California is considering major changes to the cancer risk factors used for acrolein and ethylene oxide (EtO) under the Assembly Bill 2588 Air Toxics “Hot Spots” Program (AB2588). If the draft values are adopted, facilities could see higher calculated cancer risks even when their operations and emissions have not changed.
The Office of Environmental Health Hazard Assessment (OEHHA) released draft cancer inhalation unit risk factors (IUR) for acrolein and ethylene oxide (EtO) in May 2026. As of the date of this article, the acrolein and EtO IUR values remain draft OEHHA proposals and will not be adopted until OEHHA completes the applicable review and adoption process. OEHHA’s public comment period for both proposals closed on July 29, 2026 and OEHHA is currently reviewing the comments.
Background
AB2588 requires facilities to report toxic air containment (TAC) emissions and, when warranted, assess and reduce the resulting health risk, with OEHHA setting the science and local air districts handling enforcement.
An IUR represents the excess lifetime cancer risk associated with continuous exposure to one microgram of a carcinogen per cubic meter of air (µg/m³)i. In an AB2588 health risk assessment, the IUR is combined with modeled exposure concentrations and other risk-assessment inputs to estimate the cancer risk attributable to facility emissions.
In other words, the risk number can go up simply because the underlying IUR changed, not because a facility is emitting anything new or different. So, if a more stringent IUR is adopted, a facility’s calculated cancer risk can rise even though nothing on the ground has actually changed.
Proposed Changes
AB2588 already requires air districts to evaluate acrolein for noncancer health effects and EtO for both noncancer and cancer health effects.
Acrolein
OEHHA’s draft identifies acrolein as carcinogenic to humans and discusses evidence including inhalation carcinogenicity studies in experimental animals and mechanistic evidence. This represents an important change in how acrolein could be evaluated in California. The proposed IUR would add cancer risk to future AB2588 evaluations of acrolein rather than limiting the health-risk evaluation to its existing noncancer endpoints.
This proposed change to acrolein under AB 2588 can be expected to have an impact that extends well beyond facilities that intentionally use acrolein as a raw material. OEHHA identifies acrolein sources ranging from combustion of fossil fuels, wood, and plastics (including refinery emissions, gasoline-related emissions, and cooking) to industrial uses such as paper manufacturing and upstream oil and natural gas operations.
Consequently, facilities should consider both direct use of acrolein and processes capable of generating acrolein as a combustion or process byproduct when evaluating potential impacts of the proposed change.
Ethylene Oxide
The May 2026 proposal would increase the EtO IUR 34-fold under AB 2588 from 8.8 × 10⁻⁵ (µg/m³)⁻¹ to 3.0 × 10⁻³ (µg/m³)⁻¹.
OEHHA states that its current EtO IUR was derived using animal cancer studies and the scientific evidence base has expanded substantially since development of the original IUR. The May 2026 draft relies on United States Environmental Protection Agency (U.S. EPA) analysis of occupational data and proposes an adult-based combined IUR addressing lymphoid and breast cancers.
Potential sources of EtO include sterilization and fumigation operations, chemical manufacturing, fuel combustion, mobile sources, tobacco smoke, wildfires, and natural biological processes. For facilities with EtO emissions, this change could substantially increase the EtO contribution to modeled cancer risk. The actual change in a facility’s total calculated cancer risk, however, cannot be assumed to be 34-fold because total facility risk may include multiple pollutants, exposure pathways, source locations, receptor locations, and other risk-assessment variables.
What Should Facilities Do Now?
Because the values remain proposals, facilities should not substitute the draft IUR for currently approved values in regulatory calculations unless instructed to do so by the applicable agency. However, facilities with known or potentially significant acrolein or EtO emissions can use the draft values for forward-looking sensitivity analyses.
Key Takeaways and Next Steps
Facilities that have the potential to emit acrolein or EtO should consider the following actions:
- Review existing TAC emissions inventories to determine whether acrolein or EtO is currently reported and identify the sources responsible for those emissions.
- Review recent Health Risk Assessments (HRA) and prioritization results to determine the contribution of EtO.
- Perform screening or sensitivity calculations using the proposed IUR whether your facility could experience a material increase in calculated cancer risk if the values are adopted.
- Evaluate emissions calculation methodologies, particularly for acrolein from combustion sources, to confirm that emissions factors, source testing, operating data, and other inputs are representative of facility operations. Consider whether additional measurements are warranted.
- Evaluate whether operational changes can be made to reduce or eliminate emissions of either compound.
- Review applicable air district requirements and risk-management policies. The consequences of an increased calculated risk depend on the facility’s local air district and the regulatory program under which the HRA is being performed.
- Monitor OEHHA’s review process and subsequent California Air Resource Board and air district implementation. The final values may differ from the May 2026 drafts.
Facilities whose modeled risks are already near an applicable air district notification, risk-reduction, permitting, or other risk-management threshold warrant particular attention because the adoption of these proposed health values could affect site-specific regulatory outcomes. ALL4 has significant experience working with facilities to develop strategies to respond to regulatory changes. For questions about this topic or how it may affect your facility, contact Andres Luna (aluna@all4inc.com) or Michael McHale (mmchale@all4inc.com).
iOffice of Environmental Health Hazard Assessment (OEHHA), Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments, Appendix J, Glossary of Acronyms and Definition of Selected Terms, (Feb. 2015). OEHHA Guidance Manual, Appendices G–J
