Tuberculosis (TB) is a disease caused by a bacterium called Mycobacterium tuberculosis. Although the number of TB cases has decreased over the past decades, mainly in high economic countries, it is still an important public health problem due to recently emerging bacterial strains, particularly in low- and middle-income countries, that are not responsive to commonly used drugs.
Evidence that active and passive smoking increases risk of tuberculosis
- People who smoke were 1.57 more likely to develop tuberculosis
- The risk of tuberculosis increases when people smoke more cigarettes per day and for a longer period of time
- Non-smokers who are exposed to other peoples smoke are at an increased risk of developing tuberculosis
What evidence was used?
The present review started in August 2013 and was designed to up-date previous systematic reviews relating to the increased risk of developing tuberculosis with active and passive smoking. Four systematic reviews were identified that included literature published in the period up to 2007. At the same time, an updated search for additional original studies was performed using 2006 as a starting date to present. From these searches, a total of five original studies were found that fulfilled the criteria for inclusion in the present study.
One study assessed the effects of active and passive tobacco smoking on the risk of developing tuberculosis, three studies only assessed the effect of active smoking, and the remaining study only assessed the effect of passive smoking. Two of the studies used the same population of participants; however, one paper assessed the effect of active smoking (Leung 2004) and the other assessed the effect of passive smoking (Leung 2010).
Two of the studies used the incidence of tuberculosis as the outcome, one study used death from tuberculosis as the outcome, and the remaining two studies used incidence and/or death from tuberculosis as the outcome.
None of the included studies were conducted in Europe; two studies were conducted in Hong Kong (using the same recruited population), one in Taiwan, one in India, and one in South Korea.
The quality of the five studies (as judged by the Newcastle-Ottawa scale score) ranged from 5 to 8. Three of the studies assessing the effect of active smoking on tuberculosis were deemed of high quality and both of the studies assessing the effect of passive smoking on tuberculosis were deemed of high quality (≥7).
Detailed findings and data
People who smoked tobacco were 1.57 times more likely to develop tuberculosis when compared to those who had never smoked (pooled relative risk ratio 1.57, 95% confidence interval 1.18 to 2.10). Click here to see a forest plot of the findings – Figure A.
To explore the impact of the methodological quality of the four studies included in the review, the analysis was grouped into higher (≥7) versus lower (<7) quality studies according to the Newcastle-Ottawa scale score. Significant differences were seen between the two subgroups (p<0.00001), as only a significant increase in risk of tuberculosis was seen in the three studies with a higher quality. Click here to see a forest plot of the findings – Figure B.
Number of cigarettes
All of the studies reported an increased risk of tuberculosis with increasing numbers of cigarettes smoked per day (Leung 2004; Pednekar 2007; Lin 2009; Jee 2009). Also, data from one study showed a significant trend in the increased risk of tuberculosis with increasing pack-years of smoking (Lin 2009).
The two studies that investigated passive smoking showed that people who are exposed to household tobacco smoke were 1.44 times more likely to develop tuberculosis when compared to people who were unexposed to tobacco smoke (pooled relative risk ratio 1.44, 95% confidence interval 1.02 to 2.04). Click here to see a forest plot of the findings – Figure C.
Both of the included studies were deemed to be of high quality (≥7), and similar results were seen for the study which solely recruited females (Leung 2010) when compared to the study which recruited males and females (Lin 2009).
No significant trend was seen between the increased frequency of exposure to passive smoke (numbers of days per week exposed) and the risk of tuberculosis (p=0.74) (Lin 2009).
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