Particulate matter and daily mortality and hospital admissions in the westmidlands conurbation of the United Kingdom: associations with fine and coarse particles, black smoke and sulphate

Objectives-There is considerable evidence linking ambient particles measure d as particulate matter with aerodynamic diameter < 10 mum (PM10) to daily mortality and hospital admissions but it is not clear which physical or che mical components of the particle mixture are responsible. The relative effe cts of fine particles (PM2.5), coarse particles (PM2.5-10), black smoke (ma inly fine particles of primary origin) and sulphate (mainly fine particles of secondary origin) were investigated, together with ozone, SO2, NO2, and CO, on daily mortality and hospital admissions in the west Midlands conurba tion of the United Kingdom. Methods - Time series of health outcome and environmental data were obtaine d for the period 1994-6. The relative risk of death or hospital admission w as estimated with regression techniques, controlling for long term time tre nds, seasonal patterns, influenza epidemics, effects of day of the week, an d temperature and humidity. Models were adjusted for any remaining residual serial correlation and overdispersion. The sensitivities of the estimates for the effects of pollution to the inclusion of a second pollutant and sea sonal interactions (warm or cool) were also examined. Results - Daily all cause mortality was not associated with any gaseous or particulate air pollutant in the all year analysis, although all measures o f particles apart from PM2.5-10 showed significant positive effects of the warm season. Neither respiratory nor cardiovascular admissions tall ages) w ere associated with any air pollutant, and there were no important seasonal interactions. However, analysis of admissions by age found evidence for va rious associations-notably between PM,,, PM2.5, black smoke, SO2, and ozone (negative) and respiratory admissions in the 0-14 age group. The coarse fr action, PM2.5-10 differed from PM2.5 in having smaller and less consistent associations (including several large significant negative associations) an d a different lag distribution. The results for black smoke, an indicator o f fine primary carbonaceous particles, were very similar to those for PM an d tended to be more robust in two pollutant models. The effects of sulphate , an indicator of secondary particles, also showed some similarities to tho se of PM2.5. Conclusions - Clear effects of air pollution on mortality and hospital admi ssions were difficult to discern except in certain age or diagnostic subgro ups and seasonal analyses. It was also difficult to distinguish between dif ferent measures of particles. Within these limitations the results suggest that the active component of PM,, resides mostly in the fine fraction and t hat this is due mainly to primary particles from combustion (mainly vehicle ) sources with a contribution from secondary particles. Effects of the coar se fraction cannot be excluded.