Risk Assessment


Risk assessment refers to the process used to determine short-term and long-term adverse consequences to individuals or groups in a particular area resulting from technology, chemical substances, or natural hazards.


Generally, scientists use quantitative methods to predict the number of affected individuals, illness and death (morbidity and mortality), or other outcome measures of adverse consequences. Many risk assessments have been completed to predict human and ecological impacts to the health of environmental stressors. The assessments help aid in making policy and regulatory decisions. Well-known examples of risk assessments include evaluating potential effects of herbicides and insecticides, nuclear power plants, incinerators, dams (including dam failures), automobile pollution, tobacco smoking, climate change, and such natural catastrophes as volcanoes, earthquakes, and hurricanes. Risk assessment studies often consider financial and economic factors as well.

Human health risk assessments

Human health risk assessments for chemical substances that are suspected or known to have toxic or carcinogenic effects is one critical and especially controversial subset of risk assessments. These health risk assessments study small populations exposed to a chemical. Health effects on the small group are applied to predict health impacts in large populations or to the general public who may be exposed to lower concentrations of the same chemical.

There is a mathematical formula that determines an individual's risk from chemical exposures.

Risk=(emissions)*(transport)*(loss factor)*(exposure period)*(uptake)*(toxicity factor)

The same equation is used for all individuals, but some assumptions regarding uptake and toxicity might be modified for certain populations such as pregnant women, children, or individuals who are routinely exposed to the chemicals. In some cases, monitoring might be used to verify exposure levels. The equation illustrates the complexity of the risk assessment process.

Risk assessment process

The risk assessment/management procedure consists of five steps: (1) Hazard assessment seeks to identify causative agent(s). Simply put, researchers ask whether the substance is toxic and determine if people are exposed to it. The hazard assessment demonstrates the link between human actions and adverse effects. Often, hazard assessment involves a chain of events. For example, the release of pesticide can cause soil and ground water pollution. Drinking contaminated groundwater from the site or skin contact with contaminated soils can lead to adverse health effects.

(2) Dose-response relationships describe the toxicity of a chemical using models based on human studies, including clinical and epidemiologic approaches, and animal studies. Many studies have indicated a threshold or no-effect level, that is, an exposure level where no adverse effects are observed in test populations. Some health impacts can be reversible once the chemical is removed. For potential carcinogens, assessment nearly always uses linear models. Risk or potency factors are usually set using animal data, such as experiments with mice exposed to varying levels of the chemical. With a linear dose-response model, a doubling of exposure would double the predicted risk.

(3) Exposure assessment identifies the exposed population, detailing the level, duration, and frequency of exposure. Exposure pathways of the chemical include ingestion, inhalation, and dermal (skin) contact. Human and technological defenses against exposure must be considered. For example, respirators and other protective equipment reduce workplace exposures. In the case of prospective risk assessments for facilities that are not yet constructed—for example, a proposed hazardous waste incinerator—the exposure assessment uses mathematical models to predict emissions and distribution of contaminants around the site. Probably the largest effort in the risk assessment process is in estimating exposures.

(4) Risk characterization determines the overall risk, preferably including quantification of uncertainty. In essence, the factors listed in the equation are multiplied for each chemical and for each affected population. To arrive at the total risk, risks from different exposure pathways and for different chemicals are added. Populations with the maximum risk are identified. To gauge their significance, results are compared to other environmental and societal risks. These four steps constitute the scientific component of risk assessment.

Describes a substance that potentially can cause cancer.
The study of the distribution of diseases in large groups of people.
Refers to determining an outcome using numbers to measure and quantify, compared with using observation or other unstructured data.

(5)Risk management is the final decision-making step. It encompasses the administrative, political, and economic actions taken to decide if and how a particular societal risk is to be reduced to a certain level and at what cost. Risk management in the United States is often an adversarial process involving complicated and often conflicting testimony by expert witnesses. As of 2018, a number of disputes had been resolved by mediation.

Risk management and reduction

A variety of social and political factors influence the outcome of the risk assessment/management process. Options to reduce risk, like banning a particular pesticide that is a suspected carcinogen, can decrease productivity, profits, and jobs. Furthermore, agricultural losses due to insects or other pests if pesticide is banned might increase malnutrition and death in subsistence economies. In general, risk assessments are most useful when they help weigh the benefits of alternative chemicals or agricultural practices to another. Risk management decisions must consider what degree of risk is acceptable, whether it is a voluntary or involuntary risk, and the public's perception of the risk. A risk level of one in a million is generally considered an acceptable lifetime risk by many federal and state regulatory agencies. This risk level is mathematically equivalent to a decreased life expectancy of 40 minutes for an individual with an average expected lifetime of 74 years. By comparison, the more than 40,000 traffic fatalities annually in the United States represent at least a 1% lifetime chance of dying in a wreck—10,000 times higher than acceptable for a chemical hazard. The discrepancy between what an individual accepts for a chemical hazard compared to risks associated with personal choices such as driving or smoking might indicate a need for more effective communication about risk management.

Risk assessments often are controversial. Scientific studies and conclusions about risk factors have been questioned. For example, animals are often used to determine dose-response and exposure relationships. Results from these studies are then applied to humans, sometimes without accounting for physiological differences. The scientific ability to accurately predict absolute risks is also poor. The accuracy of predictions might be no better than a factor of 10, thus 10 to 1,000 cancers or other health hazard might be experienced. The uncertainty might be even higher, a factor of 100, for example. Risks due to multiple factors are considered independent and additive. For instance, smoking and asbestos exposure together have been shown to greatly increase health risks over those associated with exposure to one factor alone. Conversely, multiple chemicals might inhibit or cancel risks. In nearly all cases, these factors cannot be modeled with the available knowledge as of 2018. Finally, assessments often use a worst-case scenario, for example, the complete failure of a pollution control system, rather than a more modest but common failure such as operator error.

See also Cancer ; Chemical poisoning .



Asanti-Duah, Kofi. Public Health Risk Assessment for Human Exposure to Chemicals, 2nd ed. Washington, DC: Springer Science, 2017.

Simon, Ted. Environmental Risk Assessment: A Toxicological Approach. Boca Raton, FL: CRC Press, 2016.


United Nations. “Stockholm Convention.” http://chm.pops.int/Countries/StatusofRatifications/Overview/tabid/3484/Default.aspx (accessed April 1, 2018).

United States Environmental Protection Agency. “Environmental Management: Risk Assessment: Human Health Risk Assessment.” https://www.epa.gov/risk/human-health-risk-assessment (accessed April 1, 2018).

United States Environmental Protection Agency. “Risk Assessment for Toxic Air Pollutants: A Citizen's Guide.” https://www3.epa.gov/airtoxics/3_90_024.html (accessed April 1, 2018).


Environmental Protection Agency, 1200 Pennsylvania Ave., NW, Washington, DC, https://www.epa.gov .

Stuart Batterman
Revised by Teresa Odle, BA, ELS

  This information is not a tool for self-diagnosis or a substitute for professional care.