factors that can be averted or reduced so as to prevent or reduce the risk of future disease and promote public health.

Types of Studies in Environmental Epidemiology

Environmental-epidemiologic studies can be classified broadly into 2 categories that are not mutually exclusive: descriptive and analytic. Typically, descriptive studies are most useful for generating hypotheses and analytic studies most useful for testing hypotheses, though each type of study can be used for both purposes. Whether a study is hypothesis-testing or hypothesis-generating depends more on the sequence of past studies and the present state of knowledge (i.e., whether a hypothesis currently under evaluation was suggested by a previous study) than on the study design. Recent innovations in descriptive studies sometimes permit refined assessments of dose-response relations and etiologic factors.

Descriptive Studies

Descriptive studies include case reports, surveillance systems, ecologic studies, and cluster studies (WHO, 1983).

Case Reports

A case report is a descriptive study of a single individual or small group in which the study of an association between an observed effect and a specific environmental exposure is based on detailed clinical evaluations and histories of the individual(s). These reports require few financial or personnel resources other than those of clinical medicine, and they may indicate whether additional study of a larger group of persons with similar health problems and exposures should be undertaken. However, the value of case reports is often limited because they lack a context of the disease in unexposed persons, variables such as time and dose of exposure are generally not known, and controls are absent. They are most likely to be useful when the disease is uncommon and when it is caused exclusively or almost exclusively by a single kind of exposure. In spite of these limitations, many known human environmental toxicants (e.g., methyl mercury, asbestos, tobacco smoke, and radon) first came to attention in case reports and series developed by astute clinicians, pathologists, and health workers. Public-health agencies must often investigate clusters of cases that are reported to them by private physicians and others. While case reports may not lead to identification of new causes of disease, they are more likely to point to specific hypotheses and to biologically meaningful associations if either the disease or the exposure is relatively rare.

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