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Epidemiology is the study of the distribution and the determinants of health-related states and events in populations, and the application of this study to control of health problems. Epidemiological methods have been widely used to generate and test hypotheses on cause-effect relationships and to evaluate interventions.

There are four main types of epidemiological studies:

Ecological studies, also called correlational studies. These studies consider populations or groups of people and not individuals. An example would be studying mortality in a city compared to air pollution levels. Such studies have been performed and can demonstrate some relationship between an increased air pollution and an increase in mortality rate in a city.

Conclusions are very difficult to draw as there are many confounding factors which are not known.

Cross-sectional studies, also called prevalence studies. These studies measure at a given time in a well-identified population the effect of one factor which can vary. The famous study among British doctors showing the close relationship between lung cancer and smoking is the most illustrative of cross-sectional studies.

Case-control studies: they study a group of people suffering from a given disease and compare this group to a reference group, as similar as possible to the studied group. Research tries to identify the possible cause for the disease. It is through studies of this type that one tries to identify the reasons behind the increased number of asthma cases among Western European populations.

Cohort studies: a group of people free of a given disease which is being investigated is selected. Exposure to the factor and disease rates which are being studied is measured during "follow-up" as in the subsequent development of the disease. These studies are very long and often expensive.

Simultaneous to these epidemiological studies, which study human beings, the scientist can study the effects of one or more pollutants on laboratory animals or bacteria. This is the major scope of toxicology.

Adapted from Basic Epidemiology

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