Learn about Risk Assessment

Commonly asked questions and answers about risk assessment are listed below, if you have other questions please use the contact us form for assistance.

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.

Risk Assessment Basics

EPA uses risk assessments 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.

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

• Human Health
• Ecological

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:

• Conducting a human health risk assessment
• Conducting an ecological risk assessment

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.

Risk managers then 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.

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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Getting Involved

• A Community Guide To Superfund Risk Assessment--What It's All About And How You Can Help
• Superfund Today: Focus on Revisions to Superfund's Risk Assessment Guidance (1999) (2 pp, 50 K, About PDF)
• Risk-Screening Environmental Indicators (RSEI) Model

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Risk Assessment Terminology

Most risk assessment terminology can be found in the Risk Assessment Glossary, but below we include the meaning behind "variability","uncertainty", and "probabilistic modeling." For these, consideration must be given to two important factors throughout the development of a risk assessment: variability and uncertaintyuncertaintyUncertainty 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..

VariabilityVariabilityThis 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. - 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.

UncertaintyUncertaintyUncertainty 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. - 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.

Probabilistic ModelingProbabilistic ModelingA 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 distribution 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., 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 distribution 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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Peer Review

Peer reviewPeer reviewA documented critical review of work by qualified individuals (or organizations) who are independent of those who performed the work, but are collectively equivalent in technical expertise. A peer review is conducted to ensure that activities are technically adequate, competently performed, properly documented, and satisfy established technical and quality requirements. The peer review is an in-depth assessment of the assumptions, calculations, extrapolations, alternate interpretations, methodology, acceptance criteria, and conclusions pertaining to specific work and of the documentation that supports them. is a documented critical review of a scientific/technical work product conducted by scientific experts who are independent of those who produced the product. 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.

• Products and Publications Related to Scientific Coordination Produced by the Office of the Science Advisor (OSA)
• Frequent Questions about Peer Review
• Peer Review Handbook, 4th Edition, 2015
• Peer Review Agenda

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) Exit for highly complex and/or critical scientific topics.

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History of Risk at EPA

EPA was involved with risk assessment practices since EPA’s early days, although risk assessment per se was not a formally recognized process then. EPA completed its first risk assessment document in December 1975 called the Quantitative Risk Assessment for Community Exposure to Vinyl Chloride (Kuzmack and McGaughy, 1975*). The next significant document appeared in 1976: Interim Procedures and Guidelines for Health Risk and Economic Impact Assessments of Suspected Carcinogens (Train, 1976*). The preamble of this document, signed by the Administrator, signaled the Agency’s intent that "rigorous assessments of health risk and economic impact will be undertaken as part of the regulatory process." A general framework described a process to be followed in analyzing cancer risks of pesticides, and the document recommended that the health data be analyzed independently of the economic impact analysis. The following links exit the EPA website Exit

• 1980s: EPA announced the availability of water quality criteria documents for 64 contaminants (USEPA, 1980*). This was the first application of quantitative procedures developed by EPA to a large number of carcinogens, and the first EPA document describing quantitative procedures used in risk assessment.
  
  Then in 1983, the National Academy of Science (NAS) published Risk Assessment in the Federal Government: Managing the Process (NRC, 1983*; commonly referred to as the “Red Book”). EPA has integrated the principles of risk assessment from this groundbreaking report into its practices to this day. The following year, EPA published Risk Assessment and Management: Framework for Decision Making (USEPA, 1984*), which emphasizes making the risk assessment process transparent, describing the assessment’s strengths and weaknesses more fully, and providing plausible alternatives within the assessment. Also in the 80's, EPA releases the Integrated Risk Information System (IRIS), a database of human health effects that may result from exposure to various substances found in the environment.
   
• 1990s: Shortly after the publication of the Red Book, EPA began issuing a series of guidelines for conducting risk assessments (e.g., in 1986 for cancer, mutagenicity, chemical mixtures, developmental toxicology, and in 1992 for estimating exposures). Although EPA efforts focused initially on human health risk assessment, the basic model was adapted to ecological risk assessment in the 1990s to deal with risks to plants, animals and whole ecosystems.

Over time, the NAS expanded on its risk assessment principles in a series of subsequent reports, including Pesticides in the Diets of Infants and Children (NRC, 1993*), Science and Judgment in Risk Assessment (NRC, 1994*; also known as the “Blue Book”), and Understanding Risk: Informing Decisions in a Democratic Society (NRC, 1996*) . For example, the NAS places equal emphasis on fully characterizing the scope, uncertainties, limitations, and strengths of the assessment and on the social dimensions of interacting with decision makers and other users of the assessment in an iterative, analytic-deliberative process. The purpose of this process is to ensure that the assessments meet the intended objectives and are understandable. EPA risk assessment practices have evolved over time along with this progression of thought, and in many cases helped drive the evolution of thinking on risk assessment.

