The hazards and benefits associated with smoking and smoking cessation in Asia: a meta-analysis of prospective studies.
To provide the most reliable evidence as to the nature of the associations between smoking and cause-specific illness, as well as the expected benefits from quitting smoking, in studies conducted in Asia, where smoking remains popular among men. Studies published between January 1966 and October 2008, identified in the Medline search strategy with medical subject headings, in addition to studies from the Asia Pacific Cohort Studies Collaboration. Studies were considered to be relevant if they were prospective studies, in an Asian setting that reported on the association between smoking, quitting and cause-specific illness. Two reviewers independently screened all identified articles for possible inclusion and extracted data. The pooled relative risks (RRs) for incidence or mortality, comparing current to never smokers were always significantly higher than unity; the highest was for lung cancer: 3.54 (95% confidence interval 3.00 to 4.17). The pooled RRs for former smokers (compared to never smokers) were also always significantly higher than unity, and were lower than in current smokers, for coronary heart disease, stroke, lung and upper aero-digestive tract cancer. Only for respiratory disease was the RR for former smokers higher than that for current smokers. This meta-analysis has shown that, despite the relative immaturity of the smoking epidemic in Asia, smoking is unquestionably a major contributor to ill health and death. However, the beneficial effects of quitting are not yet always apparent, most probably because quitting is a consequence of ill health and the relative unpopularity of smoking cessation in many Asian populations.