Positive pressure ventilation systems and indoor air quality: PM2.5 outcomes in residential buildings

Fine particulate matter (PM 2.5 ) presents a risk to residential indoor environments, particularly during winter, when occupancy is high and natural ventilation is reduced. Evidence from intervention-based field studies is limited, especially under real-world, continuously occupied conditions. This study investigates the effects of positive pressure ventilation (PPV) systems on indoor PM 2.5 concentrations in 24 homes across New Zealand. Using a pre-post intervention design, PM 2.5 concentrations, temperature and relative humidity were measured in living rooms and master bedrooms over six week periods before and after PPV installation during winter. Following PPV installation, mean indoor PM 2.5 concentrations decreased across all homes, with reductions ranging from 38% to 62%. Linear mixed effects regression modelling supported the observed reductions while accounting for outdoor PM 2.5 and building characteristics. Indoor PM 2.5 concentrations were higher in living rooms than in bedrooms, although post-intervention reductions were similar between rooms. Indoor-outdoor (I/O) ratios exceeded 1.0 in half of the homes prior to PPV installation and fell below 1.0 in most homes post-installation, indicating reduced dominance of indoor sources. Post-PPV reductions in indoor PM 2.5 were larger during peak activity periods than non-peak periods and tended to be greater in homes with larger indoor-outdoor temperature differentials, suggesting that building envelope performance influences PPV effectiveness. This study presents field-based evidence that PPV systems can reduce indoor PM 2.5 in homes during winter, especially where initial indoor concentrations are high and thermal separation from outdoors is greater. The findings highlight the combined importance of ventilation, envelope performance, and occupant behaviour in reducing indoor PM 2.5 exposure.