California Air Resources Board

What is a health risk assessment?

A health risk assessment or HRA is a tool used to estimate the adverse health effects caused by exposure to environmental pollutants in a variety of media such as air, water, and soil. In the Air Toxics Program, CARB uses HRAs to help inform communities of potential health impacts; prioritize toxic air pollutants that are the most significant; and to help weigh the benefits and costs of reducing exposure to toxic air pollutants through various emissions reduction actions such as through an Airborne Toxics Control Measure (ATCM).

An HRA is a four steps process that includes:

For more information about the four steps process, please visit the following websites:

• U.S. Environmental Protection Agency’s (USEPA) Human Health Risk Assessment Website
• Office of Environmental Health Hazard Assessment’s (OEHHA) Risk Assessment for the Layperson

Who is responsible for developing procedures for conducting HRAs?

USEPA is a leading risk assessment agency at the federal level. In California, OEHHA in the California Environmental Protection Agency (CalEPA) has the primary responsibility for developing procedures and practices for performing health risk assessments. Other agencies within CalEPA, such as the Department of Pesticide Regulation (DPR) and the Department of Toxic Substances Control (DTSC), have extensive risk assessment programs of their own but work closely with OEHHA. In the Air Toxics Program, CARB typically conducts HRAs based on the guidelines and health values adopted by OEHHA.

What do the risk results from an HRA mean?

The risk results from an HRA are typically presented as cancer and non-cancer risks. It is important to understand that HRAs cannot be used to link individual illnesses to past chemical exposures, nor can HRAs prove that a specific chemical caused an individual’s illness. However, HRAs can be used as a valuable tool for communities and regulators in evaluating the health impacts from air toxics exposures.

When looking at the risk results, keep in mind that cancer risk is a probability of developing cancer over a person’s lifetime. It does not mean it will happen or indicate when a person will develop cancer during their lifetime. Since cancer risk does not consider an individual’s health history, it is often difficult to say how the results apply to a specific person. For example, a person with a preexisting health condition may be more vulnerable. 

Local air districts typically set cancer risk levels to determine regulatory actions. Many local air districts have set this level at a cancer risk level of one chance in a million. This level generally indicates that the risk is low as compared to the overall statewide background cancer risk. In 2019, the cancer risk from breathing the top 10 pollutants in California’s ambient air are estimated to be about 720 chances in a million.

For non-cancer risk, a hazard index that is less than one is typically not treated as a level of concern. It means the exposure to a toxic air pollutant is below the reference exposure level so no adverse health effects are expected. It is important to understand that hazard indices above one do not necessarily mean there is certainty for an adverse effect; rather, it indicates there may be the potential for adverse effects that warrant further investigation.

How accurate are HRAs?

Although the HRA estimates the adverse health impacts from exposure to air pollution, there is a great deal of uncertainty associated with an HRA. Sources of uncertainty include extrapolation of toxicity data from animals to humans, uncertainty in the estimation of emissions, and uncertainty in exposure estimates to the community. However, the assumptions that are used to estimate the potential risk of harm to public health are based on the best available science and do err on the side of health protection to avoid underestimation of risk.

Which programs at CARB will you find HRAs?

• AB 1807 Toxic Air Contaminant (TAC) Identification and Control Program – The TAC Identification and Control Act established the framework of California’s Air Toxics Program, putting in place a two-step process for risk identification and management of substances. Before a substance can be controlled, it must first be formally identified as a TAC. HRAs are used in both phases of this program. The links below will provide examples of HRAs under the AB 1807 TAC Identification and Control Program.
  • TAC Identification Reports – During the identification phase, a report (a type of HRA) is developed which considers whether there are adverse health effects of a substance which may be, or is, emitted in California. After conducting a public hearing, the Board decides whether the substance should be identified as a TAC. There are currently 23 substances that have undergone this process.
  • Airborne Toxics Control Measures – Once a substance is identified as a TAC, CARB prepares a report that outlines the need and degree to regulate the TAC through an ATCM. An assessment of health risk is included as part of the ATCM development. Currently, CARB has twenty-six mobile and stationary ATCMs.

• AB 2588 Air Toxics "Hot Spots" Risk Assessments – The Air Toxics "Hot Spots" Information and Assessment Act, requires facilities that are ranked as a high priority to submit an HRA to the air pollution control and air quality management districts. A risk assessment, as defined under the Air Toxics "Hot Spots" Act, includes a comprehensive analysis of the dispersion of hazardous substances into the environment, the potential for human exposure, and a quantitative assessment of both individual and population-wide health risks associated with those levels of exposure. The risk assessments submitted by the facilities are reviewed by OEHHA and approved by the local air district. In addition, the local air district may require facilities in the intermediate and low priority categories to also submit a health risk assessment. The links below provide guidance on conducting an HRA or risk management under the AB 2588 Air Toxics “Hot Spots” Program.
  • OEHHA Health Risk Assessment Guidelines
  • CAPCOA Industrywide Guidelines
  • Risk Management Guidance for Stationary Sources of Air Toxics

• HRAs for California Railyards – CARB worked with Union Pacific (UP) and BNSF to complete an HRA to evaluate associated health impacts of toxic air contaminants emitted from 17 major railyards in California.

• HRA for West Oakland – CARB completed an HRA to study the emissions impacts and the potential public health risk from exposures to diesel particulate matter (PM). The sources of diesel PM included activities at the Maritime Port of Oakland, the UP Railyard, and other sources in and near the West Oakland community of the Bay Area.

• HRAs for Sustainable Freight Transport– CARB is working with agency partners and stakeholders to implement a broad program that includes regulations, incentives, and policies designed to support the transformation to a more sustainable freight system and reduce community impacts from freight operations in California. HRAs may be included as part of this work. A recent example of an HRA was evaluated for the Proposed Amendments to the ATCM for In-Use Diesel-Fueled Transport Refrigeration Units (TRU) and TRU Generator Sets, and Facilities Where TRUs Operate.

Besides HRAs, does CARB measure health benefits in another way?

In addition to HRAs, regulations targeting diesel PM and NOx often include an estimation of health benefits associated with diesel PM reductions in the risk of premature deaths, hospitalizations, and emergency room visits (also known as a PM2.5 mortality and illness analysis). A recent example of a PM2.5 mortality and illness analysis was evaluated for the Control Measure for Ocean‑Going Vessels At Berth. For additional information, please see Estimating the Health Benefits Associated with Reduction in Diesel PM and NOx Emissions.