The evidence on the impact of air pollution on human health shows inconsistency partly due to bias in the individual studies and in the overall field. We endorsed an umbrella review approach to evaluate the strength of the evidence on the effect of air pollution on immune-based diseases.
We searched PubMed (up to 6/2018) for meta-analyses of observational studies investigating the association between air pollution and any health outcome using a search algorithm incorporating both individual pollutants and general terms. Meta-analyses pertaining to any immune-based disease outcome were identified. The validity of the evidence was evaluated using the following criteria: statistical significance (random-effects estimate), statistical significance (largest study), cumulative number of cases, between-study heterogeneity, 95% prediction intervals, small-study effects bias, and excess significance bias.
We identified 46 associations (8 publications) pertaining to 6 pollutants. Asthma dominated the evidence-base with only 5 non-asthma associations present (allergen sensitization, atopy, eczema). ΝΟ2 was most frequently studied (12 comparisons) followed by PM10 and PM2.5 with 9 comparisons each. Only 2 associations showed strong level of evidence scrutinizing the link between NO2 and PM10 and hospital admissions or ED visits for asthma in the elderly. One additional association on O3 and adult hospital admissions or ED visits for asthma was highly suggestive. The rest of the comparisons presented suggestive (N=7), weak (N=17) or not significant (N=19) associations.
Although the detrimental effect of air pollution on immune-based diseases is frequently discussed, a very small number of associations on hard clinical outcomes are supported by strong evidence.