More on HAP risk assessment methods. See also Scorecard's Guide to Health Risk Assessment.

CANCER RISKS

Added cancer risk - the estimated individual risk of getting cancer due to a lifetime exposure to outdoor hazardous air pollutants. Note that cancer risk estimates are calculations based on models - they are useful for ranking purposes but are not necessarily predictive of any actual individual's risk of getting cancer.

Because the Clean Air Act's goal is to reduce lifetime cancer risks from hazardous air pollutants to one in one million, Scorecard expresses added cancer risk in these units: an added risk of 550 per 1,000,000, for example, is 550 times higher than the Clean Air Act goal.

Population in areas where cancer risk exceeds one in ten thousand (10-4) - the number of people living in census tracts where the added cancer risk associated with outdoor hazardous air pollutant exposures is greater than 1 in 10,000. This number of people are exposed to levels of HAPs that exceed the Clean Air Act's cancer risk goal by a factor of 100.

NONCANCER HAZARDS

Single chemical hazard index - the ratio of a hazardous air pollutant concentration divided by its Reference Concentration, or safe exposure level. If this "hazard index" exceeds one, people are exposed to levels of HAPs that may pose noncancer health risks. Note that noncancer risk estimates are calculations based on models - they are useful for ranking purposes but are not necessarily predictive of any actual individual's risk of getting a specific disease.

To attain the Clean Act's goal of "an ample margin of safety to protect public health," a chemical's hazard index should be substantially below one. A hazard index of 55, for example, is 55 times higher than the Clean Air Act goal.

Cumulative hazard index - the total hazard index, summing over all HAPs with noncancer effects in an area. Each HAP contributes its single chemical hazard index to the total. Scorecard calculates a cumulative index across all health effects, and also effect-specific hazard indices (for neurotoxicity, reproductive toxicity, etc.)

Population in areas where hazard index exceeds 1 - the number of people living in census tracts where estimated hazardous air pollutant concentrations exceeds their reference concentrations (hazard index is greater than one). This number of people are exposed to levels of HAPs that exceed the Clean Air Act's noncancer risk goal by some factor.

NOTE - SCORECARD REPORTS POPULATION-WEIGHTED AVERAGES FOR ITS RISK MEASURES
Whenever Scorecard reports risk measures for an area comprised of multiple census tracts (county, state, or national HAP reports), each tract contributes to the value for an area in proportion to its population. If a census tract has a very high added cancer risk, but no people living there, it will not contribute to the average added cancer risk for the county that contains it.