Long-term Exposure to Nitrogen Dioxide and Mortality: A Systematic Review and Meta-analysis Open Access

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 1,349 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.

Table of Contents

1.    Introduction  1

2.    Methods       2

2.1 Search strategy   2

2.2 Study selection    3

2.3 Data extraction    4

2.4 Statistical analysis      5

2.5 Risk of bias assessment   6

3.    Results  7

3.1 Characteristics of the eligible studies    7

3.2 Risk of bias assessment   8

3.3 Results of the meta-analysis    8

3.4 All-cause mortality      9

3.5 Cardiovascular mortality    10

3.6 Respiratory mortality  10

3.7 Publication bias   11

3.8 Sensitivity analysis     11

4.    Discussion   12

5.    Conclusion   16

6.    References: 17

7.    Tables   22

8.    Figures  29

9.    Appendix      35

About this Master's Thesis

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School	Rollins School of Public Health
Department	Environmental Health
Degree	M.S.P.H.
Submission	Master's Thesis
Language	English
Research Field	Environmental Health
Keyword	All-cause mortality Meta-analysis Cardiovascular mortality Air pollution Nitrogen Dioxide Respiratory mortality
Committee Chair / Thesis Advisor	Liuhua Shi, Emory University

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