General Information on 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 effects, affecting
such things as development, reproduction, respiration,
the liver, kidney or other organs. Health effects of chemicals
are discovered in a number of ways. Researchers can study
groups of people that have been exposed to the chemicals
in the past, usually at the workplace. They can also expose
volunteers to specific amounts of a chemical and record
the effects. 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.
Using Health Benchmarks
Groups of experts at government agencies, such as USEPA
and California EPA, look at all of the studies done on
the health effects of a chemical, and recommend measures
of toxicity, known as unit risk factors and reference
concentrations, that can be used to evaluate public exposure
to those chemicals.
Unit risk factors are measures used for carcinogens
that estimate the increased risk of getting cancer associated
with the concentration of the chemical in the air that
you are breathing. A risk of less than one in a million
is considered to be negligible.
Reference concentrations are measures developed
for noncarcinogens. Exposure to a chemical below the 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, to evaluate the potential
health effects of air toxic concentrations. 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. Air concentrations that
are below these health benchmarks are 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
health effects and actual harmful levels will vary substantially
from pollutant to pollutant.
