METHODOLOGICAL ISSUES IN STUDIES OF AIR-POLLUTION AND DAILY COUNTS OFDEATHS OR HOSPITAL ADMISSIONS

Study objective - To review the issues and methodologies in epidemiolo gic time series studies of daily counts of mortality and hospital admi ssions and illustrate some of the methodologies. Design - This is a re view paper with an example drawn from hospital admissions of the elder ly in Cleveland, Ohio, USA. Main results - The central issue is contro l for seasonality. Both over and under control are possible, and the u se of diagnostics, including plots, is necessary. Weather dependence i s probably non-linear, and adequate methods are necessary to adjust fo r this. In Cleveland, the use of categorical variables for weather and sinusoidal terms for filtering season are illustrated. After control for season, weather, and day of the week effects, hospital admission o f persons aged 65 and older in Cleveland for respiratory illness was a ssociated with ozone (RR = 1.09, 95% CI 1.02, 1.16) and particulates ( PM(10) (RR = 1.12, 95% CI 1.01, 1.24), and marginally associated with sulphur dioxide (SO2) (RR = 1.03, 95% CI = 0.99, 1.06). All of the rel ative risks are for a 100 mu g/m(3) increase in the pollutant. Conclus ions - Several adequate methods exist to control for weather and seaso nality while examining the associations between air pollution and dail y counts of mortality and morbidity. In each case, care and judgement are required.