In 1995, EPA updated and issued the current Agency-wide Risk Characterization Policy (USEPA, 1995a*). The Policy calls for all risk assessments performed at EPA to include a risk characterization to ensure that the risk assessment process is transparent; it also emphasizes that risk assessments be clear, reasonable, and consistent with other risk assessments of similar scope prepared by programs across the Agency. Effective risk characterization is achieved through transparency in the risk assessment process and clarity, consistency, and reasonableness of the risk assessment product-TCCR. EPA’s Risk Characterization Handbook (USEPA, 2000a*) was developed to implement the Risk Characterization Policy.

The Congressional/Presidential Commission on Risk Assessment and Risk Management (CRARM) was created by the Clean Air Act Amendments of 1990 and formed in 1994. Its mandate was to make a full investigation of the policy implications and appropriate uses of risk assessment and risk management in regulatory programs, under various federal laws, designed to prevent cancer and other chronic health effects that may result from exposure to hazardous substances. More specifically, its mandate was to provide guidance on how to deal with residual emissions from Section 112 hazardous air pollutants (HAPs) after technology-based controls have been placed on stationary sources of air pollutants. In 1997, the Commission published its report in two volumes (CRARM, 1997a*; CRARM, 1997b*). These discussed the importance of better understanding and quantification of risks, as well as the importance of evaluating strategies to reduce human and ecological risks.

• 2000s: EPA’s risk assessment principles and practices build on our own risk assessment guidances and policies — such as the Risk Characterization Policy; Guidance for Cumulative Assessment, Part 1: Planning and Scoping (USEPA, 1997a*); the Risk Assessment Guidance for Superfund, or RAGS (USEPA, 1989a*, and subsequent updates); EPA's Information Quality Guidelines (USEPA, 2002a*); and A Summary of General Assessment Factors for Evaluating the Quality of Scientific and Technical Information (USEPA, 2003a*) — as well as the NAS, the CRARM, and others’ concepts. It is understood that risk assessment provides important information about the nature, magnitude, and likelihood of possible environmental risks to inform decisions — principles that evolved out of these many efforts.
   

  Science and Decisions: Advancing Risk Assessment (NRC, 2009) (commonly referred to as the “Silver Book”) provided updated recommendations from the NAS aimed at improving technical analysis (by incorporating improvements in scientific knowledge and techniques) and utility of risk assessment for decision making. The Silver Book recommends that risk assessment should be viewed as a method for evaluating the relative merits of various options for managing risk, and not as an end in itself. This has a number of implications for the practice of risk assessment. It implies a greater need for upfront planning of the risk assessment, and the involvement of risk managers, risk assessors, and other stakeholders helps to determine the risk-management questions that risk assessment should address. They also recommended the technical analyses within the risk assessment should be more closely aligned with the questions to be answered.

  For example, the level of detail of uncertainty and variability analyses should be determined by what is needed to inform risk management decisions. EPA used some of the recommendations from the Silver Book to support the development of the Human Health Risk Assessment Framework, and is currently working to incorporate other recommendations into its risk assessment policies and practices.

*Source: EPA Staff Paper on Risk Assessment Principles & Practices

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Getting Help with Risk Assessment Issues

EPA is dedicated to helping you with whatever risk assessment issues you come across, but we recommend you try these first 2 steps before using the "contact us" option.

1. Contact the EPA hotline(s). EPA has several topic specific hotlines, we recommend you try these if you know what type of issue you are requesting assistance with.
2. Contact the Program Office or other Federal Agency. Some risk assessment related issues may actually be handled by other parts of the US Government.
3. Contact your local EPA Regional Office. EPA has in-house risk assessors on hand to assist our local offices and can direct you to the proper channels you need to report or receive assistance with a risk assessment issue. Visit our Where you Live page for contact information.

EPA Hotlines

This is a short list of hotlines related to risk:

For the full list of visit EPA Hotlines.

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Program Offices

The table below outlines several EPA offices or other federal agencies that are responsible for assessing and managing risks associated with particular stressors. Though the EPA Office contacts listed below go to the head of each office, you will need to request a "risk assessment specialist" so you are put in touch with the appropriate EPA staff.

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