Health risk assessment is a process that combines available information
on chemical toxicity and exposure to estimate the probability that
someone will experience adverse health effects as a result of exposure
to a chemical. The goal is to characterize health risks in an
understandable way, so that decisions can be made about the public
health significance of chemical exposures and the need for taking action
to control pollution.
Risk assessment is typically broken down into four, sequential
steps: (1) hazard identification, (2) dose-response
assessment, (3) exposure assessment, and (4) risk
characterization. Scorecard's About the Chemicals section can help you
with each of these steps if you are concerned about exposure to toxic
IDENTIFY MAJOR HEALTH EFFECTS OF CONCERN
Every chemical profile in Scorecard identifies the kinds of health
effects a chemical is recognized to or suspected of causing. This
information is the first step of risk assessment, because it can help
focus the analysis on the health effects of greatest concern and
identify the types of information needed from the dose-response and
exposure assessment steps. One chemical may cause several different
types of adverse health effects (ranging from relatively minor skin
irritation to life-threatening cancer), and these effects may manifest
themselves at different exposure levels. Since there is rarely
sufficient information to conduct a thorough risk assessment on every
health effect associated with a chemical, it is generally necessary to
focus on the most important specific forms of toxicity (e.g.,
neurotoxicity, or carcinogenicity) that can be caused by a chemical, and
then evaluate whether these forms of toxicity might be expressed in exposed humans.
SELECT AN APPROPRIATE RISK ASSESSMENT VALUE
Chemicals vary dramatically in their potency, or ability to cause
serious health effects at a given dose. Scientists analyze both animal
and human studies to define the relationship between chemical doses and
toxic responses. Separate risk assessment values are calculated
for carcinogens (potencies) and non-carcinogens (reference doses
or concentrations). Cancer potencies express how much added cancer risk
is associated with lifetime exposure to a unit dose of a chemical
(presented as the additional cancer risk associated with an average
daily dose of one milligram of a chemical per kilogram of bodyweight).
Reference doses and concentrations are estimates of the daily
exposure to the human population (including sensitive subgroups) that is
likely to be without an appreciable risk of deleterious effects over a
Scorecard lists the risk assessment values available for a chemical in
the Information Needed for Safety Assessment section of every chemical
profile. Select a risk assessment value based on the health effects you are concerned about (cancer or noncancer health effects).
ESTIMATE THE CHEMICAL DOSE PEOPLE RECEIVE FROM AN EXPOSURE
The dose of a chemical is the amount a person takes in as a result of
exposure to a chemical in contaminated air, water, soil or food. It is
estimated by combining information about a chemical's concentration in
the environment (such as parts per million in drinking water) with
information about intake rates (such as average daily drinking water consumption). Although it is often difficult to obtain the required data, chemical risk assessments should be based on total exposure assessments, which attempt to include all the different ways someone could be exposed to a chemical.
The magnitude, duration, and timing of the doses that people receive as a result of chemical exposures are critical to evaluating potential health risks. Note that Scorecard currently only has risk assessment values that have been derived to protect people against chronic (prolonged) exposures to a chemical. These values are not generally appropriate for assessing the impact of acute (short-term) exposures.
Scorecard indicates whether national exposure monitoring data are
available for a chemical in the Information Needed for Safety Assessment
section of every chemical profile. References for this section lead
directly to available online sources of information about chemical exposure levels in the U.S. Unfortunately, detailed data on local concentrations of toxic chemicals are rarely available. If you are concerned about the health impact of toxic chemical releases, your first request of polluting facilities and local regulators should be to provide monitoring data or exposure estimates.
CHARACTERIZE THE HEALTH RISKS OF CHEMICAL EXPOSURE
Risk characterization integrates the information from the first three
steps of risk assessment to develop a qualitative or quantitative
estimate of the likelihood that any of the hazards associated with a
chemical will be realized in exposed people.
For chemicals that cause noncancer health effects, risks are typically
characterized using a measure called the hazard index. The hazard index
is calculated by dividing the dose received as a result of some exposure
scenario by the reference dose for a chemical:
HI = Dose from Exposure/Reference Dose
If the HI is greater than 1, an individual is at some risk of adverse health effects, because their dose exceeds a regulatory agency's estimate of the allowable daily intake. To characterize population risks, the number of people receiving doses greater than the reference dose is often calculated to obtain an exceedance count. Note that noncancer risk characterization does not generally involve quantitative predictions of how much someone's risk of adverse effects are increased when their exposures exceed a reference dose.
For carcinogens, individual risks are typically characterized by
estimating the increased risk of cancer associated with an exposure. Risks to an exposed population are characterized by estimating the additional number of additional cancer cases that may occur.
Individual's Increased Cancer Risk = Cancer Potency Value x Lifetime Average Daily Dose
Predicted Additional Cancer Incidence = Average Individual Cancer Risk x Number of People Exposed
CONDUCTING A RISK ASSESSMENT IS NOT ROCKET SCIENCE
As this guide illustrates, it is not difficult to conduct a screening-level risk assessment if data are available on a chemical's health hazards and exposure. While the risk assessment process can be made much more complicated and intimidating, Scorecard makes the basic tools of risk assessment available to everyone.
IMPORTANT LIMITS TO HEALTH RISK ASSESSMENT
Risk assessment combines what is currently known about chemical toxicity
and exposure to characterize potential health problems, but there are
significant uncertainties in each step of the process. Some
uncertainties are addressed by regulatory agencies through the use of
health-protective assumptions that may result in overestimates of risk.
These include assuming that animal toxicity test results are predictive
of human responses, and that there is some risk of a carcinogenic
response at even extremely low doses. These health-protective
assumptions are often criticized by chemical defenders as leading to
biased risk assessments. However, there are other uncertainties that
are currently ignored in conventional risk assessment, which may result
in underestimating health risks. Important factors that could affect
health outcomes are often ignored because critical data are lacking. For
example, risk assessment values are derived based on the assumption that
people are exposed to a single chemical at a time, and that there is no
significant interaction between chemicals that heightens the probability
of adverse outcomes. Variations in susceptibility to a toxic chemical
between people are often ignored, even though it is known that factors
such as health status or genetic characteristics can greatly affect how
someone responds to chemical exposure.
MORE INFORMATION ABOUT HEALTH RISK ASSESSMENT
Scorecard recommends U.S. EPA's Risk
Assessment for Toxic Air Pollutants: Citizen's Guide as an
accessible introduction to the practice of health risk assessment.
Further reading on the debate over whether risk assessment is biased:
Nichols A.L. and R.J. Zeckhauser. The perils of prudence: How
conservative risk assessments distort regulation. Regulatory
Toxicology and Pharmacology 8: 61-75. 1988.
Finkel A.M. Is risk assessment really too conservative? Revising the
revisionists. Columbia Journal of Environmental Law 14:427-467. 1989.