Notice of Adoption of Cancer Inhalation Unit Risk Factor for Isoprene

Jan 3, 2025

The Office of Environmental Health Hazard Assessment (OEHHA) is adopting a new cancer inhalation Unit Risk Factor (IUR) for Isoprene for use in the Air Toxics Hot Spots Program.

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 requirement, OEHHA develops new unit risk factors and revises existing unit risk factors for many air pollutants. The IUR for isoprene was developed using the most recent guidance, “Air Toxics Hot Spots Program Technical Support Document for Cancer Potency Factors,” finalized by OEHHA in 2009. IURs are used to estimate lifetime cancer risks associated with inhalation exposure to a carcinogen.

A draft document for the isoprene IUR was released on February 16, 2024 to solicit public comment and was discussed at two public workshops 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 on Toxic Air Contaminants (SRP) in August 2024 before being finalized.

The Isoprene IUR values are as follows:

Inhalation Unit Risk Factor (IUR):

5.4 × 10^–6 per microgram per cubic meter (µg/m³)^–1;

1.5 × 10^–5 per part per billion (ppb)^–1

Cancer Slope Factor (CSF):

1.9 × 10^–2 per milligram per kilogram of body weight per day (mg/kg-d)^–1

Adoption Memo