Background

Ambient air pollution is among the greatest environmental risks to human health. However, little is known about the health effects of nitrogen dioxide (NO₂), a traffic-related air pollutant. Herein, we aimed to conduct a meta-analysis to investigate the long-term effects of NO₂ on mortality.

Methods

We conducted a systematic search for studies that were published up to February 2020 and performed a meta-analysis of all available epidemiologic studies evaluating the associations between long-term exposure to NO₂ with all-cause, cardiovascular, and respiratory mortality. Overall pooled effect estimates as well as subgroup-specific pooled estimates (e.g. location, exposure assessment method, exposure metric, study population, age at recruitment, and key confounder adjustment) and 95% confidence intervals were calculated using random-effects models. Risk of bias assessment was accessed by following WHO global air quality guidelines. Publication bias was accessed by visually inspecting funnel plot and Egger's liner regression was used to test of asymmetry.

Results

Our search initially retrieved 1349 unique studies, of which 34 studies met the inclusion criteria. The pooled hazard ratio (HR) for all-cause mortality was 1.06 (95%CI: 1.04–1.08, n = 28 studies, I² = 98.6%) per 10 ppb increase in annual NO₂ concentrations. The pooled HRs for cardiovascular and respiratory mortality per 10 ppb increment were 1.11 (95%CI: 1.07–1.16, n = 20 studies, I² = 99.2%) and 1.05 (95%CI: 1.02–1.08, n = 17 studies, I² = 94.6%), respectively. The sensitivity analysis pooling estimates from multi-pollutant models suggest an independent effect of NO₂ on mortality. Funnel plots indicate that there is no evidence for publication bias in our study.

Conclusion

We provide robust epidemiological evidence that long-term exposure to NO₂, a proxy for traffic-sourced air pollutants, is associated with a higher risk of all-cause, cardiovascular, and respiratory mortality that might be independent of other common air pollutants.