Background:
Previous systematic reviews examining the impact of smoke-free legislation (SFL) suggest more beneficial impacts in higher socioeconomic status (SES) groups. However, these have focused on associated changes in smoking behaviour due to a paucity of studies examining health outcomes. This review includes more recent evidence to assess the impact of comprehensive and partial SFL on socioeconomic inequalities in smoking-related: a) disease-specific morbidity and mortality and b) all-cause mortality among adults.

Methods:
We searched Medline and fourteen other databases up to September 2016. Studies in English, reporting the differential impacts by SES of SFL on smoking-related morbidity and mortality among adults (≥18 years) were eligible. Following data extraction, quality and risk of bias among the included studies were assessed using the Cochrane EPOC and EPHPP criteria. Harvest plot and narrative methods were used to illustrate either i)neutral effect of SFL on inequalities, ii) positive effect (greater benefits among lower SES groups, iii) negative effect (greater benefits among higher SES groups) or iv) mixed/unclear effect.

Results:
Eight studies (six interrupted time-series and two before-after studies), all from high-income countries were included; five studies examined the impact of comprehensive SFL and three the impact of partial SFL. Implementation of comprehensive SFL led to either a positive effect (three studies) or no effect (two studies) on socioeconomic inequalities in health outcomes; partial SFL led to either no effect (two studies) or unclear effect (one study). The choice of the socioeconomic indicator may have had an important influence on findings. Except for the before-after studies, most studies were judged to be high quality with a suitable study design.

Conclusions:
Comprehensive SFL potentially reduces inequalities in smoking-related health outcomes, partial SFL may not. All countries should implement comprehensive SFL to achieve the Sustainable Development Goal targets to reduce premature NCD mortality and inequalities.