Is remaining indoors an effective way of reducing exposure to PM 2.5 during biomass burning events?

Bushfires, prescribed burns and residential wood burning are a significant source of fine particles (PM2.5 ) affecting the health and well-being of many communities. Despite the lack of evidence, a common public health recommendation is to remain indoors assuming that the home provides a protective barrier against ambient PM2.5 . The study aimed to assess to what extent houses provide protection against peak concentrations of outdoor PM2.5 and whether remaining indoors is an effective way of reducing exposure to PM2.5 . The effectiveness of this strategy was evaluated by conducting simultaneous week-long indoor and outdoor measurements of PM2.5 at 21 residences in regional areas of Victoria, Australia. During smoke plume events, remaining indoors protected residents from peak outdoor PM2.5 concentrations, but the level of protection was highly variable ranging from 12-76%. Housing stock (e.g. age of the house) and ventilation (e.g. having windows/doors open or closed) played a significant role in the infiltration of outdoor PM2.5 indoors. The results also showed that leaving windows and doors closed once the smoke plume abates trapped PM2.5 indoors and increased indoor exposure to PM2.5 . Furthermore, for approximately 50% of households, indoor sources such as cooking activities, smoking and burning candles or incense contributed significantly to indoor PM2.5 .