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This case-control study examined the smoking habits and medical causes of mortality in men from southern India. The death rates of current or former smokers were double those of never smokers (RR = 2.1 (95% CI 2.0 to 2.2)). Of this excess mortality among smokers, a third involved respiratory disease. Tuberculosis was the chief cause of respiratory mortality (4.5 (95% CI 4.0 to 5.0), smoking attributed fraction 61%). A separate survey of 250 000 men living in the urban study area found that smokers are three times more likely than never smokers to report a history of tuberculosis, corresponding to a higher rate of progression of chronic subclinical infection to clinical disease. The authors conclude that smoking, which is a cause of half the deaths from tuberculosis in men in India, increases the incidence of clinical tuberculosis.
This and similar studies will no doubt stimulate research into the mechanisms behind reactivation of tuberculosis by smoking. Some clues may be derived from the reported effects of smoking on other diseases. Smokers have a reduced incidence of sarcoidosis and extrinsic allergic alveolitis, diseases in which tumour necrosis factor-α (TNF-α) plays a key role. Moreover, the use of anti-TNF-α agents for conditions such as rheumatoid arthritis leads to reactivation of tuberculosis. Collectively, these data converge on the hypothesis that smoking might oppose the release or function of TNF-α in the lungs. Recent work on the effects of nicotine has revealed likely pathways (