SDA Volume 4, Number 1
Dear Perplexed,
Risk analysis was originally a technique used by French matchmakers to predict the extent of marital harmony. Hence a risqué person was one prone to discord due to an adventurous temperament.
In modern times risk analysis has been reformulated as a relatively new discipline that has come to be a crucial part of public debate and decision-making on a wide variety of environmental issues. It attempts to quantify the hazards posed by dangerous substances and/or processes. At its core, risk analysis relies on probability; it seeks to quantify both the probability and the magnitude of adverse consequences that individuals, populations, or ecosystems might suffer from specific hazards.
There are several steps in assessing risk that range from determining the nature of the hazard to estimating exposure and actual effects.
Determining the nature of hazard
First, one must decide whether and how a particular substance, process, or event could be harmful. For substances, it is necessary to determine the doses at which harm occurs and factors that could influence how harm occurs. For instance, a substance may be acutely toxic or poisonous only upon prolonged exposure. When hazards involve an event (an accidental release), one must also calculate the probability of the accident occurring. A series of failures may be needed for an accident to occur. In such cases, risk analysis typically involves the construction of "fault trees", which are diagrams that show the sequence(s) of failures in sub-systems that could lead to an overall system failure. When the data are available, this analysis enables the computation of an overall probability of failure.
Determining exposure
To estimate a person's or population's exposure as a result of environmental contamination (called "dose reconstruction"), it is crucial to know the amount of the pollutant (called a "source term") released to a particular medium, such as air or water. Alternatively, an accurate history of concentrations of pollutants in air, water, and soil is necessary.
Discharges to one medium can affect another medium. If particles of a radioactive material are released to the air, they will also be deposited in soil as "fallout." Pollutants on the soil surface may percolate into the groundwater or be washed into surface waters by rain and melting snow. Radionuclides like cesium-137, strontium-90, tritium, and carbon-14 and many organic toxic compounds can be incorporated from air, water, and soil into vegetation and crops.
"Pathway analysis" clarifies the often complex ways in which pollutants reach people via the environment. This analysis enables release estimates to be converted to dose estimates. Worker exposures, in principle, can be ascertained more directly. For instance, workers in nuclear plants wear film badges that record levels of exposure to gamma and beta radiation. Internal exposure to radioactive materials can be determined from urine samples and whole body counting.
Harmful substances may also be contained in consumer products, in which case sampling of the products and patterns of use and consumption are needed to estimate exposure.
Assessing the Damage
Once levels of exposure to off-site populations and to workers have been determined, adverse health consequences can be estimated, if the effects of exposure to the substance are known. Another way to assess damage to health in many circumstances is to conduct an epidemiological study, if suitable exposed and control groups can be established.
Risks can be expressed in absolute or relative terms and on an individual or population basis. An "absolute risk" specifies the actual number of bad outcomes (like cancers) that would occur as a result of the exposure. To say that an individual's risk of getting a cancer as a result of a given level of exposure is 1 in 100,000 means that one "excess cancer" in a population of 100,000 would be expected if each person were exposed to the same degree.
A "relative risk" shows the risk in the exposed population compared to the risk in the unexposed population. For instance, the relative individual risk of a particular cancer has doubled as a result of an exposure. This means that one would expect to find twice the number of cancers in the exposed population as in a comparable, but unexposed "control" population.
Limitations of risk analysis
Uncertainties are inherent in risk analysis, since risk estimates are probabilistic statements. It is good practice to estimate uncertainties and state them explicitly. When data are reasonably good, uncertainty calculations are quite straightforward. However, when data are poor or non-existent, such calculations are far more problematic and controversial, since they involve personal judgments of "experts" in place of real data and analysis. The range of uncertainty in such cases can be quite enormous.
Risk analysis can be a useful quantitative guide to decision-making if sound science underlies it, and if it is complemented by social and political decision-making processes that take into account its inherent limitations. It should not be used to impose risk without informed consent and full democratic debate. Its inherent uncertainties mean that complementary techniques are generally desirable or necessary to determine environmental hazards and evaluate effectiveness of policies.
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An example of risk assessment and dose reconstruction: a uranium processing plant
The following example is an invented scenario of accidents at an industrial plant and how to calculate exposure to the surrounding population. Suppose the chance of an accidental release from the plant is roughly 1 in 10 per week (10 percent), and the plant operates for 50 weeks per year. One would expect 5 accidents per year. Suppose each of these accidents releases 400 kilograms (almost 900 pounds) of uranium for a total source term for air releases for that year of 2,000 kilograms. (An uncertainty range for probable emissions can be calculated if the range of releases in accidents and the variability in accident frequency is known). The next step in the analysis would be to assume various prevailing weather conditions and calculate uranium concentrations in air at various locations. Once these concentrations are estimated, we can calculate the amount of uranium inhaled by someone living a certain distance from the plant. We do this calculation based on the breathing rate of the average person (about 20 cubic meters of air per day), and the size of the uranium particles that were inhaled. With this information, we can then estimate the total dose to the body. A certain fraction of the inhaled uranium is retained in the lung, irradiating the lung and migrating from there to other organs, like bones, also irradiating them. The total dose from uranium depends on how long it stays in the body, which in turn depends on the solubility of the chemical form of inhaled uranium. Inhaled uranium is excreted via urine. The uncertainties in such calculations are typically very large, especially if the weather, chemical form, location of the exposed person, and particle size are uncertain. This is often the case. Estimated offsite doses in such cases can range from a fraction of a millirem to many rem (a rem is one thousand times bigger than a millirem), even when the source term is known. |
Institute for Energy and Environmental Research
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Takoma Park, Maryland, USA
Last updated: August, 1996