Analyzing Risks

Why Is Risk Assessment Important?

To make an effective risk management decision, risk managers and other stakeholders need to know what potential harm a situation poses and how great is the likelihood that people or the environment will be harmed. Gathering and analyzing this information is referred to as risk assessment.

The nature, extent, and focus of a risk assessment should be guided by the risk management goals. The results of a risk assessment—along with information about public values, statutory requirements, court decisions, equity considerations, benefits, and costs—are used to decide whether and how to manage the risks.

Risk assessment can be controversial, reflecting the important role that both science and judgment play in drawing conclusions about the likelihood of effects on human health and the environment. Often, the controversy arises from what we don’t know and from what risk assessments can’t tell us, because our knowledge of human vulnerability and of environmental impacts is incomplete, especially at the relatively low levels of chemical exposure commonly encountered in the general community.

A good risk management decision is based on a careful analysis of the weight of scientific evidence that supports conclusions about a problem’s potential risks to human health and the environment.


How Should Risk Be Characterized?

Risk results from a combination of hazard and exposure. Hazard is an intrinsic property of a substance or situation: for example, benzene can cause leukemia but not lung cancer; DDT can prevent eagles from reproducing in the wild, but does not affect prairie dogs; a rattlesnake bite can kill but a garter snake bite does not. Exposure means contact between the hazardous substance and a person, population, or ecosystem. The more exposure, the greater the risk. When there is no current or potential exposure, there is no risk.

Risk assessment is performed by considering intrinsic hazards, the extent of exposure to the hazards, and information about the relationship between exposures and responses. Unfortunately, we seldom have enough information to accurately determine hazards, exposures, or exposure-response relationships, so risk assessors must use a combination of scientific information and their best judgment to characterize risks. Making judgments about risk on the basis of scientific information is called "evaluating the weight of the evidence." For example, considerations involved in analyzing the weight of the evidence associated with identifying a hazard using toxicity studies in rodents include the:

It is important that risk assessors respect the objective scientific basis of risks and procedures for making inferences in the absence of adequate data. Risk assessors should provide risk managers and other stakeholders with plausible conclusions about risk that can be made on the basis of the available information, along with evaluations of the scientific weight of evidence supporting those conclusions and descriptions of major sources of uncertainty and alternative views.

Risk is determined by considering the nature, likelihood, and severity of adverse effects on human health or the environment.


The outcome of a risk assessment is called a risk characterization. Typically a risk characterization should address the following:

The level of detail considered in a risk assessment and included in a risk chacterization should be commensurate with the problem’s importance, expected health or environmental impact, expected economic or social impact, urgency, and level of controversy, as well as with the expected impact and cost of protective measures.

Risk characterizations should include sufficient information to enable:

We lack sufficient animal data on many
substances, however, drawing conclusions about
human risks from laboratory animals is uncertain.


Stakeholders’ perception of a risk can vary substantially depending on such factors as the extent to which they are directly affected, whether they have voluntarily assumed the risk (as in choosing not to wear a seatbelt) or had the risk imposed on them (as in exposure to air pollutants), and whether they are connected with the cause of the risk. For this reason, the Commission recommends that a risk assessment characterize the scientific aspects of a risk and note its subjective, cultural, and comparative dimensions (see "How Should Risks Be Analyzed?" on page 24). While this expands risk assessment beyond its traditional, more narrowly scientific scope, including these additional dimensions will help educate all stakeholders about key factors affecting the perception of risk. Such education is likely to reduce controversy and litigation and to improve communication during the risk management process.

Risk characterizations must include information that is useful for all stakeholders.


Risk characterization should form a common basis for the understanding of a problem among stakeholders. Stakeholder involvement within the Risk Management Framework should enhance the integrity of the risk assessment. Stakeholders play an important role in providing information that should be used in risk assessments and in identifying specific health and ecological concerns they would like to see addressed. For example, community stakeholders consulted at this stage can help identify groups with high exposures so that appropriate exposure assessments can be designed. Industry stakeholders can provide important information about a substance’s toxicity and lifecycle.

The integrity of a risk assessment is best assured if it is carried out or peer-reviewed independently, for example, by scientists at regulatory agencies, universities, and research institutions. To relieve some of the burden on regulatory agencies and other public institutions, however, certification, auditing, and oversight programs should be considered, so that companies, industry organizations, and other organizations or individuals can provide risk assessments that are considered credible by all stakeholders. For example, in order to place greater responsibility on the private sector for cleaning up contaminated sites, the state of Massachusetts has instituted a successful program for certifying Licensed Site Professionals to oversee or perform site assessments or cleanups.

The Need for More Data

Lack of data is a major barrier to reliable risk assessments.

We lack data on the hazards that chemicals and other stressors pose, largely because of:

As a result, many chemicals are never properly tested at all.

We lack data on actual human and ecological exposures to agents of concern, largely due to:

Because of the difficulties involved in studying chemical hazards and exposures, risk assessors cannot always accurately determine the health risks of an exposed population or the ecologic risks of an exposed ecosystem, the contribution of each individual source of exposure to the overall risk, or the success of risk management actions in reducing the risk from existing sources of exposure. (See also "Ecological Risk Assessment and Risk Management.")

Risk assessment will be greatly improved if risk assessors and other members of the scientific and risk management communities can work to develop and validate new toxicity tests in laboratory animals, investigate similarities and differences in laboratory animals and humans, obtain data on exposures, and develop and validate models to help fill toxicity and exposure data gaps.

The Importance of Comprehensive, Multimedia Risk Analysis

Risk assessment provides the scientific foundation for risk management decision-making. Traditionally, risk assessments, like risk management, have largely focused on assessing the risks of just one chemical in one medium at a time. However, to achieve comprehensive, multimedia risk management, risk managers will need comprehensive, multimedia risk assessments. Thus, to improve risk management, the risk assessment paradigm must be expanded.

Scientists must develop methods to assess multimedia, multisource, multichemical risks


A number of EPA offices conduct more comprehensive risk assessments. Specifically, when establishing a standard for exposure to a chemical in drinking water, EPA accounts for nondrinking water sources of exposure to that chemical. When considering whether to reregister a pesticide, EPA now considers other sources of exposure to that pesticide and to similar pesticides. In addition, some total exposure and cumulative exposure studies have been performed. However, few other regulatory agencies consider exposures or risks this comprehensively, and EPA often does not do so because of resource or statutory limitations. Failure to account for multiple and cumulative exposures is one of the primary flaws of current risk assessment and risk management.

Assessing aggregate risks from multiple exposures is an area in which risk assessors and risk managers need both methods and experience.


To the greatest extent possible, EPA and other regulatory agencies must work to develop and refine techniques for comprehensive risk assessment. In addition to the work already being done by EPA, a number of other efforts provide useful models. One example of a technique for assessing aggregate or cumulative risks from multiple pollutants and multiple sources is the method for regional risk assessment of air pollution developed by the Air and Waste Management Association. This method was used in San Diego as part of California’s "hot spots" program, which examines the potential for cumulative pollution from multiple facilities to impact neighborhoods in a county. The method generates a contour map of estimates of the maximum cancer risks associated with industrial facilities throughout the county using meteorological data and information on contaminants, emission rates, and risks from individual facilities. The results can be used to: