IntroductionThe Surgeon General reports that there is no risk-free level of exposure to secondhand smoke and that it can cause premature death and disease in nonsmokers.1 Previous research has demonstrated that levels of fine particulate matter (PM 2.5 ) in the air are three times higher in smoke-permitted homes than in smoke-free homes and that confining smoking to certain living spaces does not offer protection from AbstractIntroduction. Secondhand smoke remains a health concern for individuals living in multiunit housing, where smoke has been shown to easily transfer between units. Building-wide smoke-free policies are a logical step for minimizing smoke exposure in these settings. This evaluation sought to determine whether buildings with smoke-free policies have less secondhand smoke than similar buildings without such policies. Furthermore, this study assessed potential secondhand smoke transfer between apartments with and without resident smokers. Methods. Fine particulate matter (PM 2.5 ), airborne nicotine, and self-reported smoking activity were recorded in 15 households with resident smokers and 17 households where no one smoked in 5 Boston Housing Authority developments. Of these, 4 apartment pairs were adjacent apartments with and without resident smokers. Halls between apartments and outdoor air were also monitored to capture potential smoke transfer and to provide background PM 2.5 concentrations. Results. Households within buildings with smoke-free policies showed lower PM 2.5 concentrations compared to buildings without these policies (median: 4.8 vs 8.1 µg/m