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California Air Resources Board

Toxics Risk Estimation
ARB's ADAM Air Quality Database
Toxics Risk Assessment Methodology

The potential cancer risk estimates presented on this website reflect the risk assessment methodology finalized by the Office of Environmental Health Hazard Assessment (OEHHA) on March 6, 2015. Information on the OEHHA risk assessment methodology and the public process used to develop them can be found at Over the last 25 years, toxic emissions and ambient concentrations have declined and California's Air Toxics Program continues to effectively reduce emissions and associated potential health impacts from sources of air toxics.

The estimated cancer risk is calculated using advances in science showing that early-life exposures to cancer-causing air toxics contribute to an increased lifetime risk of developing cancer, or other adverse health effects, compared to exposures that occur in adulthood. The risk assessment methodology addresses this greater sensitivity and incorporates the most recent data on childhood and adult exposure to air toxics.

The cancer risk estimates on these web pages are intended to place the monitoring results into perspective if they are used for comparison with other cancer risk estimates. Cancer risk estimates should not be interpreted as expected rates of disease, but rather as estimates of potential for disease based on current knowledge and a number of assumptions. In general, cancer risk estimates are based on the best science available, assumptions of long-term exposure activities and estimated annual concentrations that may, or may not, vary in real time. The data on this website are taken from ARB's ambient air monitoring stations located in urban areas of California. Localized cancer risk estimates may be higher or lower. For example, cancer risk estimates may be lower in rural locations since they are typically less impacted by mobile sources, industrial sources, freight hubs, etc.

With ambient data and population risk estimates, cancer risk is typically expressed as the chance that people within the population may have of developing cancer from exposure to the reported annual mean concentration over 70 years. Cancer risk estimates are expressed as a probability; typically, either in scientific notation (e.g., 1x10-5) or as a whole number (e.g., 10 chances per million). At times, they may also be expressed as a percentage (e.g., 0.001%). For example, 10 chances per million cancer risk estimate indicates there are 10 chances in a million (or a 0.001% chance) that people in the population may develop cancer from exposure to the monitored air toxics.

For more information on how to interpret cancer risk estimates, and to read about ARB and District plans for addressing air toxics, please see the Air Resources Board and California Air Pollution Control Officer's Association Risk Management Guidance for Stationary Sources of Air Toxics (July 23, 2015). This document is available at, and can be found in the "Risk Management Policy for Risk Assessment Using the Inhalation Pathway" section.