The methodology of risk assessment has developed rapidly in recent decades and is increasingly relied on as a tool in making decisions about activities that pose risks. Since various techniques of risk assessment are widely used by decision-makers, and as their findings often form the substance of what is communicated to the public, it is important that the limitations of risk assessment as currently practised are fully recognized and understood. Otherwise, its use can result in misplaced confidence in potentially hazardous activities, and risk communication based exclusively on it can prove to be misleading.
The practice of risk assessment as carried out over the past few decades has accompanied, and to some extent legitimized, many environmentally destructive practices, resulting for example in the introduction and build-up of toxic, bioaccumulative chemicals in the environment over the same period. It is therefore not surprising that the process is regarded by the public with some scepticism.
This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities. Agenda 21 recommends building on past, present and future risk assessment work at an international level so as to support countries, particularly developing countries, in developing and strengthening risk assessment capabilities at national and regional levels to minimize risk in the manufacture and use of toxic chemicals.
One of the key issues in assessing risks is the treatment of scientific uncertainty. As modern society becomes increasingly complex, there are major difficulties in making detailed, accurate assessments of risks and hazards. There may be uncertainty both in the probability of an event occurring and in the scale and nature of the consequences if it does occur. These uncertainties may arise from, or be compounded by, a number of factors: (a) lack of data: e.g. the sheer volume of new chemicals coming on to the market makes comprehensive testing a remote aspiration; (b) biased sources of data: sometimes the main information available on the risks posed by a technology comes from those with an interest in promoting it; (c) the sheer complexity of interactions between humans and the environment: too many possible causes for any given effect, too many parameters to monitor for any given cause; (d) the emergence of new technologies (e.g. genetic engineering) for which there is no accumulated body of experience or data; (e) separation of cause and effect over space (e.g. widely dispersed pollution) and time (e.g. intergenerational effects), making it difficult to prove causal connections; (f) synergistic, additive and cumulative effects (e.g. failure to take account of pre-existing body burdens of toxic substances); (g) unpredicted or unidentified sources of hazards; and (h), varying susceptibilities among populations. To the extent that risk assessment fails to explicitly acknowledge and address these uncertainties, it gives the illusion of a precision and objectivity which is not justified.