The study involved 2,401 adults from two urban communities in Cape Town, South Africa. Participants were surveyed about their smoking habits and underwent a tuberculin skin test. Factors such as age, sex, education level, body mass index, and income were taken into account.

Smoking was defined as having ever smoked for at least one year. The average number of cigarettes smoked per day was recorded for all ex-smokers and current smokers, and the number of pack-years smoked was calculated.

Of 1,309 current smokers or ex-smokers, 1,070 (82%) had a positive skin test. This was significantly higher than for never smokers. A positive relation with pack-years was also observed, with those smoking more than 15 pack-years having the highest risk.

The reason for the increased risk of infection in smokers is unclear, but may be explained by the effects of smoking on the ability of the lungs to fight off infection, say the authors.

They conclude that smoking may increase the risk of tuberculosis infection, and propose that further studies be conducted to investigate this association and to establish whether smoking reduction strategies contribute to tuberculosis control.

An accompanying editorial discusses whether this is a chance or causal association. The evidence from the literature does not support an effect of smoking on tuberculin responses, but perhaps the likelihood of developing active tuberculosis is greater in those who have smoked for a long time, says Dr Graham Bothamley. Social factors are significant confounding factors in such studies.; Source: British Medical Journal