Short-term exposure to ozone, nitrogen dioxide, and sulphur dioxide and emergency department visits and hospital admissions due to asthma: A systematic review and meta-analysis

Abstract

Background: Air pollution is a major environmental hazard to human health and a leading cause of morbidity for asthma worldwide.

Objectives: To assess the current evidence on short-term effects (from several hours to 7 days) of exposure to ozone (O₃), nitrogen dioxide (NO₂), and sulphur dioxide (SO₂) on asthma exacerbations, defined as emergency room visits (ERVs) and hospital admissions (HAs).

Methods: We searched PubMed/MEDLINE, EMBASE and other electronic databases to retrieve studies that investigated the risk of asthma-related ERVs and HAs associated with short-term exposure to O₃, NO₂, or SO₂. We evaluated the risks of bias (RoB) for individual studies and the certainty of evidence for each pollutant in the overall analysis. A subgroup analysis was performed, stratified by sex, age, and type of asthma exacerbation. We conducted sensitivity analysis by excluding the studies with high RoB and based on the E-value. Publication bias was examined with the Egger's test and with funnel plots.

Results: Our literature search retrieved 9,059 articles, and finally 67 studies were included, from which 48 studies included the data on children, 21 on adults, 14 on the elderly, and 31 on the general population. Forty-three studies included data on asthma ERVs, and 25 on asthma HAs. The pooled relative risk (RR) per 10 µg/m³ increase of ambient concentrations was 1.008 (95%CI: 1.005, 1.011) for maximum 8-hour daily or average 24-hour O₃, 1.014 (95%CI: 1.008, 1.020) for average 24-hour NO₂, 1.010 (95%CI: 1.001, 1.020) for 24-hour SO₂, 1.017 (95%CI: 0.973, 1.063) for maximum 1-hour daily O₃, 0.999 (95%CI: 0.966, 1.033) for 1-hour NO₂, and 1.003 (95%CI: 0.992, 1.014) for 1-hour SO₂. Heterogeneity was observed in all pollutants except for 8-hour or 24-hour O₃ and 24-hour NO₂. In general, we found no significant differences between subgroups that can explain this heterogeneity. Sensitivity analysis based on the RoB showed certain differences in NO₂ and SO₂ when considering the outcome or confounding domains, but the analysis using the E-value showed that no unmeasured confounders were expected. There was no major evidence of publication bias. Based on the adaptation of the Grading of Recommendations Assessment, Development and Evaluation, the certainty of evidence was high for 8-hour or 24-hour O₃ and 24-hour NO₂, moderate for 24-hour SO₂, 1-hour O₃, and 1-hour SO₂, and low for 1-hour NO₂.

Conclusion: Short-term exposure to daily O₃, NO₂, and SO₂ was associated with an increased risk of asthma exacerbation in terms of asthma-associated ERVs and HAs.

Keywords: Air pollutants; Asthma; Meta-analysis; Observational study; Systematic review; Time series studies.