Prop 65 Updates: New MFH Listing and BPS Developmental Toxicity Endpoint Added

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On December 5, 2025, California’s Office of Environmental Health Hazard Assessment (OEHHA) announced the addition of N-methyl-N-formylhydrazine (MFH) to the Prop 65 list as a carcinogen.  On the same day, OEHHA also announced that it is adding the developmental toxicity endpoint to the listing for bisphenol S (BPS).  Both listings took effect December 8, 2025.

MFH (CAS RN 758-17-8)

MFH was added to the Prop 65 list via the “state’s qualified experts” mechanism following a November 18, 2025, determination by the Carcinogen Identification Committee (CIC).  The warning requirement for significant exposures to MFH will take effect one year after its listing, on December 8, 2026.

According to an August 2025 OEHHA document describing the evidence of MFH’s carcinogenicity, MFH is naturally occurring in edible Gyromitra mushrooms.  MFH is also used in the production of marbofloxacin (an antibacterial agent used in veterinary medicine) and for research purposes.

BPS (CAS RN 80-09-1)

The developmental toxicity endpoint for BPS was also added via the state’s qualified experts mechanism after an October 9, 2025, meeting by the Developmental and Reproductive Toxicant Identification Committee (DRTIC).  OEHHA’s evidence document for BPS’s developmental toxicity can be found here.

BPS has been included on the Prop 65 list since December 2023, when it was listed for female reproductive toxicity.  The warning requirement for significant exposures to BPS took effect in 2024.  In January 2025, OEHHA also added the male reproductive toxicity endpoint for BPS.

OEHHA has not established a maximum allowable dose level (MADL) safe harbor for BPS (under which a warning is not required).  As of the date of this post, private enforcers have sent over nine hundred 60-day notices to companies, alleging that customers may be receiving significant exposures to BPS from receipt papers without warning.

In recent out-of-court settlements, including an October 2025 settlement with Build-A-Bear Workshop, Inc., companies have committed to using “BPS free” thermal receipt paper.  In these settlements, BPS free is typically defined as less than 100 or 200 parts per million BPS and no intentionally added BPS, with some agreements also requiring no intentionally added bisphenol A (BPA).

The complete, updated Prop 65 list can be found here.