Notice of Adoption of Reference Exposure Levels for Trimethylbenzenes

The Office of Environmental Health Hazard Assessment (OEHHA) is adopting new Reference Exposure Levels (RELs) for Trimethylbenzenes (TMB) for use in the Air Toxics Hot Spots Program. RELs are airborne concentrations of a chemical that are not anticipated to result in adverse noncancer health effects for specified exposure durations in the general population, including sensitive subpopulations. The adopted RELs cover different types of exposure to TMB in air: infrequent 1-hour exposures, repeated 8-hour exposures, and continuous long-term exposure.

OEHHA is required to develop guidelines for conducting health risk assessments under the Air Toxics Hot Spots Program (Health and Safety Code Section 44360(b)(2)). In response to this statutory requirement, OEHHA develops RELs for many air pollutants, including TMB. The TMB RELs were developed using the most recent “Air Toxics Hot Spots Program Technical Support Document for the Derivation of Noncancer Reference Exposure Levels” (OEHHA, 2008). TMB will also be added to the list of Toxic Air Contaminants that may disproportionately impact children, pursuant to Health and Safety Code Section 39669.5(b)(1).  

A draft document for the TMB RELs was released on January 27, 2023 to solicit public comment and was discussed at two public workshop (February 23, 2023 and March 2, 2023) during the subsequent 45-day public review period. No public comments were received. The document was peer reviewed by the State’s Scientific Review Panel (SRP) on Toxic Air Contaminants in June 2023 before being finalized.

The TMB REL values are as follows:

Acute REL (for infrequent 1–hour exposures): 2,400 micrograms per cubic meter (μg/m³) (490 parts per billion (ppb))
Chronic REL (for long–term exposures): 4 µg/m³ (1 ppb)
8–Hour REL (for repeated 8–hour exposures): 8 µg/m³ (2 ppb)

Adoption Memo

• Trimethylbenzenes Reference Exposure Levels Technical Support Document (October 6, 2023)

SRP Draft (May 22, 2023)

• Notice of Public Meeting
  • SRP Slides 
  • Meeting Transcript

First Draft (January 27, 2023)

• Notice of Public Comment Period
• Amendment to Notice of Public Comment Period
• Public Comments – none received
• Public Workshop Slides

Please direct any inquiries concerning technical matters or document availability to:

Rima Woods, Ph.D., Chief
Air Toxicology and Risk Assessment Section
Office of Environmental Health Hazard Assessment

Rima.Woods@oehha.ca.gov

Related Information

• OEHHA Acute, 8-hour and Chronic Reference Exposure Level (REL) Summary
• Air Toxics Hot Spots - OEHHA (ca.gov)