Association between air pollution and acute childhood wheezy episodes: prospective observational study

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Abstract
Objective: To examine the association between the air pollutants ozone, sulphur dioxide, and nitrogen dioxide and the incidence of acute childhood wheezy episodes. Design: Prospective observational study over one year. Setting: District general hospital. Subjects: 1025 children attending the accident and emergency department with acute wheezy episodes; 4285 children with other conditions as the control group. Main outcome measures: Daily incidence of acute wheezy episodes. Results: After seasonal adjustment, day to day variations in daily average concentrations of ozone and sulphur dioxide were found to have significant associations with the incidence of acute wheezy episodes. The strongest association was with ozone, for which a non-linear U shaped relation was seen. In terms of the incidence rate ratio (1 at a mean 24 hour ozone concentration of 40 µg/m3 (SD=19.1)), children were more likely to attend when the concentration was two standard deviations below the mean (incidence rate ratio=3.01; 95% confidence interval 2.17 to 4.18) or two standard deviations above the mean (1.34; 1.09 to 1.66). Sulphur dioxide had a weaker log-linear relation with incidence (1.12; 1.05 to 1.19 for each standard deviation (14.1) increase in sulphur dioxide concentration). Further adjustment for temperature and wind speed did not significantly alter these associations. Conclusions: Independent of season, temperature, and wind speed, fluctuations in concentrations of atmospheric ozone and sulphur dioxide are strongly associated with patterns of attendance at accident and emergency departments for acute childhood wheezy episodes. A critical ozone concentration seems to exist in the atmosphere above or below which children are more likely to develop symptoms. This study shows that after seasonal adjust- ment the incidence was found to be high on days when ozone concentrations were very low or very high The log incidence increased in a linear manner with increasing concentrations of sulphur dioxide Nitrogen dioxide was not found to have a significant effect on the incidence of wheezy episodes The non-linear U shaped relation between ozone and incidence suggests that at low ozone concentrations either other factors are more important in determining wheezy episodes or an optimum protective concentration of ozone exists in the atmosphere