The association between indoor and outdoor NO2 levels: A case study in 50 residences in an urban neighbourhood in New Zealand

Background In the context of inner-city housing intensification, exploring the association between outdoor and indoor air quality is important to quantify any potential health risks. Methods We undertook a cross-sectional case study on 50 residences in an inner-city neighbourhood scheduled for urban development, bisected by a busy road. We measured indoor and outdoor NO2, indoor temperature, and undertook a survey on demographic factors and household NO2 sources. We performed a linear regression to examine the association between indoor and outdoor NO2 (measured at the building facade) adjusting for potential confounders. Results Mean indoor and outdoor levels of NO2 were 8.4 and 12.5 μg/m3 respectively. The unadjusted model showed no significant association between indoor and outdoor NO2. In the fully-adjusted model, a 1 % increase in outdoor NO2 was associated with a 5 % increase in indoor NO2 (95 %CI: 1.2–8.8%). Conclusion A weak association between indoor and outdoor NO2 was found. More intensive housing development in a transport corridor may be possible; however given the health risks of induced by air pollutants starts even at levels below WHO recommended levels, urban policy measures to reduce indoor and ambient air pollution are required.