How We Estimate Risk From Air Toxics

HEALTH EFFECTS OF AIR TOXICS

Air toxics can be broadly grouped into two categories according to their health effects: carcinogens (cancer-causing) or noncarcinogens. Carcinogens are those chemicals that have been shown to cause cancer, either in people or animals. Noncarcinogens have other kinds of health impacts, affecting development, reproduction, respiration, the liver, kidney or other organs.

Health effects of specific chemicals are determined in a number of ways. Researchers can study groups of people that have been exposed to the chemicals in the past, usually in a workplace. They can also expose volunteers to specific amounts of a chemical and record the effects. However, most health effects information comes from studies of animals that are exposed in the laboratory to specific doses of a chemical for specific periods of time.

DEVELOPING HEALTH BENCHMARKS

Government agencies, such as USEPA and the California Office of Environmental Health Hazard Assessment (OEHHA), have established procedures for developing human toxicity values from chemical studies in order to determine a “safe” level of human exposure. Groups of experts look at all of the available health effects studies done on a chemical, and refine the information into a dose-response value that can be used to estimate the risk to public health from exposure to that chemical. These toxicity values are referred to as unit risk factors and reference concentrations.

Unit risk factors are toxicity values used for carcinogens, which estimate the increased risk of getting cancer that is associated with the concentration of the chemical in air that you are breathing. A cancer risk of less than one in a million is usually considered to be negligible.

Reference concentrations are toxicity values developed for noncarcinogens. Exposure to a chemical below its reference concentration, even over a long period of time, is not expected to have any negative effect on health.

These unit risk factors and reference concentrations can be used as health benchmarks. For carcinogens, the health benchmark is the air concentration that would result in a one in a million increase in the risk of getting cancer if a person inhaled that concentration over a whole lifetime. For noncarcinogens, health benchmarks are set at the reference concentration.

RISK ASSESSMENT

The process of estimating the chance of developing health problems as a result of exposure to a chemical in the environment is called risk assessment. Risk assessment can be used to evaluate the potential health effects of air toxic concentrations measured by air monitors, or predicted by air pollution models such as those used by USEPA for the AirToxScreen Assessment.

One way of doing a risk assessment is to compare a chemical’s health benchmark to a monitored or modeled air concentration to calculate a risk ratio. The risk ratio just shows how much higher the air concentration is than the health benchmark, and indicates how much reduction may be needed. A risk ratio that is equal to or less than one (below the health benchmark) is not expected to be harmful to human health.

It is not always clear, however, how far above the health benchmark an air concentration has to be before it becomes harmful. Types of harmful effects and actual harmful levels will vary from pollutant to pollutant, and person-to-person, and some chemicals have more than one effect. Still, comparison to a health benchmark is a useful tool for evaluating air concentrations like those predicted in AirToxScreen. If the modeled air concentration is below the health benchmark (the risk ratio is less than or equal to one) there is likely no need for further concern. If the risk ratio is greater than one (the air concentration is above the health benchmark), there may be some cause for concern, and further assessment is warranted.

SOURCES OF INFORMATION

The health benchmarks used by NJDEP to evaluate the AirToxScreen data are also used by the NJDEP Air Quality Permitting Program in a routine risk assessment screening process that evaluates potential health effects from facilities seeking permits to emit air toxics. The unit risk factors and reference concentrations that are used as the basis for the health benchmarks and risk assessment can be found at:

Risk Screening Tools

TECHNICAL SUPPORT DOCUMENTS

For more information on compiling a risk assessment, see:

Technical Manual 1003: Guidance on Preparing a Risk Assessment Protocol for Air Contaminant Emissions.

For more information on how HAP reporting thresholds are updated, see:

Technical Support Document: Updating Hazardous Air Pollutant Reporting Thresholds