Steps of Assessment

There are two major components in risk assessment. Basically, the hazardous substance must be characterized to determine its toxicity and the person or ecosystem must be assessed to determine the degree of exposure. A substance may be highly toxic, but if there is no exposure to the ecosystem (animal and plant populations and their habitats) or a human, then the risk is very low. Conversely, a substance may be less hazardous but will be toxic if there is a large amount of exposure over an extended period of time.

There are four primary steps included in making a risk assessment: hazard identification, exposure assessment, toxicity assessment, and risk characterization.

Hazard identification is the process that determines whether or not a substance causes harm. There are four main methods of hazard identification. These methods are animal studies, test tube studies, comparison studies or epidemiological studies. Animal studies are most common. Test tube (in vitro) studies are experiments conducted in an isolated, highly controlled environment. Comparison studies judge one chemical against a similar substance that is known to be harmful. Epidemiological studies identify trends of illness within a population that can be correlated to exposures to a substance. These studies are the most reliable, but are difficult to obtain because exposures can vary greatly. In an ecosystem risk assessment, the stressor and the ecosystems potentially at risk are identified.

Exposure assessment evaluates the route of exposures (inhalation, ingestion, absorption), the length of exposure, whether exposure was continuous or intermittent, and how susceptible the person exposed was to the substance. Exposure levels vary with each individual, complicating the ability of risk assessment to have pinpoint accuracy. To determine if human health is at risk of disease from contaminants, two things must occur: there must be an exposure to a contaminant; and the contaminant must be toxic and be in sufficient quantity to cause harm. A complete exposure pathway must be present for illness to occur. The elements of an exposure pathway are:

  1. the source - how the contaminant got into the environment,
  2. the transport media - how the contaminant moves through the environment,
  3. the exposure point - how people came in contact with the contaminant,
  4. the exposure route - how the contaminant entered the body,
  5. the receptor population - how susceptible the population is to the contaminant (adapted from ATSDR, 1994).

Toxicity assessment extrapolates from animal studies the quantity of a substance that is needed to cause harm in a human. Assumptions about the size and other differences between animals and humans and the expected doses encountered in the environment are used to estimate toxicity. This assessment is often called a "dose-response" curve.

Risk characterization is the combination of toxicity and exposure assessments to try to predict what will happen to people that are exposed to a hazardous substance. Such a characterization is an estimate and not a fact. The uncertainty of toxicity assessments and exposure assessments makes it difficult to provide an exact risk characterization. A final description of the risk provides information on the confidence the risk assessor has in the results, and identifies a threshold for adverse effects.

For the other risk-related pages, please see:

  • Risk Assessment Standards This page provides a discussion on standards for risk assessment based on the American Society for Testing and Materials.
  • Ecological Risk Assessment This page focuses on the unique aspects of ecological risk assessment.
  • Risk Management This page provides information about plans for managing and/or preventing risk.
  • Risk Communication This page provides information about communicating with communities facing contaminated sites.
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