Abstract
Background: Although nitrogen dioxide (NO2) is one of the most common air pollutants encountered indoors, and extensive literature has examined the link between NO2 exposure and duration causing adverse respiratory effects in susceptible populations, information about global and local exposure to NO2 in different indoor environments is limited. To synthesize the existing knowledge, this review analyzes the magnitude of and the trends in global and local exposure to NO2 in schools and offices, and the factors that control exposure.
Methods: For the literature review, Web of Science, SCOPUS, Google Scholar, and PubMed were searched using 42 search terms and their combinations to identify manuscripts, reports, and directives published between 1971 and 2019. The search was then extended to the reference lists of relevant articles.
Results: The calculated median, as well as the mean, concentration of NO2 in school (median 21.1 mu g/m(3); mean 29.4 mu g/m(3)) and office settings (median 22.7 mu g/m(3); mean 25.1 mu g/m(3)) was well below the World Health Organization (WHO) guideline of 40 mu g/m(3) for the annual mean NO2 concentration. However, a large range of average concentrations of NO2 were reported, from 6.00 to 68.5 mu g/m(3) and from 3.40 to 56.5 mu g/m(3) for school and office environments, respectively, indicating situations where the WHO guidelines are exceeded. Outdoor levels of NO2 are a reliable predictor of indoor NO2 levels across seasons, with mean and median Indoor/Outdoor (I/O) ratios of 0.9 and 0.7 in school and 0.9 and 0.8 in office environments, respectively. The absence of major indoor NO2 emission sources and NO2 sinks, including chemical reactions and deposition, are the reasons for lower indoor NO2 concentrations. During the winter, outdoor NO2 concentrations are generally higher than during the summer. In addition, various building and indoor environment characteristics, such as type of ventilation, air exchange rates, airtightness of the envelope, furnishing and surface characteristics of the building, location of the building (urban versus suburban and proximity to traffic routes), as well as occupants' behavior (such as opening windows), have been statistically significantly associated with indoor NO2 levels in school and office environments.
Conclusions: Indoor exposure to NO2 from the infiltration of ambient air can be significant in urban areas, and in the case of high traffic volume. Although reducing transportation emissions is challenging, there are several easier means to reduce indoor NO2 concentrations, including a ventilation strategy with suitable filters; location planning of new schools, classrooms, and ventilating windows or intakes; traffic planning (location and density); and reducing the use of NO2-releasing indoor sources.