Combined Effect of Air Pollution with Other Agents

Air pollution has been linked to several malignancies, including lung cancer, urinary bladder cancer, and acute leukaemia; the evidence is most abundant for lung cancer, for which several causal factors are well established (Samet and Cohen, 2006). Worldwide, tobacco use, and in particular cigarette smoking, is the dominant cause of lung cancer and accounts for the majority of cases; in fact, most cases in many countries (IARC, 2004). Other well-characterized causes of lung cancer include occupational agents and indoor radon (Alberg and Samet, 2003). When considering air pollution and its sources in the causation of lung cancer, its combined effects with these other causes of lung cancer are inevitably an issue. At present in the USA and some other developed countries, the relative risk of lung cancer in regular smokers compared with never-smokers ranges from 10 to > 20. These extremely high relative risk values indicate a need to consider the potential modification by tobacco smoking of the risk of lung cancer (and possibly other cancers) associated with air pollution, and to assess the potential for residual confounding by tobacco smoking to explain risks associated with air pollution in epidemiological studies. In addition, information on tobacco smoking and cancer is relevant to the collective body of evidence for an evaluation of air pollution by the International Agency for Research on Cancer (IARC), since some specific carcinogens in air pollution are also found in tobacco smoke, which similarly to ambient and indoor air pollution, is a complex mixture that contains a multitude of carcinogens, and, similarly to ambient air pollution, elicits an inflammatory response in the lung and systemically (Lewtas, 2007).