006; Accepted: November 30, 2006; Published: January 16, 2007
Copyright: © 2007 Lin et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This review was supported by The International Union Against Tuberculosis and Lung Disease through a grant from the World Bank. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Abbreviations: AM, alveolar macrophage; CI, confidence interval; IAP, indoor air pollution from biomass fuels; OR, odds ratio; TB, tuberculosis; TST, tuberculin skin test
Tobacco smoking has been identified by the World Health Organization as one of the leading causes of death worldwide. Smokers are at higher risk than nonsmokers for a very wide variety of illnesses, many of which are life-threatening. Inhaling tobacco smoke, whether this is active (when an individual smokes) or passive (when an individual is exposed to cigarette smoke in their environment) has also been associated with tuberculosis (TB). Many people infected with the TB bacterium never develop disease, but it is thought that people infected with TB who also smoke are far more likely to develop the symptoms of disease, and to have worse outcomes when they do.
Why Was This Study Done?
The researchers were specifically interested in the link between smoking and TB. They wanted to try to work out the overall increase in risk for getting TB in people who smoke, as compared with people who do not smoke. In this study, the researchers wanted to separately study the risks for different types of exposure to smoke, so, for example, what the risks were for people who actively smoke as distinct from people who are exposed to smoke from others. The researchers also wanted to calculate the association between TB and exposure to indoor pollution from burning fuels such as wood and charcoal.
What Did the Researchers Do and Find?
In carrying out this study, the researchers wanted to base their conclusions on all the relevant information that was already available worldwide. Therefore they carried out a systematic review. A systematic review involves setting out the research question that is being asked and then developing a search strategy to find all the meaningful evidence relating to the particular question under study. For this systematic review, the researchers wanted to find all published research in the biomedical literature that looked at human participants and dealt with the association between active smoking, passive smoking, indoor air pollution and TB. Studies were included if they were published in English, Russian, or Chinese, and included enough data for the researchers to calculate a number for the increase in TB risk. The researchers initially found 1,397 research studies but then narrowed that down to 38 that fit their criteria. Then specific pieces of data were extracted from each of those studies and in some cases the researchers combined data to produce overall calculations for the increase in TB risk. Separate assessments were done for different aspects of “TB risk,” namely, TB infection, TB disease, and mortality from TB. The data showed an approximately 2-fold increase in risk of TB infection among smokers as compared with nonsmokers. The researchers found that all studies evaluating the link between smoking and TB disease or TB mortality showed an association, but they did not combine these data together because of wide potential differences between the studies. Finally, all studies looking at passive smoking found an association with TB, as did some of those examining the link with indoor air pollution.
What Do These Findings Mean?
The findings here show that smoking is associated with an increased risk of TB infection, disease, and deaths from TB. The researchers found much more data 