In 1956 Doll and Hill¹ wrote that “the relationship between smoking and mortality from pulmonary tuberculosis is distinct, but with a disease so influenced by social factors more precise data are needed to justify a direct cause and effect hypothesis”.

The essential risk factors for human tuberculosis are (1) the tubercle bacillus, (2) a susceptible host, and (3) an environment which allows the tubercle bacilli to survive transit from one host to the next. All other risk factors are subsumed under these headings. If smoking is a risk factor for tuberculosis, then it must act by increasing the susceptibility of the human host or the probability of transmission by encouraging infectious individuals to cough (this requires smoking to be a social as much as an individual pursuit). If the association between smoking and tuberculosis is more apparent than real, then smoking may be a pointer to other risk factors. These include social class—itself a marker for overcrowding, poor ventilation and rooms with no natural light as well as poor nutrition—general ill health and, increasingly, HIV infection with prostitution and intravenous drug use.

TUBERCULIN SENSITIVITY AND SMOKING

Tuberculous infection and tuberculosis as a disease are entirely different states. The former is commonly characterised by tuberculin reactivity (despite the known problem of exposure to non-tuberculous mycobacteria and the cross reactivity of the mixture of antigens²). Approximately one third of the world’s population may be infected with the tubercle bacillus, but only eight million (0.4%) develop tuberculosis each year.

Studies conflict as to whether smoking affects delayed hypersensitivity to tuberculin. Kuemmerer and Comstock³ noted that …