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Health Risk Assessment

including Air Dispersion Modeling

In compliance with the California Environmental Quality Act (CEQA), a Lead Agency will often require that a health risk assessment (HRA) be performed if a project will generate substantial emissions of toxic air contaminants (TACs). An HRA involves evaluation of emission sources to determine whether emission levels of certain substances will cause public health effects. The California Office of Environmental Health Hazard Assessment (OEHHA) has compiled an extensive list of TACs that must be analyzed for impacts on public health, and a CEQA analysis may require the inclusion of TAC emissions from construction equipment and vehicles as well as operational stationary sources, as applicable. Typical TACs quantified and evaluated in an HRA include byproducts of fossil fuel combustion, commonly natural gas, gasoline, or diesel fuel, as diesel particulate matter (DPM) from diesel-fueled construction equipment, trucks, and emergency generators is an especially potent carcinogen. Other common TACs include volatile ingredients in commercial products such as coatings, adhesives, sealants, and solvents, which evaporate into the air when used.

An HRA estimates the public health impacts of TAC emissions within the vicinity of a proposed project for both short-term construction and long-term operation. In particular, an HRA evaluates the potential for impacts to sensitive receptors, which include residences, schools, daycare centers, hospitals, nursing homes, and assisted living facilities. In some air districts, emissions of fine particulate matter (PM2.5), a criteria pollutant, must also be assessed. At a minimum, cancer and non-cancer (e.g., respiratory impairment) acute (short-term) and chronic (long-term) impacts to the nearest residents and neighboring workers are assessed. However, the level of health risk modeling – screening or refined – can vary depending on the TACs being emitted, mass emission rates, and distance to the nearest receptors.

In 2015, the OEHHA published its new Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments. Yorke Engineering, LLC has implemented the methodologies and toxicity values contained in this document, including subsequent updates, in our CEQA risk assessment procedures for both short-term construction and long-term project operations. Typically, conservative assumptions are applied in the quantification of emissions, pollutant dispersion, hazard identification, exposure duration, and toxicity assessment.

Consistent with the new guidance from the OEHHA, Yorke performs HRAs using the Hotspots Analysis and Reporting Program, version 2, (HARP2). HARP2 is distributed by the California Air Resources Board (CARB) and is intended to facilitate the exposure duration and toxicity assessment elements of HRAs in a consistent manner. As input data, HARP2 utilizes ground-level concentrations of TACs predicted by the U.S. Environmental Protection Agency (EPA) dispersion models AERSCREEN or AERMOD, expressed in units of micrograms per cubic meter, at the nearest residential receptor, the nearest workplace receptor, and all identified sensitive receptors. In turn, HARP2 calculates the maximum individual cancer risk (MICR), chronic hazard index (HIC), and acute hazard index (HIA) for these locations. Yorke also performs TAC emissions calculations, Air Quality Impact Analyses, and odor analysis for CEQA.

For minor or temporary sources, such as construction-only projects, a screening HRA performed with AERSCREEN and HARP2 is normally sufficient to demonstrate that the resultant MICR, HIC, and HIA are below CEQA significance thresholds. Thus, the analysis would be deemed complete and a refined HRA would not be required. A screening approach is also used for initial assessments of stationary source projects to determine whether a refined HRA is necessary. For larger projects involving permanent stationary sources that may have long-term impacts, a refined HRA performed with AERMOD and HARP2 is sometimes required to demonstrate that risks are below thresholds. Refined modeling may be needed because AERSCREEN uses simulated meteorological data to calculate more conservative (higher) results than AERMOD, which uses actual meteorological data for the vicinity of the project site.

For some projects, it may be necessary to conduct a mobile source HRA in accordance with Air District procedures. For a mobile source HRA, local ambient air quality data are used if the project location (vicinity) would have proximate TAC emissions that contribute to risk in a designated impacted area, commonly referred to as an air toxic “hot spot”. In general, this would be the case if a project places a sensitive land use – often caused by mixed-use residential infill developments – within a specified distance of a major freeway, a major arterial roadway, and/or railroad tracks. In such situations, the CARB Air Quality and Land Use Handbook should be consulted for recommendations on siting distances for sensitive land uses near sources of air contaminants consistent with applicable guidance from the local Air District. Similar to a stationary source HRA, AERSCREEN would be used initially, and AERMOD would be used if necessary.

Once the health risk determinations are complete, the results are compared to Lead Agency-defined significance criteria. If construction and operation of a project will not expose sensitive receptors to substantial pollutant concentrations (risks), the project has a less than significant impact on public health. If a project will cause cancer or non-cancer risks to residents, workers, and identified sensitive receptors in excess of numeric thresholds, then the project is required to incorporate mitigation measures (controls) to reduce emissions and resultant risks to below thresholds. The results are typically compiled in a Technical Report that supports the CEQA document prepared by or under the auspices of the Lead Agency.

Main Office:

31726 Rancho Viejo Rd. Suite 218
San Juan Capistrano, CA 92675

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