IARC Classifies Herbicides Atrazine and Alachlor as 'Probably Carcinogenic' Group 2A

The International Agency for Research on Cancer (IARC), part of the World Health Organization (WHO), classified the herbicides atrazine and alachlor as 'probably carcinogenic to humans (Group 2A)' and the fungicide vinclozolin as 'possibly carcinogenic to humans (Group 2B).' A working group of 22 experts from 12 countries evaluated them in Lyon, France, and a summary appeared in The Lancet Oncology (IARC Monographs Volume 140).

For atrazine, human evidence was rated 'limited' for an association with non-Hodgkin lymphoma carrying a specific chromosomal translocation, t(14;18). The pooled odds ratio from two case-control studies was 1.99 (95% confidence interval 1.13 to 3.53), and there was no association with lymphoma without the translocation. For alachlor, 'limited' evidence for laryngeal cancer was presented from a large cohort of pesticide applicators. For all three substances, evidence of carcinogenicity in animals was 'sufficient,' and mechanistic evidence, such as disruption of endocrine pathways and oxidative stress, was judged 'strong.'

The strength of evidence, not the size of risk

An IARC classification indicates only the strength of the evidence on whether a substance can cause cancer, and says nothing about how large the risk is from everyday exposure. IARC explained that exposure of the general public occurs mainly through drinking water and food and is generally estimated to be low. The groups with the highest exposure are workers who manufacture and apply pesticides. Atrazine is banned in the European Union but is widely used elsewhere on corn, sorghum and sugarcane. Judging risk at actual exposure levels requires assessments by national regulatory agencies.