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California Announces Antimony (Trivalent Compounds) Does Not Meet Prop 65 Listing Criteria

California State House

On June 8, 2023, the California Office of Environmental Health Hazard Assessment (OEHHA) announced it will not proceed with the Proposition 65 listing process for antimony (trivalent compounds) as a substance known to cause cancer. OEHHA had announced on September 30, 2022, that it intended to list antimony (trivalent compounds) as a carcinogen under Proposition 65 pursuant to the “Labor Code” listing mechanism.

Also known as the Safe Drinking Water and Toxic Enforcement Act of 1986, Proposition 65 prohibits knowingly exposing any individual to a listed chemical without first providing a “clear and reasonable warning.” Under the Labor Code, chemicals are added to the list if they are identified by the World Health Organization’s International Agency for Research on Cancer (IARC) as causing cancer in humans or laboratory animals.

OEHHA’s 2022 announcement that it intended to list antimony (trivalent compounds) under the Labor Code listing mechanism was based on IARC’s conclusion that antimony (trivalent compounds) is “probably carcinogenic to humans” (Group 2A); at the time, IARC’s summary conclusion was merely published in a March 2022 Lancet article.

However, after review of volume 131 of the IARC 2023 Monographs on the Identification of Carcinogenic Hazards to Humans, which was published in May of 2023, OEHHA determined that existing data on antimony (trivalent compounds), other than antimony (III) trioxide, does not meet the criteria for listing as causing cancer via the Labor Code mechanism. Thus, while the Proposition 65 listing for antimony trioxide remains in place, OEHHA has determined not to list the broader category of all trivalent antimony compounds, despite the updated IARC designation.