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Risk Assessment

Basic Information

Before finding out about risk assessment there are some fundamental principles you need to understand:

What is risk? What is a stressor?

While there are many definitions of the word risk, EPA considers risk to be the chance of harmful effects to human health or to ecological systems resulting from exposure to an environmental stressor.

A stressor is any physical, chemical, or biological entity that can induce an adverse response. Stressors may adversely affect specific natural resources or entire ecosystems, including plants and animals, as well as the environment with which they interact.

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What is risk assessment?

EPA uses risk assessment to characterize the nature and magnitude of health risks to humans (e.g., residents, workers, recreational visitors) and ecological receptors (e.g., birds, fish, wildlife) from chemical contaminants and other stressors, that may be present in the environment.  Risk managers use this information to help them decide how to protect humans and the environment from stressors or contaminants. Note that “risk managers” can be:

  • federal or state officials whose job it is to protect the environment,
  • business leaders who work at companies that can impact the environment, or
  • private citizens who are making decisions regarding risk.

At EPA, environmental risk assessments typically fall into one of two areas:

Risk assessment is, to the highest extent possible, a scientific process.  In general terms, risk depends on the following factors:

  • How much of a chemical is present in an environmental medium (e.g., soil, water, air),
  • How much contact (exposure) a person or ecological receptor has with the contaminated environmental medium, and
  • The inherent toxicity of the chemical.

Following a planning and scoping stage where the purpose and scope of a risk assessment is decided, the risk assessment process usually begins by collecting measurements that characterize the nature and extent of chemical contamination in the environment, as well as information needed to predict how the contaminants behave in the future.  Here are some useful links to get started:

Based on this, the risk assessor evaluates the frequency and magnitude of human and ecological exposures that may occur as a consequence of contact with the contaminated medium, both now and in the future. 

This evaluation of exposure is then combined with information on the inherent toxicity of the chemical (that is, the expected response to a given level of exposure) to predict the probability, nature, and magnitude of the adverse health effects that may occur. In the ideal world, all risk assessments would be based on a very strong knowledge base (i.e., reliable and complete data on the nature and extent of contamination, fate and transport processes, the magnitude and frequency of human and ecological exposure, and the inherent toxicity of all of the chemicals).  However, in real life, information is usually limited on one or more of these key data needed for risk assessment calculations.  This means that risk assessors often have to make estimates and use judgment when performing risk calculations, and consequently all risk estimates are uncertain to some degree.  For this reason, a key part of all good risk assessments is a fair and open presentation of the uncertainties in the calculations and a characterization of how reliable (or how unreliable) the resulting risk estimates really are.

Developing a risk assessment is often an iterative process, which involves researchers identifying and filling data gaps in order to develop a more refined assessment of the risk.  This in turn may influence the need for risk assessors and risk managers to refine the scope of the risk assessment further triggering the need for more data or new assumptions.

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What is risk management?

As described in EPA’s Risk Characterization Handbook (PDF) (89 pp, 8.9MB, about PDF), "Risk Management" is the process which evaluates how to protect public health. Examples of risk management actions include deciding how much of a substance a company may discharge into a river; deciding which substances may be stored at a hazardous waste disposal facility; deciding to what extent a hazardous waste site must be cleaned up; setting permit levels for discharge, storage, or transport; establishing national ambient air quality standards; and determining allowable levels of contamination in drinking water.

Risk assessment provides "INFORMATION" on potential health or ecological risks, and risk management is the "ACTION" taken based on consideration of that and other information, as follows:

  • Scientific factors provide the basis for the risk assessment, including information drawn from toxicology, chemistry, epidemiology, ecology, and statistics - to name a few.
  • Economic factors inform the manager on the cost of risks and the benefits of reducing them, the costs of risk mitigation or remediation options and the distributional effects.
  • Laws and legal decisions are factors that define the basis for the Agency’s risk assessments, management decisions, and, in some instances, the schedule, level or methods for risk reduction.
  • Social factors, such as income level, ethnic background, community values, land use, zoning, availability of health care, life style, and psychological condition of the affected populations, may affect the susceptibility of an individual or a definable group to risks from a particular stressor.
  • Technological factors include the feasibility, impacts, and range of risk management options.
  • Political factors are based on the interactions among branches of the Federal government, with other Federal, state, and local government entities, and even with foreign governments; these may range from practices defined by Agency policy and political administrations through inquiries from members of Congress, special interest groups, or concerned citizens.
  • Public values reflect the broad attitudes of society about environmental risks and risk management.

