No Significant Risk Level for Antimony Trioxide: Modification to Proposed Regulation, Addition of Documents to Rulemaking File, and 15-Day Public Comment Period.

The proposed Proposition 65 No Significant Risk Level (NSRL) for antimony trioxide in Title 27, California Code of Regulations, section 25705(b)(1) has been modified, and documents have been added to the rulemaking file, according to the Office of Environmental Health Hazard Assessment (OEHHA). On August 26, 2022, OEHHA first published a Notice of Proposed Rulemaking in the California Regulatory Notice Register (CRNR), introducing the regulation. An Initial Statement of Reasons for the proposal was released concurrently by OEHHA. The initial plan was open comments for 45 days, from August 26 to October 11, 2022, according to OEHHA. (See below for the proposed rulemaking and the public comments received.) 

The OEHHA is restricting the proposed NSRL to the inhalation route after carefully examining the feedback received on this proposed regulation as well as the existing scientific research. The suggested NSRL’s daily dosage of 0.13 micrograms remains the same. The notice includes a description of the proposed alteration as well as a list of the documents that have been added to the record.  

The Rationale for the Proposed Modification

OEHHA received feedback on the route of antimony trioxide exposure throughout the comment period for the initial proposal. The rate of absorption by oral and inhalation exposure appears to vary, according to remarks. According to the Initial Statement of Reasons, antimony trioxide (particles) aerosols were administered to animals during the carcinogenicity experiments to determine the cancer potency. Antimony trioxide’s carcinogenicity in inhalation studies considers the effects of both the inhaled particles and, after inhalation, the dissolved form. 

After carefully examining reviews of the scientific literature that are currently available, such as those by the European Commission (2008), ATSDR (2019), WHO (2003), IARC (1989), and Boreiko and Rossman (2020), as well as toxicokinetic data from studies like Gross et al. (1955), Groth et al. (1986), Hiraoka (1986), Newton et al. (1994), Sunagawa (1981), and Westrick (1953), OEHHA determined that Due to this ambiguity and the apparent changes in absorption between inhalation and oral exposure, OEHHA concluded that it was not possible to accurately calculate the NSRL of this chemical for oral exposures. 

Therefore, by adding “(inhalation)” to the proposed NSRL, OEHHA is restricting the NSRL to exposure through inhalation. This does not rule out the idea of creating an NSRL for the oral route down the road once enough data is gathered. 

Submission of Public Comments

The regulatory text has been modified, and the OEHHA is asking for feedback on it. Comments must be submitted to OEHHA by July 27, 2023, at 11:59 p.m., which is the deadline for submissions. The OEHHA strongly encourages electronic submission of comments rather than paper ones using our website at The contribution may also be made on paper and submitted in person or via mail.   


Reach out to our regulation experts on chemical and product regulatory compliances