Assessment of exposure to environmental pollutants.

Accepted 26 May 1995 Environmental hazards to health have a high public profile, but assessment of the risks that they pose is not easy. What is the morbidity, if any, caused by living near a hazardous waste incinerator? Do oestrogenic alkylphenolic pollutants in drinking water affect reproductive function, and if so, how much? Such questions have important implications for public health policy, but epidemiologists find them hard to answer. One difficulty is the need to detect risks that are only marginally increased. In the occupational setting, uncertainties about low levels of risk can often be tolerated. We accept that many jobs carry a toll of injury or illness, provided that the risks are small and are controlled as far as is reasonably practical. The same levels of risk are not acceptable when the cause is pollution of the general environment. For one thing such pollution affects many more people, including potentially vulnerable groups such as children, pregnant mothers, and elderly people. Also, people are thought to have more choice about the job that they do than about the general environment to which they are exposed. The problem for the epidemiologist is that, even with large studies, small increases in risk are difficult to disentangle from the effects of bias and uncontrolled confounding. Another major limitation is the lack of reliable information about people's exposure to environmental pollutants. Many epidemiological investigations rely on proxy measures of exposure such as distance of residence from a point source of pollution, but do not assess their validity. Techniques for measuring personal exposures in the workplace are now relatively well developed, but they have not been applied so widely in environmental studies. When errors in the assessment of exposure are unrelated to health outcome (as is most often the case) the effect is to attenuate risk estimates, and make hazards more difficult to detect. In response to these problems, the Medical Research Council (MRC) convened a working group at the end of 1993 to examine ways of improving the assessment of environmental exposures, and to help formulate its research strategy on environmental pollution. The group focused on chemical pollutants, although many of its conclusions could be applied also to physical and biological hazards. It identified three main needs.