Notice of Public Comment for Draft Air Toxics Hot Spots Program Guidance Manual for the Preparation of Risk Assessments 2014

The Office of Environmental Health Hazard Assessment (OEHHA) is releasing a draft document, Air Toxics Hot Spots Program Guidance Manual for the Preparation of Risk Assessments to solicit public comment. This draft Guidance Manual has been developed by OEHHA, in conjunction with the Air Resources Board, for use in implementing the Air Toxics Hot Spots Program (Health and Safety Code Section 44360 et. seq.). OEHHA is required to develop guidelines for conducting health risk assessments under Section 44360(b)(2).

OEHHA has developed three Technical Support Documents (TSDs) that provide the scientific basis for numeric values used in assessing health risks from exposures to facility emissions. The three TSDs describe non-cancer risk assessment (Technical Support Document for the Derivation of Noncancer Reference Exposure Levels, 2008), derivation of cancer potency factors (Technical Support Document for Cancer Potency Factors: Methodologies for derivation, listing of available values, and adjustments to allow for early life stage exposures, 2009), and exposure assessment methodology including stochastic risk assessment (Technical Support Document for Exposure Assessment and Stochastic Analysis, 2012). These TSDs underwent public and peer review, were approved by the State’s Scientific Review Panel on Toxic Air Contaminants, and adopted by OEHHA for use in the Air Toxics Hot Spots program. The draft Guidance Manual combines the critical information from the three TSDs into a guidance manual for the preparation of health risk assessments. The Guidance Manual will also undergo review by the Scientific Review Panel.

We are seeking comment on the guidance itself including clarity, and on items related to implementing the information in the three TSDs, including items noted below that the TSDs were silent on or that represent slight corrections. These items are related to the mechanics of conducting the risk assessment. We are not seeking comments on the underlying scientific information that originally appeared in the TSDs, which are finalized documents. Issues that are not directly addressed in the three Technical Support Documents, or are clarified in the Guidance Manual include:

  • Chapter 4
    • In Section 4.3.1 and, the text clarifies examples of “release types” for point, area, or volume sources and modeling selection related to screening or refined air dispersion modeling.
    • In Section 4.7.3, the text clarifies the method for spatial averaging at a fence line receptor when a portion of the grid is within the facility boundary.
  • Chapter 5
    • In Section 5.3.2, the text clarifies that in Tier 1 assessments, for pathways involving dermal exposure to contaminated soil or soil ingestion, the concentration in soil reflects accumulation over 70 years.
    • In Section, we provide guidance on how to use the mother’s milk biotransfer coefficients shown in Table 5.5.
    • In Table 5.4, footnotes were added to clarify how to use intake point estimates for food animals in the food animal pathway.
    • In section (and again in Chapter 8, Section 8.3.4), the text clarifies application of the 8-hour Reference Exposure Levels for offsite workers and residences, and for continuously emitting versus non-continuously emitting facilities.
    • In section, there is a small correction in the equation for calculating dermal dose. There was an extra parameter (year) in the equation in the 2012 TSD; it has now been removed.
    • In the 2012 TSD for Exposure Assessment, there were no equations shown to determine the weighted average dose used for calculating a chronic noncancer hazard quotient for the non-inhalation pathways of exposure. This needs to be done across age-groupings to estimate a chronic Hazard Quotient. Algorithms were added (pp. 5-40-42, 5-44-45, 5-47-48, 5-53-54, 5-57-58, 5-60-61) to make this clearer.
  • Chapter 6
    • On page 6-7, we clarified that for determining Hazard Index by target organ system, reproductive and developmental toxicants are combined into one Hazard Index.
  • Chapter 8
    • In section 8.2.7, we clarified that for estimating non-inhalation pathway cancer risk for the mother’s milk pathway, the exposure duration is one year in the 0 < 2yr age groups and 2 years for the other non-inhalation pathways.
    • In section 8.2.10, we included more detailed text regarding cancer risk assessment for short-term projects, relative to the description on page 61 in the May 2009 Technical Support Document for Cancer Potency Factors: Methodologies for derivation, listing of available values, and adjustments to allow for early life stage exposures. This issue is also discussed in Chapter 11, pages 11-17 and 11-18 of the Technical Support Document for Exposure Assessment and Stochastic Analysis.
  • Appendix E
    • We added language on page E-2 regarding estimating noncancer impacts from unspeciated polychlorinated biphenyl mixtures.

The distribution of the document commenced a 45-day public review period that ends on August 4, 2014 August 18, 2014. Please direct your comments on the documents, in writing or by e–mail, and any inquiries concerning technical matters or availability of the documents to:



Air Community and Environmental Research Branch
Office of Environmental Health Hazard Assessment
1001 I St
Sacramento, CA, 89814
Telephone: (916) 324-7572

Public workshops will be held in Northern and Southern California at the following locations and times:

July 15, 2014, 1:00 to 5:00 pm
South Coast Air Quality Management District
21865 E. Copley Dr,
Diamond Bar, CA 91765

July 16, 2014, 1:00 to 5:00 pm
Sierra Hearing Room
Cal/EPA Building
1001 I Street,
Sacramento, CA

Information about dates and agenda for meetings of the Scientific Review Panel can be obtained from the California Air Resources Board web page at