Factors affecting the indoor concentration of nitrogen dioxide in school and office environments

Potential health hazards caused by nitrogen oxides (NOx), especially nitrogen dioxide (NO2), are being intensively discussed, particularly as regards the release of NOx from diesel engines. Moreover, several epidemiological studies conducted in residential environments have examined the link between NO2 exposure and respiratory diseases. The World Health Organization (WHO) has published guidelines for indoor areas of 40 mg/m³ (annual) and 200 mg/m³ (1 h). The new NO2 guideline values derived for indoor areas in Germany are 0.25 mg/m³ (1h, precautionary) and 0.08 mg/m³ (1 h, health hazard). However, NO2 pollution of the indoor and outdoor atmosphere can only be considered and evaluated under regional aspects. As NO2 is a traffic- and industry-related pollutant, emissions are generally higher in urban than in rural areas and generally higher in outdoor than indoor if there are no specific indoor sources available. In this case I/O ratios strongly depend on the air exchange rate and on the NO2 removal efficiency. Various building and indoor environment characteristics, such as type of ventilation, air-tightness of the envelope, furnishing and surface characteristics of building, location of the building (urban versus suburban and proximity to traffic routes), as well as occupants behavior such as window opening, have been significantly associated with indoor NO2 levels. NO2 is effectively degraded on surfaces. Taking into account the materials used in indoor areas, degradation rates between 1.04 1/h and 1.45 1/h are common. Typical primary sources of indoor NO2 in residential buildings, are gas-fueled cooking, heating appliances, candles, and ethanol fireplaces. Our study focuses on the situation in schools and offices, where the information about global and local exposures to NO2 is limited. Data were extracted from the available literature and evaluated by means of existing criteria.