Penetration of nitrogen oxides and particles from outdoor into indoor air and removal of the pollutants through filtration of incoming air

We studied the effect of ventilation and air filtration systems on indoor a ir quality in a children's day-care center in Finland. Ambient air nitrogen oxides (NO, NO2) and particles (TSP, PM10) were simultaneously measured ou tdoors and indoors with automatic nitrogen oxide analyzers and dust monitor ing. Without filtration nitrogen oxides and particulate matter generated by nearby motor traffic penetrated readily indoors. With chemical filtration 50-70% of nitrogen oxides could be removed. Mechanical ventilation and filt ration also reduced indoor particle levels. During holidays and weekends wh en there was no opening of doors and windows and no particle-generating act ivity indoors, the indoor particle level was reduced to less than 10% of th e outdoor level. At times when outdoor particle concentrations were high du ring weekdays, the indoor level was about 25% of the outdoor level. Thus, t he possible adverse health effects of nitrogen oxides and particles indoors could be countered by efficient filtration. We also showed that inclusion of heat recovery equipment can make new ventilation installations economica l.