Epidemiologic Evidence and Possible Mechanisms for the Association Between Cigarette Smoking and Injuries. Part 2: Is the Relationship Between Smoking and Injuries Causal?

Part 1 of this series reviewed the epidemiologic evidence for the association between cigarette smoking and injuries and possible biological and psychosocial mechanisms to account for this relationship. In the present article, nine criteria are explored to determine if smoking is a direct cause of injuries (i.e., a causal relationship). There is substantial evidence that individuals who smoked in the past have a higher subsequent risk of injury. A recent meta-analysis found that smokers in the military were 1.31 times more likely to be injured than nonsmokers and Servicemembers with low, medium, and high levels of smoking had 1.27, 1.37, and 1.71 times, respectively, the risk of injury compared with nonsmokers. The association between smoking and injuries has been reported in at least 18 US military studies and in 14 civilian studies in seven countries. The biological plausibility of the association was discussed extensively in part 1 of this series. A possible alternative explanation with sufficient data was that smokers may be risk takers and it is the risk-taking behavior that increases injury risk (not smoking per se). Once an individual no longer smokes, a decrease in injury risk has been reported for at least bone health and wound healing. The effects of smoking do not appear to be specific to one type of injury, possibly because of the numerous compounds in tobacco smoke that could affect tissues and physiological processes, with evidence provided for bones, tendons, and healing processes. The association was consistent with other knowledge, with some evidence provided from other types of medical problems and trends in smoking and injury-related mortality. In summary, the association between smoking and injuries appears to meet many of the criteria for a causal relationship.