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

•Inner-city residential intensification comes with risks of air pollution exposure (84 char with spaces).•Indoor- outdoor air pollution associations in residences are important to quantify.•We researched NO2 pollution in an inner-city suburb bisected by a busy road.•An increase in outdoor NO2 of 1 % was associated with a 5 % increase in indoor NO2.•Indoor and outdoor NO2 pollution sources need to be reduced.