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Case-crossover design in air pollution epidemiology

CORRESPONDENCE: J.J.K. Jaakkola, Institute of Occupational Health, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. E-mail: j.jaakkola@bham.ac.uk. Fax: 358 919127570

Keywords: air pollution, epidemiologic methods, case-crossover study, case-control study, crossover study

Received: April 12, 2002
The case-crossover design was developed to study the effects of transient, short-term exposures on the risk of acute events, such as myocardial infarction, in the early 1990s. This paper illustrates how the principles of case-crossover design are related to the principles of crossover and case-control designs and stipulates the possibilities of case-crossover design in air pollution epidemiology.

In the case-crossover design, the study population consists of subjects who have experienced an episode of the health outcome of interest. Similar to a crossover study, each subject serves as his or her own control. As in a matched case-control study, the inference is based on a comparison of exposure distribution rather than the risk of disease. The case-crossover study is most suitable for studying relations with the following characteristics: 1) the individual exposure varies within short time intervals; 2) the disease has abrupt onset and short latency for detection; and 3) the induction period is short.

Case-crossover design allows use of routinely monitored air pollution information and at the same time makes it possible to study individuals rather than days as the unit of observation. Case-crossover design is amenable for studying the effects of varying short-term air pollution exposure on health outcomes with an abrupt onset, such as myocardial infarction or asthma attack.