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Who evaluates the risks?

The table below outlines which EPA office or other federal agency is responsible for assessing and managing risks associated with particular stressors.

Stressor EPA Office Other Federal Agencies
Air Pollution Office of Air and Radiation  
Hazardous substances, pollutants, and waste Office of Solid Waste and Emergency Response  
Pharmaceuticals   FDA’s Center for Drug Evaluation and Research
Pesticides Office of  Pesticide Programs U.S. Consumer Product Safety Commission (toys and other consumer products)
FDA's Center for Food Safety and Applied Nutrition
Radiation including radon Radiation Programs  
Toxic substances, human exposure, environmental exposure Office of Pollution Prevention and Toxics

Office of Research and Development
Vaccines   FDA’s  Center for Biologics Evaluation and Research
Water pollution Office of Water  

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How does EPA conduct risk assessments?

At EPA, environmental risk assessments typically fall into one of two areas: human health risk assessments or ecological risk assessments. These are described in steps or parts due to the differences in how each of these are conducted at EPA.

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Where do I find EPA risk assessments?

Because risk assessments are performed all over EPA (see the EPA Organization Chart for other EPA Offices and Regions), risk assessments are produced by many of EPA's Regions and Program Offices. Here is a list of primary risk assessment sources:

See Tools & Guidance for a list of more resources.

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Where can I find additional information on risk assessment for the public?

EPA has posted a few citizen guides that may be of help for those new to risk assessment. Here is a list of available publications:

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What can I do? Participating in risk assessments

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What does EPA mean by "variability","uncertainty", and "probabilistic modeling"?

Consideration must be given to two important factors throughout the development of a risk assessment: variability and uncertainty.

Variability - Refers to the range of toxic response or exposure. For example, the dose that might cause a toxic response can vary from one person to the next depending on factors such as genetic differences, preexisting medical conditions, etc. Exposure may vary from one person to the next depending on factors such as where one works, time spent indoors or out, where one lives, how much people eat or drink, etc.

Uncertainty - Refers to our inability to know for sure - it is often due to incomplete data. For example, when assessing the potential for risks to people, toxicology studies generally involve dosing of sexually mature test animals such as rats as a surrogate for humans. Since we don't really know how differently humans and rats respond, EPA often employs the use of an uncertainty factor to account for possible differences. Additional consideration may also be made if there is some reason to believe that the very young are more susceptible than adults, or if key toxicology studies are not available. [Learn more about determining uncertainty]

Probabilistic Modeling, a related term, is a technique that utilizes the entire range of input data to develop a probability distribution of exposure or risk rather than a single point value. The input data can be measured values and/or estimated distributions. Values for these input parameters are sampled thousands of times through a modeling or simulation process to develop a distibution of likely exposure or risk. Probabilistic models can be used to evaluate the impact of variability and uncertainty in the various input parameters, such as environmental exposure levels, fate and transport processes, etc.

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What is peer review?

Peer review is a documented critical review of a scientific/technical work product which is conducted by scientific experts who are independent of those who performed the work.  Peer review can provide an independent evaluation of the assumptions, calculations, extrapolations, alternate interpretations, methodology, acceptance criteria, and conclusions pertaining to the scientific/technical work product.

When evaluating the scientific rigor of our risk assessments, EPA utilizes both standing federal advisory groups of experts such as the Science Advisory Board (SAB) and the FIFRA Scientific Advisory Panel, as well as ad hoc panels to provide peer review.  EPA will occasionally seek peer review from outside expert groups such as the National Academy of Science (NAS) for highly complex and/or critical scientific topics.

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