Combined effects of aerobiological pollutants, chemical pollutants and meteorological conditions on asthma admissions and A & E attendances in Derbyshire UK, 1993-96

Background The effect of outdoor aeroallergen exposure in asthma may be enh anced by air pollutants, including ozone, nitrogen dioxide and particulates , and by certain weather conditions. It is not yet established whether thes e interactions are important in determining asthma morbidity at the populat ion level. Objective We have investigated the joint effects of aeroallergens, rainfall , thunderstorms and outdoor air pollutants on daily asthma admissions and A ccident and Emergency (A & E) attendance using routinely collected data bet ween 1993 and 1996 from Derby in central England. Methods Daily counts during the aeroallergen season of grass and birch poll en, basidiospores, Didymella, Alternaria and Cladosporium, maximum 1 hour o zone and nitrogen dioxide and daily average black smoke measurements, all m ade in the vicinity of the city centre, were categorized in tertiles. Rainf all was classified as dry, light (less than or equal to 2 mm) or heavy (> 2 mm). The modifying effect of outdoor pollutant levels, and rainfall or the occurrence of a thunderstorm, upon the effects of individual aeroallergens on asthma admissions and A & E attendance were investigated by fitting app ropriate interactions in log linear autoregression models with adjustment f or potential confounders. Results We found a significant interaction between the effects of grass pol len and weather conditions upon A & E attendance, such that the increase wi th grass pollen count was most marked on days of light rainfall (adjusted r ate ratio for greater than or equal to 50 vs < 10 grains/m(3) at lag 2 days = 2.1, 95% CI 1.4, 3.3). Asthma admissions increased with Cladosporium cou nt. We found no statistically significant interactions between effects of a ny individual aeroallergen and outdoor air pollutant upon either measure of asthma morbidity. Conclusions Rainfall and thunderstorms are important effect modifiers in th e relation between grass pollen and measures of acute asthma morbidity. Int eractions between ambient levels of aeroallergens and chemical pollutants i n the Derby area do not play a major role in determining asthma admissions and A & E attendance.