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Meta-Analysis
. 2021 Jul 1:776:145968.
doi: 10.1016/j.scitotenv.2021.145968. Epub 2021 Feb 19.

Long-term exposure to nitrogen dioxide and mortality: A systematic review and meta-analysis

Shiwen Huang  1 , Haomin Li  2 , Mingrui Wang  1 , Yaoyao Qian  1 , Kyle Steenland  1 , William Michael Caudle  1 , Yang Liu  1 , Jeremy Sarnat  1 , Stefania Papatheodorou  3 , Liuhua Shi  4
Affiliations
Meta-Analysis

Long-term exposure to nitrogen dioxide and mortality: A systematic review and meta-analysis

Shiwen Huang et al. Sci Total Environ. .

Abstract

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.

Keywords: Air pollution; All-cause mortality; Cardiovascular mortality; Meta-analysis; Nitrogen dioxide; Respiratory mortality.

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Conflict of interest statement

Declaration of competing interest The authors declare no competing interests.

Figures

Figure 1.
Figure 1.
Flow chart of the study selection process.
Figure 2.
Figure 2.
Traffic light plot of risk of bias assessment for each study.
Figure 3.
Figure 3.
Forest plot of study-specific hazard ratio (HR) of all-cause mortality associated with a 10-ppb increase in exposure to NO2. The meta-estimate and weights in the forest plot are estimated from random effects meta-analyses.
Figure 4.
Figure 4.
Forest plot of study-specific hazard ratio (HR) of cardiovascular mortality associated with a 10-ppb increase in exposure to NO2. The meta-estimate and weights in the forest plot are estimated from random effects meta-analyses.
Figure 5.
Figure 5.
Forest plot of study-specific hazard ratio (HR) of respiratory mortality associated with a 10-ppb increase in exposure to NO2. The meta-estimate and weights in the forest plot are estimated from random effects meta-analyses.
Figure 6.
Figure 6.
Funnel plots for (a) all-cause mortality, (b) cardiovascular mortality, and (c) respiratory mortality.
See this image and copyright information in PMC

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