Epidemiological studies have repeatedly established adverse health effects due to long-term exposure to ambient air pollution. The Swiss Study on Air Pollution and Lung Disease in Adults (SAPALDIA) published a -3.14% decrease in forced vital capacity (FVC) per 10 microg x m(-3) increment in particulate matter (particles with a 50% cut-off aerodynamic diameter of 10 microm (PM(10)). Compared to the within-subject variability of FVC, the effect may be considered small. This individual (or clinical) perspective is, however, misleading. The purpose of this study was to demonstrate the public health relevance of apparently "small" effects, using the impact of PM10 on FVC as an example. The scenario compares a population A, exposed to an annual mean PM10 of 20 microg x m(-3), with a population B exposed to 30 microg x m(-3) mean PM10. A shift of -3.14% in the population distribution of FVC increases the number of subjects in the lower tail of the distribution. In population B a relative increase was expected of 47% (16-91%) in the prevalence of "FVC <80% predicted", (i.e., from 5.17 to 7.59% and 5.88 to 8.65% among males and females, respectively). The relative increase in the prevalence of "FVC <70% predicted" (approximately 1% of population) was 63% (30-98%, males) and 57% (21-86%, females). An epidemiological estimate of a change in the mean value of the population distribution should not be misinterpreted as an effect on the individual level. However, the impact of a 10 microg x m(-3) increase in particles with a 50% cut-off aerodynamic diameter of 10 microm (PM10) on the number of subjects with a clinically relevant reduction in lung function is quantitatively important.
