Background: Some countries have recently extended smoke-free policies to particular outdoor settings; however, there is controversy regarding whether this is scientifically and ethically justifiable.
Objectives: The objective of the present study was to review research on secondhand smoke (SHS) exposure in outdoor settings.
Data sources: 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.
Study selection: 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.
Results: 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.
Conclusions: 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.