HOW WE ESTIMATE RISK FROM AIR TOXICS
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.
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.
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
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.
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.
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.
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 National-Scale Air Toxics Assessment (NATA).
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. 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 NATA. If the modeled air concentration is below the health benchmark (the risk ratio is less than or equal to one) there is probably 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. The risk ratio also indicates just how much higher the air concentration is than the health benchmark, and indicates how much reduction may be needed.
SOURCES OF INFORMATION
The health benchmarks used by NJDEP to evaluate the NATA 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:
For more information on compiling a risk assessment,
Manual 1003: Guidance on Preparing a Risk Assessment
Protocol for Air Contaminant Emissions.
Risk Assessment for Toxic Air Pollutants: A Citizen's Guide
Evaluating Exposures to Toxic Air Pollutants: A Citizen's Guide
Air Pollution and Health Risk