Pesticide Exposure & Cancer Risk Evidence

Pesticides are not a single substance. They are hundreds of unrelated chemicals. Each has its own toxicity. Each lasts a different length of time. Each has its own body of evidence. Asking whether pesticides cause cancer is like asking whether medications cause side effects. The answer depends on which one, how much, and how you were exposed.

So the useful question is always the specific one. Which agent? At what exposure? And what does the evidence for that agent say?

An IARC working group reviewed five agents in Monographs Volume 112. It did not give them all the same answer.

• Glyphosate: Group 2A, probably carcinogenic to humans. The human evidence for non-Hodgkin lymphoma was limited. The evidence in laboratory animals was convincing.
• Malathion: Group 2A, with links reported for non-Hodgkin lymphoma and prostate cancer.
• Diazinon: Group 2A, with links reported for non-Hodgkin lymphoma and lung cancer.
• Tetrachlorvinphos: Group 2B, possibly carcinogenic to humans.
• Parathion: Group 2B.

As always, these Groups rank how strong the evidence is that something can cause cancer. They do not rank how dangerous a substance is at real-world exposure. Two Group 2A agents can differ enormously in how much risk they create.

IARC also separated the settings. Occupational exposure happens during production and application. For everyone else, exposure comes mainly from living near sprayed areas, home use and diet. Those levels are described as generally low.

This one deserves to be reported as the open dispute it is.

IARC, 2015: Group 2A, probably carcinogenic to humans.

EPA, 2020 interim decision: glyphosate is not likely to be carcinogenic to humans.

The courts, June 2022: the Ninth Circuit vacated the human health portion of EPA's interim decision. It ordered further analysis. EPA then withdrew the interim decision. It began updating its carcinogenicity evaluation. That review is still open.

Part of the disagreement is built into the two jobs. IARC asks a hazard question: can this agent cause cancer under any circumstances? EPA asks a risk question: does it create unreasonable risk at expected exposure levels? Those two questions can honestly give different answers about the same chemical. The rest of the disagreement is a real scientific fight over which studies to weight.

The Agricultural Health Study has followed more than 54,000 licensed pesticide applicators in Iowa and North Carolina. About 83 percent of them used glyphosate. Its 2018 analysis looked hard for a glyphosate link. It found none for solid tumors overall. It found none for lymphoid malignancies either, non-Hodgkin lymphoma included. Among the most heavily exposed applicators, acute myeloid leukaemia was raised. But that result was not statistically significant, and the authors said it needs confirmation.

The cohort's wider results are worth knowing too. After 20 years of follow-up, overall cancer incidence was lower than in the general population. It came to 0.91 for private applicators, 0.89 for spouses and 0.83 for commercial applicators. Less smoking and more physical activity are the likely reasons. Within that lower total, some cancers were raised. Those were prostate, multiple myeloma, and acute myeloid leukaemia. Thyroid, testicular, peritoneal and lip cancer were raised too. Both halves of that picture are true. Reporting only one of them misleads.

If you apply pesticides for work, this is where the exposure that matters actually sits. The label is a legal document. Follow its personal protective equipment requirements and its re-entry intervals. Mix in ventilated areas. Wash before eating or smoking. Change out of work clothes. Wash them separately, so residue does not travel home. Keep a written record of products, years and jobs.

If your exposure is dietary, wash produce under running water and rub the surface. Do not let worry about residues cut how much fruit and veg you eat. The benefits of eating them are far better established than the harms of trace residues. Organic produce is grown with pesticides too, just different ones. There is no strong evidence that switching lowers cancer risk.

• EPA — Glyphosate: current regulatory status
• Lerro et al., Cancer incidence in the Agricultural Health Study after 20 years of follow-up (PMC)
• Andreotti et al., Glyphosate use and cancer incidence in the Agricultural Health Study (PMC)
• Agricultural Health Study — publications
• NCI DCEG — Occupational exposures to chemicals and cancer risk