Human immunodeficiency virus (HIV) infection
HIV infection has been observed to be statistically associated with an increased lung cancer risk; for example, the results of a meta-analysis of 13 studies indicated HIV-infected individuals had a 2.6-fold higher risk of lung cancer than non-HIV-infected individuals (standard incidence ratio = 2.6; 95% confidence interval [CI] 2.1–3.1). The clinical significance of this association remains to be elucidated, as it raises the possibility that HIV infection increases susceptibility to lung cancer, but may merely reflect the high smoking prevalence (study estimates ranged from 59% to 96%) among those infected with HIV compared with the general population (smoking prevalence approximately 20%).
Other environmental causes of lung cancer
Occupational exposures to lung carcinogens
Several environmental exposures other than tobacco smoke are causally associated with lung cancer, but the proportion of the lung cancer burden due to these exposures is small compared with cigarette smoking. Many lung carcinogens have been identified in studies of high occupational exposures. Considered in total, occupational exposures have been estimated to account for approximately 10% of lung cancers. These carcinogens include asbestos, radon, tar and soot (source of PAHs), arsenic, chromium, and nickel. For many of these workplace carcinogens, cigarette smoking interacts synergistically to increase the risk. In developed countries, workplace exposures to these agents have largely been controlled.
Approximately 10% of all lung cancer deaths and 30% of lung cancer deaths in lifetime nonsmokers are estimated to be attributable to indoor exposure to radon, estimations that are supported by meta-analysis and pooled-analyses of case-control studies of lung cancer and indoor radon exposure.[29,30] Due to a synergistic interaction between cigarette smoking and radon exposure, the radon-associated risk of lung cancer among smokers is considerably greater than for nonsmokers. The prevention strategy for residents of homes with high radon concentrations is to have the basement sealed to prevent radon gas from leaking into the home.
Early evidence from case-control and cohort studies did not support an association between air pollution and lung cancer; however, the evidence now points to a genuine association. In particular, two prospective cohort studies provide evidence to suggest that air pollution is weakly associated with the risk of lung cancer. In an extended follow-up of a study of six U.S. cities, the adjusted relative risk (RR) of lung cancer mortality for each 10 µg/m3 increase in concentration of fine-particulate was 1.27 (95% CI, 0.96–1.69). Using data from the American Cancer Society's Cancer Prevention Study II, it was observed that compared with the least polluted areas, residence in areas with high sulfate concentrations was associated with an increased risk of lung cancer (adjusted RR = 1.4; 95% CI, 1.1–1.7) after adjustment for occupational exposures and the factors mentioned above. In a subsequent update to this report, the risk of lung cancer was observed to increase 14% for each 10 μg/m3 increase in concentration of fine particles. The evidence indicating an association between constituents of ambient air pollution and increased lung cancer mortality continues to strengthen, with reports from Asia [37,38] and New Zealand  documenting increased risks with exposure to measures of particulate matter, sulfur dioxide, and nitrogen dioxide.