Although outdoor air pollution has been identified as carcinogenic to humans, the magnitude of the relative risk (RR) and the 95% confidence interval (CI) for lung cancer in relation to outdoor air pollution remain uncertain. On a global scale, we quantified the risk of lung cancer associated with long-term exposure to outdoor air pollution using a meta-analytic approach. Relevant cohort studies from two databases (PubMed and Web of Science) through 31 May 2014 were searched, and a total of 21 cohort studies were identified in the analysis. The risk of lung cancer mortality or morbidity increased 7.23 (95% CI: 1.48-13.31)%/10 μg/m increase in fine particles (PM2.5), 13.17 (95% CI: 5.57-21.30)%/10 parts per billion (ppb) increase in nitrogen dioxide (NO2), 0.81 (95% CI: 0.14-1.49)%/10 ppb increase in nitrogen oxides (NOx), and 14.76 (95% CI: 1.04-30.34)%/10 ppb increase in sulfur dioxide (SO2). These positive associations remained when analysis was restricted to never-smokers or studies with high methodological quality, and showed no difference by sex. In addition, the association of fine particles with lung cancer was suggestively stronger among never-smokers (RR per each 10 μg/m=1.18, 95% CI: 1.06-1.32). There was a null association for carbon monoxide and ozone. Our study indicated that long-term exposure to PM2.5, NO2, NOx, and SO2 may be associated with an increased risk of lung cancer. Although the magnitude of the RR is relatively small, our finding, if validated, may be of public health importance because a large proportion of the population is exposed to air pollution globally.