Basics of Risk Assessment

Perhaps the easiest definition of ecological risk assessment is the probability of an effect occurring to an ecological system. Note that the word probability is key here. Important components of a risk assessment are the estimations of hazard and exposure due to a stressor. 

A stressor is a substance, circumstance, or energy field that causes impacts, either positive or negative, upon a biological system. Stressors could be as wide ranging as chemical effects, ionizing radiation, or rapid changes in temperature. 

Hazard is the potential of a stressor to cause particular effects upon a biological system. The determination of an LD50 or the mutagenicity of a material is an attempt to estimate the hazard posed by a stressor. 

Exposure is a measure of the concentrations or persistence of a stressor within the defined system. Exposure can be expressed as a dose, but in environmental toxicology it is often possible to measure environmental concentration. One of the advantages of determining tissue concentrations in fish and mammals is that it is possible to estimate the actual dose of a chemical to the organism. Biomarkers may also provide clues to dosage. 

A stressor poses no risk to an environment unless there is exposure. This is an extremely crucial point. Virtually all materials have, as a characteristic, some biological effect. However, unless enough of the stressor interacts with biological systems, no effects can occur. Risk is a combination of exposure and effects expressed as a probability. In contrast, hazard assessment does not deal with concentration and is not probabilistic in nature. Table 12.1 compares the two assessments as outlined by Suter (1990). 

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Kolluru, R. V. "Understand the basics of risk assessment." CEP. March 1991. 

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