Given the strong relationship between cigarette smoking and lung cancer risk,
evidence of an association between PM2.5 and lung cancer is more convincing when observed among never-smokers,
compared with current or former smokers,
because of possible residual confounding by cigarette smoking (4, 5).
A previous analysis of the American Cancer Society Cancer Prevention Study-II (CPS-II),
based on 16 years of follow-up data of approximately 500,000 included participants controlling for measured parameters of active smoking,
found an 8% (95% confidence interval [CI], 1–16%) increase in lung cancer mortality for each 10 μg/m3 increase in PM2.5 concentrations (6).
The risk was somewhat higher,
although statistically insignificant,
when restricted to the subgroup of never-smokers.
An extended analysis of the Harvard Six Cities Study (n = 8,096) found a positive association between PM2.5 and lung cancer mortality (hazard ratio [HR] per each 10 μg/m3 = 1.27; 95% CI, 0.96–1.69) controlling for active smoking (7).
Naess and coworkers (8) observed significant positive associations between PM2.5 and lung cancer mortality among Oslo women in a recent register-based study; however, no data on smoking history were available in this study.