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Risk Analysis

Risk analysis

Risk is the chance that something undesirable will happen. Everyone faces personal risks daily; we all have a chance of being struck by a car or by lightning or of catching a cold. None of these are certain to happen today, but they all can and do happen occasionally, some more frequently than others. Even though all risk is unpleasant, the consequences of being struck by an automobile are much more serious than those of catching a cold. Most people would do more and pay more to avoid the risks they consider most serious. Thus, risk has two important components: (1) the consequences of an event and (2) its probability . In addition, while the threat of lightning has always been present, the possibility of being struck by a car emerged only in the last century. Modern risks are constantly evolving.

Events that challenge the health of ecological systems are also becoming apparent. Ecosystems have always faced the risk of severe damage from fire, flooding , and volcanoes. More recently, however, population increases, especially in cities; global climate change; deforestation ; acid rain ; pesticides; and sewage, garbage , and industrial waste disposal have all threatened ecosystems.

Healthy ecosystems supply air, water, food, and raw materials that make life possible; they also process the wastes human societies produce. For these compelling reasons, we must protect them. In the United States, the National Environmental Policy Act , the Toxic Substances Control Act, the Clean Water Act , the Federal Insecticide, Fungicide , and Rodenticide Act, and other legislation has been passed by Congress to protect the environment .

Risk analysis can determine whether proposed actions will damage ecosystems. This screening process evaluates plans that might prove destructive, and allows people to make informed decisions about which would be most environmentally sound. Proposed utilities, roads, waste-disposal sites, factories, and even new products can use risk analysis to address environmental concerns during planning stages, when changes are most easily made. The process also allows people to rank environmental problems and allocate attention, resources, and corrective efforts.

Risk analyses are made by both scholars and government decision makers. Scientists are interested in a thorough understanding of the way ecosystems function, but decision makers need quick and efficient tools for making choices.

Scholarly approaches take many forms: Synoptic surveys assess the characteristics of stressed and unstressed natural systems. Experiments determine how the whole or one part of an ecosystem (such as fish) will respond to stress. Extrapolations apply specific observations to other ecosystems, chemicals ,or properties of interest by analyzing dose-response curves, establishing relationships between the molecular structure of a chemical and its likely environmental effects, or simulation of entire ecosystems on a computer. Using these tools, scientists can also measure change in ecosystems and translate the results for the general public.

A less precise method, often used by public officials and other nonscientists, is called ecosystem risk assessment . It uses available toxicological, ecological, geological, geographical, chemical, and sociological information to estimate possible damage. The process has three steps:

  • The problem is identified. For example: could nutrient runoff from local agriculture affect commercial fishing on a nearby lake?
  • Available scientific information is gathered to predict both the level of stress that could result and the likely ecosystem response. Where do the nutrients go and how are organisms exposed to them? How does increasing nutrient levels affect biological systems, especially fish?
  • Risk is quantified by comparing exposure and effects data. If the predicted stress level is lower than those known to cause serious damage, risk is low. On the other hand, if the predicted stress is higher, risk is high.

Despite the reams of data available, there is never enough information about the possible effects of any stress. An assessment based squarely on facts is more reliable than one that uses scarce, preliminary information. Anyone charged with making decisions must weigh all options.

The possibility of an undesirable occurrence, the seriousness of the consequences, and the uncertainty involved in any prediction all factor into the estimate. Alternative actions and their risks and benefits must also be considered, and the consistency of the action balanced with other societal goals. An action's risks and benefits are often not distributed evenly in society. For example, people sharing the water table with a proposed landfill may shoulder more risk, while those who use many nonrecyclable consumer products may benefit disproportionately. Who benefits and who loses can also affect decisions.

Risk predictions are similar to weather forecaststhey are based on careful observation and are useful, but they are far from perfect. They indicate useful precautionswhether these involve carrying an umbrella or treating waste before it enters the water. They help us understand ecosystems and allow us to consider the environment before potentially harmful action is taken. If ecological risks are considered when decisions are being made, ecosystems on which people depend can be protected.

See also Environmental policy; Environmental stress; Greenhouse effect; Industrial waste treatment; Sewage treatment; Solid waste

[John Cairns Jr. and B. R. Neiderlehner ]



Bartell, S. M., R. H. Gardner, and R. V. O'Neill. Ecological Risk Estimation. Chelsea, MI: Lewis Publishers, 1992.

Ehrlich, P. R., and A. H. Ehrlich. Healing the Planet: Strategies for Solving the Environmental Crisis. New York: Addison-Wesley Publishing, 1992.

National Research Council. Risk Assessment in the Federal Government: Managing the Process. Washington, DC: National Academy Press, 1983.


Norton, S. B., et al. "A Framework for Ecological Risk Assessment at the EPA." Environmental Toxicology and Chemistry 1112 (1992): 1663672.


