CHEMICAL PROFILES|Scorecard's Guide to Health Risk Assessment

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 chemicals.

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.

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 lifetime.

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).

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.

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

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.

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.

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.