an elevated risk for some cancers, but not for others.^[39] After following more than 84,000 pesticide applicators and their wives in Iowa and North Carolina from 1993 to 2006, the researchers reported a significantly lower incidence of cancer overall—specifically for oral, colon, lung, bladder, and kidney cancer—compared with the general populations of those states. But the researchers did find an elevated risk for a range of other cancers, including multiple myeloma, prostate cancer, lip cancer, and certain subtypes of non-Hodgkin lymphoma.

In a follow-up study, the AHS group tried to determine the risk for prostate cancer associated with specific pesticides. The 2013 study published in the American Journal of Epidemiology revealed that four insecticides—fonofos, malathion, terbufos, and aldrin—were associated with significantly increased risk for aggressive prostate cancer in farmers and commercial pesticide applicators.^[40]

Two subsequent studies from the AHS group found associations between several commonly used pesticides and an elevated risk for specific cancers. One study reported an association between the pesticides DDT, lindane, permethrin, diazinon, and terbufos and certain subtypes of non-Hodgkin lymphoma,^[41] whereas another study reported an elevated risk for colorectal and lung cancer risk in the AHS cohort exposed to acetochlor.^[42] The authors of the acetochlor study arrived at a cautious conclusion, however, stating that "due to lack of exposure-response trend, small number of exposed cases, and relatively short time between acetochlor use and cancer development, these findings warrant caution in interpretation and further investigation."

Yet other studies have revealed no discernable link between certain pesticides and cancer risk. A 2012 review of epidemiologic studies did not find evidence of increased cancer risk in people exposed to the popular herbicide glyphosate,^[43] and a 2013 review of epidemiologic evidence examining a link between atrazine and cancer risk concluded that "there is no causal association between atrazine and cancer and that occasional positive results can be attributed to bias or chance."^[44]

What the experts say: According to Paolo Boffetta, MD, MPH, director of the Institute for Translational Epidemiology and professor in the Division of Hematology and Medical Oncology and the Department of Preventive Medicine at Mount Sinai School of Medicine, "For those with high pesticide exposure levels, the link is very plausible for some cancers, such as lymphoma or myeloma and possibly prostate cancer as well. But for other cancers, I don't think evidence is very strong. For consumers, the low level of exposure probably does not increase cancer risk."

The link between pesticides and cancer risk is complex. All pesticides are tested for general toxicity, and most linked to cancer in animals have been banned. "What we have left are chemicals that do not show an effect in animal models," Dr Boffetta said.

In addition, it is difficult to study the effects of individual pesticides in humans. For instance, farmers and applicators apply a range of chemicals in different combinations, and thus are exposed to many agents over time. "It becomes very challenging to identify people exposed to only one pesticide, and it is therefore difficult to identify which individual pesticide or chemicals may put people at risk," Dr Boffetta said. "I don't think for any specific agent, we can state that there is conclusive evidence it causes cancer, but some studies do show an elevated risk for cancer in those with significant pesticide exposure."

Although Dr Boffetta believes further research is needed to determine a causal link, he thinks it is important to reduce exposure levels overall. "I would recommend lowering exposure where possible, especially for applicators and farmers. For consumers, it may be worth erring on the side of caution, even though their risk is likely to be very low."

Ms Zoumas noted that in consumers, there is no clear association between pesticide exposure and cancer risk. "There does not appear to be a benefit of organic produce," Ms Zoumas said. "It may be more important to wash fruit well and to be careful about specific fruits, known as the 'dirty dozen,' that have higher levels of pesticides."

Verdict: Plausible in people with high exposure levels (farmers and pesticide applicators), but unlikely in consumers with very low levels of exposure.

Medscape Oncology © 2015  WebMD, LLC

Cite this: Victoria Stern. Mythbusters: Does This Cause Cancer? - Medscape - Mar 04, 2015.