Long-term exposure to outdoor air pollution and the prevalence of asthma: meta-analysis of multi-community prevalence studies

The effect of outdoor air pollution on variations in asthma prevalence is unclear. We have conducted a meta-analysis of multi-community studies to evaluate and quantify the evidence for an association between community levels of pollution and asthma prevalence. We identified peer-reviewed articles reporting associations between community levels of air pollution and the prevalence of wheeze symptom or asthma diagnosis which were based on five or more communities. Associations were investigated by meta-analysis and by analysis of the direction and statistical significance of estimates. We identified 21 studies of more than five communities (range 6 to 62). The proportion of studies reporting at least one significantly positive association was 43% but, of the total of 178 pollution-outcome estimates, only 11% were statistically significantly positive. Thirteen studies reported associations with pollution analysed as a quantitative variable, and these results were meta-analysed. For period prevalence (mainly wheeze symptom), the results were null: Random effects estimates (95% confidence intervals) for the odds ratios of nitrogen dioxide, particulate matter with aerodynamic diameter < 10 mu m, ozone and sulphur dioxide were 1.00 (0.95, 1.06), 1.00 (0.94, 1.07), 1.01 (0.96, 1.07) and 1.03 (0.97, 1.09), respectively. For lifetime prevalence (mainly asthma diagnosis), the random effects estimates were similar: 1.00 (0.99, 1.01), 0.99 (0.96, 1.02), 1.06 (0.98, 1.14) and 1.00 (0.96, 1.05), respectively. We found no evidence of an association between community levels of outdoor air pollution and asthma prevalence.