There are few studies measuring exposure to outdoor tobacco smoke (OTS). Tobacco users often gather at the boundaries of tobacco-free campuses, resulting in unintended consequences. The objective of this study was to measure exposure levels from OTS on sidewalks bordering a tobacco-free university campus. Data were collected while walking along a sidewalk adjacent to a medium traffic road between May and August 2011. Monitoring occurred during "background," "stop," and "walk-through" conditions at and near hot spot area to measure fine particulate matter (< 2.5 microm; PM2.5) from OTS using a portable aerosol monitor The average PM2.5 levels during stop and walk-through conditions were significantly higher than during background conditions. PM2.5 peak occurrence rate and magnitude of peak concentration were significantly different depending on smoking occurrence. The peak occurrence rate during the stop condition was 10.4 times higher than during the background condition, and 3.1 times higher than during the walk-through condition. Average peak PM2.5 concentrations during the stop condition were 48.7% higher than during the background condition. In conclusion, individuals could be exposed to high levels of PM2.5 when stopping or even passing by smokers outdoors at the perimeter of tobacco-free campuses. The design and implementation of tobacco-free campus policies need to take into account the unintended consequences of OTS exposure at the boundaries. Implications: In this study, outdoor tobacco smoke (OTS) exposure was measured at the perimeter of tobacco-free campus. OTS exposure could be determined by peak analysis. Peak occurrence rate and peak concentration for OTS exposure were identified by using peak analysis. People could be exposed to high levels of PM2.5 when standing or even passing by smokers at the perimeter of tobacco-free campus. OTS exposure measurement in other outdoor locations with smokers is needed to support outdoor smoking regulation.