Notice of Adoption of Reference Exposure Levels for Toluene Diisocyanate and Methylene Diphenyl Diisocyanate

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The Office of Environmental Health Hazard Assessment (OEHHA) is adopting new and revised Reference Exposure Levels (RELs) for toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) 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 TDI and MDI 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 TDI and MDI. The TDI and MDI 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). These chemicals 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).  

Draft documents for the TDI and MDI RELs were released on July 4, 2014, to solicit public comment and were discussed at public workshops in Sacramento and Diamond Bar, CA during the subsequent 75-day public review period. The documents were revised to reflect public comments, and peer-reviewed by the State’s Scientific Review Panel on Toxic Air Contaminants (SRP) in February 2015, and again in August 2015 before being finalized.

The REL values are as follows:

Toluene Diisocyanate:

  • Acute REL (for a 1-hour exposure): 2 µg/m3 (0.3 ppb)
  • 8-Hour REL (for repeated 8-hour exposure): 0.015 µg/m3 (0.002 ppb)
  • Chronic REL (for long-term exposure): 0.008 µg/m3 (0.001 ppb)

Methylene Diphenyl Diisocyanate:

  • Acute REL (for a 1-hour exposure): 12 µg/m3 (1.2 ppb)
  • 8-Hour REL (for repeated 8-hour exposure): 0.16 µg/m3 (0.015 ppb)
  • Chronic REL (for long-term exposure): 0.08 µg/m3 (0.008 ppb)

Please direct any inquiries concerning technical matters or availability of the documents to:

Dr. John Faust, Chief
Air, Community, and Environmental Research Branch
Office of Environmental Health Hazard Assessment
1515 Clay Street, 16th Floor
Oakland, CA 94612

E–mail: john.faust@oehha.ca.gov