Secondhand tobacco smoke exposure in open and semi-open settings: a systematic review

Some countries have recently extended smoke-free policies to particular outdoor settings; however, there is controversy regarding whether this is scientifically and ethically justifiable.

The objective of the present study was to review research on secondhand smoke (SHS) exposure in outdoor settings.

We conducted different searches in PubMed for the period prior to September 2012. We checked the references of the identified papers, and conducted a similar search in Google Scholar.

Our search terms included combinations of "secondhand smoke," "environmental tobacco smoke," "passive smoking" OR "tobacco smoke pollution" AND "outdoors" AND "PM" (particulate matter), "PM(2.5)" (PM with diameter ≤ 2.5 µm), "respirable suspended particles," "particulate matter," "nicotine," "CO" (carbon monoxide), "cotinine," "marker," "biomarker" OR "airborne marker." In total, 18 articles and reports met the inclusion criteria.

Almost all studies used PM(2.5) concentration as an SHS marker. Mean PM(2.5) concentrations reported for outdoor smoking areas when smokers were present ranged from 8.32 to 124 µg/m(3) at hospitality venues, and 4.60 to 17.80 µg/m(3) at other locations. Mean PM(2.5) concentrations in smoke-free indoor settings near outdoor smoking areas ranged from 4 to 120.51 µg/m(3). SHS levels increased when smokers were present, and outdoor and indoor SHS levels were related. Most studies reported a positive association between SHS measures and smoker density, enclosure of outdoor locations, wind conditions, and proximity to smokers.

The available evidence indicates high SHS levels at some outdoor smoking areas and at adjacent smoke-free indoor areas. Further research and standardization of methodology is needed to determine whether smoke-free legislation should be extended to outdoor settings.