Impact of second-hand smoke exposure followed by cigarette smoking quitting on subsequent risk of coronary heart disease: Evidence from the population-based cohort of Tehran Lipid and Glucose Study (TLGS).

Cigarette smoking with its various phenotypes is an established, strong, and modifiable risk factor for coronary heart disease (CHD). Little research has been conducted on the effect of former smokers who have quitted smoking but are exposed to others’ cigarette smoke (former & secondhand smokers) on CHD risk. Limitations of published data have left this important question.
A prospective population-based cohort (TLGS) was conducted (n=20069) with median follow-up time of 14.6 years. A subset of 8050 participants ≥30 years old (first CHD events as study outcome) were considered. Participants were categorized as never, former, current, secondhand, and former & secondhand smokers. Data on smoking intensity (cigarettes/day) were also collected. Cox proportional hazards regression model was applied to estimate the risk of CHD taking into account the main potential confounders.
The mean (SD) age of individuals was 46.10 (11.38) years that experienced 1118 first CHD events (most CHD cases in former smokers) during follow-up. The risk of CHD was higher in current, former & secondhand, former, and secondhand smokers (HR: 1.99, 95% CI: 1.65-2.39; HR: 1.55, 95% CI: 1.15-2.08; HR: 1.39, 95% CI: 1.12-1.72; HR: 1.27, 95% CI: 1.07-1.51) respectively than never smokers. The risk of CHD increased with rising the smoking intensity (as a better proposed smoking phenotype) indicating a dose-response pattern.
The risk of CHD in former & secondhand smokers was an interesting and remarkable finding which need to further research to establish and approve in future studies to transfer to health policy makers.