PFAS defendants can use the International Agency for Research on Cancer’s (IARC’s) recent statement on the carcinogenicity of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in support of causation and notice defenses. PFOA and PFOS are part of the group of compounds known as per- and polyfluoroalkyl substances (PFAS), which have been disparagingly labeled “forever chemicals” due to their alleged capacity to remain in the environment for long periods.

IARC’s statement is limited to two of the thousands of PFAS compounds, and courts, like the Sixth Circuit in its recent decision in Hardwick, have rejected allegations that conflate these distinct substances (see Sixth Circuit Rejects Overly Ambitious PFAS Class Action). Furthermore, IARC recognized there is only “limited evidence” of PFOA’s ability to cause just two cancers in humans, testicular and kidney cancer, and the evidence it causes other cancers in humans is “inadequate.” IARC labeled as “inadequate” the evidence PFOS causes any cancer in humans.

IARC’S Classification of PFOA and PFOS

IARC is the cancer-research branch of the World Health Organization (WHO). IARC classifies substances in one of four groups: Group 1 (carcinogenic to humans), Group 2A (probably carcinogenic to humans), Group 2B (possibly carcinogenic to humans), and Group 3 (not classifiable as to its carcinogenicity to humans). In November 2023, an IARC working group met in France to finalize its evaluation of PFOA and PFOS. In early December, the WHO announced that IARC’s findings will be published in 2024 as Volume 135 of the IARC Monographs. In the interim, IARC released a summary of its findings.

In a 2016 Monograph, IARC concluded PFOA is “possibly carcinogenic to humans.” At its November 2023 conference, IARC changed PFOA’s classification to “carcinogenic to humans.” IARC based its decision on evidence in experimental animals and “strong mechanistic evidence” of immunosuppression and alteration of gene expression in humans.

However, IARC acknowledged the dearth of evidence PFOA causes cancer in humans. According to IARC, there is only “limited evidence” PFOA causes kidney and testicular cancer, and IARC noted that while some human studies have found an association, the same number or more studies have not. IARC called the evidence for all other human cancers “inadequate.” IARC reported an animal study published since its 2016 conclusions, but it focused on liver, pancreatic, and uterine cancer and related effects in rats (i.e., neither kidney nor testicular cancer).

At its November 2023 conference, IARC classified PFOS as “possibly carcinogenic to humans” due to “strong mechanistic evidence” it is immunosuppressive and induces genetic alterations. However, IARC acknowledged the evidence that PFOS causes cancer in humans is “inadequate” because the few available studies showed only sporadic and inconsistent positive findings for breast, testicular, and thyroid cancer. IARC also acknowledged the evidence that PFOS causes cancer in animals is “limited,” citing a study showing increased liver cancer and related effects in the highest exposed rats.

Conclusion

Defendants in the widespread PFAS personal injury, consumer class action and other litigation should be aware of IARC’s determinations and how they support the defense position that the available scientific evidence is insufficient to demonstrate causation. For example, IARC’s conclusions are restricted to two among thousands of PFAS compounds, and courts have rejected allegations that conflate these distinct substances (see Sixth Circuit Rejects Overly Ambitious PFAS Class Action). Furthermore, IARC acknowledges that the evidence either PFOA or PFOS cause cancer in humans is inadequate, comprising studies with inconsistent results and, insofar as associations have been found in humans, they are limited to cancers at specific sites: for PFOA, testicular and kidney cancer, for PFAS, testicular, breast, and thyroid cancer. Defendants in PFAS lawsuits in which IARC’s findings are raised should distinguish which PFAS compounds and cancers are allegedly at issue, and emphasize that even for PFOA and PFOS, IARC acknowledges a lack of evidence for cancer causation in humans.