Objective: To study whether ambient air levels of sulphur dioxide (SO2), nitrogen dioxide (NO2), total suspended particulates (TSP), and ozone (O3) affect the number of hospital admissions for asthma.
Design: The associations between the daily number of admissions and air pollutants were analysed with Poisson regression, taking into account potential confounding factors by using the standardised protocol of the APHEA project.
Setting: Helsinki, Finland, 1987-89.
Patients and measurements: Patients admitted to hospitals through emergency rooms because of asthma (n = 2421). The daily mean concentration was 13-25 micrograms/m3 for SO2, 33-41 micrograms/m3 for NO2, 19-41 micrograms/m3 for O3, and 58-109 micrograms/m3 for TSP during the different seasons. Values are means for various stations. The daily mean temperature during the three year period was +5.4 degrees C (range -37-27 degrees C).
Main results: Positive associations with admissions were observed for O3 levels in all children under 14 years, and for SO2 levels in 15-64 year olds and among those older than 64. Significant associations were also seen between admissions for digestive tract diseases (the control) and O3 levels. This suggests that the modelling, which proved to be problematic, was unsatisfactory, or it may be a statistical coincidence. A rise in temperature was associated with a low number of admissions for asthma among 0-14 year olds and among 15-64 year olds, whereas humidity did not have a significant effect on the number of admissions.
Conclusions: It is possible than even low level pollution may increase hospital admissions for asthma. However, definitive conclusions will be justified only after meta-analysis comprising several studies. The methodology was seen to have a strong effect on the results and standardised methods, possibly differing from those for the study concerning mortality, are needed to investigate the association between morbidity and ambient air pollutants.