Cairns Jr., J., K. L. Dickson, and A. W. Maki, eds. Estimating the Hazard of Chemical Substances to Aquatic Life, STP 657. Philadelphia: American Society for Testing and Materials, 1978.

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Risk Assessment (Public Health)

Risk assessment (public health)

Risk assessment refers to the process by which the short and long-term adverse consequences to individuals or groups in a particular area resulting from the use of specific technology, chemical substance, or natural hazard is determined. Generally, quantitative methods are used to predict the number of affected individuals, morbidity or mortality , or other outcome measures of adverse consequences. Many risk assessments have been completed over the last two decades to predict human and ecological impacts with the intent of aiding 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, 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 that have been exposed to the chemical in question. Health effects are then extrapolated to predict health impacts in large populations or to the general public who may be exposed to lower concentrations of the same chemical.

One mathematical formula that determines an individual's risk from chemical exposures is:

For the case of a hazardous waste incinerator, emissions might be average smoke stack emissions of gas; the transport term represents dilution in the air from the stack to the community; the loss factor might represent chemical degradation of reactive contaminants as stack gases are transported in the atmosphere ; the exposure period is the number of hours that the community is downwind of the incinerator; uptake is the amount of contaminants absorbed into the lung (a function of breathing rate and other factors); and toxicity is the chemical potency. Multiplying these factors indicates the probability of a specific adverse health impact caused by contaminants from the incinerator. In typical applications, such models give the incremental lifetime risk of cancer or other health hazards in the range of one in a million (equivalent to 0.000001). Cancers currently cause about one-third of deaths, thus, a one in a million probability represents a tiny increase in the total cancer incidence. However, calculated risks can vary over a large range0.001 to 0.00000001.

While the same equation is used for all individuals, some assumptions regarding uptake and toxicity might be modified for certain individuals 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, is the substance toxic and are people 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 may cause soil and ground water pollution . Drinking contaminated groundwater from the site or skin contact with contaminated soils may therefore result in 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 may be reversible once the chemical is removed. In the case of potential carcinogens, linear models are used almost exclusively. 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 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 constructedfor example, a proposed hazardous waste incineratorthe 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. (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. In recent years, a number of disputes have been resolved by mediation.

Risk management and risk reduction

Options that result from the risk management step include performing no action, product labeling, and placing regulations and bans. Examples of product labeling include warning labels for consumer products, such as those on tobacco products and cigarette advertising, and Material Safety Data Sheets (MSDS) for chemicals in the workplace. Regulations might be used to set maximum permissible levels of chemicals in the air and water (e.g., air and water quality criteria is set by the U.S. Environmental Protection Agency ). In the workplace, maximum exposures known as Threshold Limit Values (TLVs) have been set by the U.S. Occupational Safety and Health Administration . Such regulations have been established for hundreds of chemicals. Governments have banned the production of only a few materials, including DDT and PCBs, and product liability concerns have largely eliminated sales of some pesticides such as Paraquat and most uses of asbestos .

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 , may 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 used in a relative or comparative fashion, weighing 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 40,000 traffic fatalities annually in the United States represent over a 1% lifetime chance of dying in a wreck10,000 times higher than acceptable for a chemical hazard. The discrepancy between what an individual accepts for a chemical hazard in comparison to risks associated with personal choices like driving or smoking might indicate a need for more effective communication about risk management.

Risk assessments are often 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 than exposure to one factor alone. Conversely, multiple chemicals might inhibit or cancel risks. In nearly all cases, these factors cannot be modeled with our present knowledge. 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 like operator error.

Ecological risk assessment

Ecological risk assessments are similar to human health risk assessments but they estimate the severity and extent of ecological effects associated with an exposure to an anthropogenic agent or a perturbational change. Again, the risk estimate is stated in probability terms that reflect the degree of certainty. Ecological assessments tend to be more complex than human health assessments since a variety of dynamic ecological communities or systems may be involved, and these systems have important but often poorly understood interactions and feedback loops. In addition the current status and health of ecological systems must be defined by measurements and analysis before an assessment can begin. In some cases, animal species or ecosystems may be more sensitive than humans. Contingency or hazard assessment resembles that made for human health but focuses on low probability events such as failure of dams, nuclear power plants, and industrial facilities that have the potential for significant public health and welfare damage. Finally, risk reduction approaches have been suggested that shift focus from end-of-pipe controls, for instance, pollution control equipment, to preventing pollution in the first place by minimizing waste and recycling .

[Stuart Batterman ]



Chemical Risk: A Primer. Washington, DC: American Chemical Society, 1984.


Naugle, D. F., and T. K. Pierson. "A Framework for Risk Characterization of Environmental Pollutants." Journal of the Air and Waste Management Association 4 (1992): 12981307.


U.S. Environmental Protection Agency. Integrated Risk Information System Background Document. Washington, DC: U. S. Government Printing Office, 1991.

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"Risk Assessment (Public Health)." Environmental Encyclopedia. . 24 Apr. 2017 <>.

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