Background and Objective: Exposure to traffic-related air pollutants is an important public health issue. Here, we present a systematic review and meta-analysis of research examining the relationship of measures of nitrogen oxides (NO_x) and of various measures of traffic-related air pollution exposure with lung cancer.

Methods: We conducted random-effects meta-analyses of studies examining exposure to nitrogen dioxide (NO₂) and NO_x and its association with lung cancer. We identified 20 studies that met inclusion criteria and provided information necessary to estimate the change in lung cancer per 10-μg/m³ increase in exposure to measured NO₂. Further, we qualitatively assessed the evidence of association between distance to roadways and traffic volume associated with lung cancer.

Results: The meta-estimate for the change in lung cancer associated with a 10-μg/m³ increase in exposure to NO₂ was 4% (95% CI: 1%, 8%). The meta-estimate for change in lung cancer associated with a 10-μg/m³ increase in NO_x was similar and slightly more precise, 3% (95% CI: 1%, 5%). The NO₂ meta-estimate was robust to different confounding adjustment sets as well as the exposure assessment techniques used. Trim-and-fill analyses suggest that if publication bias exists, the overall meta-estimate is biased away from the null. Forest plots for measures of traffic volume and distance to roadways largely suggest a modest increase in lung cancer risk.

Conclusion: We found consistent evidence of a relationship between NO₂, as a proxy for traffic-sourced air pollution exposure, with lung cancer. Studies of lung cancer related to residential proximity to roadways and NO_x also suggest increased risk, which may be attributable partly to air pollution exposure. The International Agency for Research on Cancer recently classified outdoor air pollution and particulate matter as carcinogenic (Group 1). These meta-analyses support this conclusion, drawing particular attention to traffic-sourced air pollution.

Citation: Hamra GB, Laden F, Cohen AJ, Raaschou-Nielsen O, Brauer M, Loomis D. 2015. Lung cancer and exposure to nitrogen dioxide and traffic: a systematic review and meta-analysis. Environ Health Perspect 123:1107–1112;  http://dx.doi.org/10.1289/ehp.1408882

Address correspondence to G.B. Hamra, Department of Environmental and Occupational Health, Drexel University School of Public Health, 3215 Market St., Philadelphia, PA 19104 USA. Telephone: (267) 359-6034. E-mail: ghassan.b.hamra@drexel.edu

A.J.C. is employed by the Health Effects Institute (HEI). The HEI receives about half of its core funds from the U.S. Environmental Protection Agency and half from the worldwide motor vehicle industry, although other public and private organizations periodically support special projects or certain research programs.

The views expressed in this article do not represent those of the HEI or its sponsors.

The authors declare they have no actual or potential competing financial interests.

Received: 27 June 2014 Accepted: 9 April 2015 Advance Publication: 14 April 2015 Final Publication: 1 November 2015

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