CEQA HRA work is the slow link in many EIRs.
Here is where the workflow usually breaks.

The California Environmental Quality Act applies to discretionary projects approved by state or local agencies in California. A CEQA review is triggered when a public agency considers approving a project that could have a physical effect on the environment. Where the project is sited near sensitive receptors (homes, schools, daycares, hospitals) and emits toxic air contaminants or diesel particulate matter, the lead agency typically requires a Human Health Risk Assessment as part of the air quality analysis in the Environmental Impact Report. The HRA is the section that quantifies cancer risk and chronic hazard from project emissions at the nearest exposed individuals.

A CEQA HRA is typically required when a project will emit toxic air contaminants or diesel particulate matter, and sensitive receptors are located within the zone of impact of the project. Lead agencies and air districts apply their own significance thresholds to decide when the analysis is required, but the trigger is almost always the combination of TAC or DPM emissions plus nearby sensitive land uses. Both construction-phase emissions (haul truck DPM, on-site equipment) and operational-phase emissions count. The HRA quantifies the resulting cancer risk and hazard index at the maximally exposed individual.

Significance thresholds for CEQA HRAs vary by lead agency. The South Coast Air Quality Management District (SCAQMD) typically uses 10 in a million (1 x 10 to the negative 5) for project-level cancer risk and a chronic hazard index of 1.0. Bay Area AQMD, San Joaquin Valley APCD, and other districts publish their own thresholds, and some lead agencies layer additional cumulative thresholds on top. The threshold the analysis is measured against should be confirmed with the lead agency and the relevant air district before the HRA is finalized.

Both rely on the OEHHA 2015 Air Toxics Hot Spots Program Risk Assessment Guidelines as their methodological backbone, but they live in different regulatory tracks. An AB2588 Hot Spots HRA is a recurring facility-level filing under the California Air Toxics Hot Spots Information and Assessment Act, submitted to the local air district for an existing operating facility. A CEQA HRA is a project-level analysis prepared as part of an Environmental Impact Report for a proposed project, including construction-phase emissions and operational-phase emissions. LSARS supports both because the underlying exposure math is the same.

In practice, yes. California air districts and lead agencies expect CEQA HRAs to follow the OEHHA 2015 Air Toxics Hot Spots Program Risk Assessment Guidelines, including the Age Sensitivity Factors that increase exposure assumptions for the third trimester through age 16. The CARB HARP2 software implements OEHHA 2015 and is the de facto reference implementation reviewers use to reproduce results. LSARS implements the same OEHHA 2015 methodology and is parity-validated against HARP2 to five significant figures.

The pollutants that drive a CEQA HRA are the toxic air contaminants on the OEHHA list of substances with health values, plus diesel particulate matter where on-site or mobile diesel equipment is involved. For most projects, DPM dominates the cancer risk number because of its high cancer unit risk factor and its prevalence in construction equipment, haul trucks, and on-site generators. The full TAC list, with cancer unit risk factors and chronic and acute reference exposure levels, is maintained by OEHHA and is built into the LSARS HRA engine.

Construction-phase HRA is required when project construction is long enough and intense enough that emissions from equipment, haul trucks, and other on-site sources produce meaningful exposure at nearby sensitive receptors. The analysis combines a construction emissions inventory (often from CalEEMod), dispersion modeling (typically AERMOD or AERSCREEN), and OEHHA 2015 exposure math with construction-specific exposure durations. LSARS treats construction-phase and operational-phase HRA as a single workflow so cancer risk and hazard index can be summed across both phases without rebuilding the calculation.

When a CEQA HRA shows that project cancer risk or chronic hazard exceeds the lead agency significance threshold, CEQA requires the lead agency to identify and adopt feasible mitigation measures to reduce the impact to less than significant where possible. If the impact cannot be mitigated below the threshold, the lead agency must adopt a Statement of Overriding Considerations to approve the project. The HRA itself does not impose mitigation. It quantifies the impact so the lead agency can make a defensible significance determination and the public can review the analysis.