The effect on health and disease of substances polluting the urban air is difficult to assess. Despite many epidemiological surveys, it is not known whether particulates and gaseous pollutants are harmful in the concentrations in which they are frequently found, whether they cause specific diseases, or if they exert a differential effect which may increase reactions to some diseases more than others. It is possible, however, to observe acute effects, possibly induced by small changes in air-pollution levels, in a population temporarily at a low level of tolerance—hospitalized patients.
The hospitalized person is in the physical condition most likely to be influenced by additional stresses. Also, reliable measures of increased morbidity can be obtained relatively easily. Of a number of measures of prevalence and severity of morbidity, hospital admissions and the length of stay may be the most available. Although factors other than the clinical state of the disease operate