The 2015 smoke haze episode was one of the most severe and prolonged transboundary air pollution events ever seen in Southeast Asia (SEA), affecting the air quality of several countries within the region including Indonesia, Malaysia and Singapore. The 24 h mean outdoor PM2.5 (particulate matter (PM) with aerodynamic diameter ≤ 2.5 μm) concentrations ranged from 72–157 μg m−3 in Singapore during this episode, exceeding the WHO 24 h mean PM2.5 guidelines (25 μg m−3) several times over.