with, the interpretation of results. Few studies meet this ideal; this limitation is also common in peer-reviewed, published reports.

The committee relied on a combination of evidence from different sources to assess the impact on public health of exposures suspected of causing symptoms or disease. The types of information from these sources are discussed at length in chapter 2 of volume 1.

Small Relative Risks, but Large Numbers of Cases

Increasingly, environmental epidemiology concerns the search for factors that might moderately affect the risk of common multifactorial diseases. The effect of an individual environmental exposure on the relative risk of a disease may be small, but this does not mean that it is inconsequential; it can affect very large numbers of people and thus be associated with large numbers of cases of disease. For example, the risk of death in males aged 45-74 years with a diastolic blood pressure of 95 mm Hg in the Framingham study was only about 1.15 times the risk in those with a diastolic blood pressure of 85 mm Hg, yet the amount of disease that could be prevented in the population by reducing diastolic pressures to 85 mm Hg would be substantial. Increased use of hypertension medication, along with improvements in diet and exercise, is thought to be responsible for some part of the substantial decline in cardiovascular mortality in the last 20 years.

Large sample sizes and long-term followup studies are generally necessary to demonstrate potentially serious effects that involve small increases in relative risk. The chronic effects of ozone exposure and the acute effects of particulate air pollution are instances in which the relative risk may be small, but the population disease burden may be substantial. Ozone levels are often excessively high in many urban and coastal areas in the summer, when millions of people are outdoors, so that even a 10% increase in relative risk will produce a large number of cases of disease. For further discussion on this point, readers are referred to chapter 4.

The Need for More-General Monitoring

Monitoring systems utilizing existing data sources, as we discuss in chapter 5, will be of increasing importance. The first report also noted that monitoring may be the only way to determine the extent to which disease rates have changed as a result of changes in environmental contamination.

The use of aggregate statistics is also critical to detect trends or patterns in environmental pollution and health consequences that are not apparent at a local level. For example, what was initially thought to be laboratory drift in the measurement of blood lead concentrations in the

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