Relationship between indoor and outdoor NO2: A review

Studies have proved that outdoor nitrogen dioxide (NO2) has adverse health effects for people. Nowadays, most people spend much more time indoors than outdoors, and the indoor NO2 concentration is affected by a variety of indoor and outdoor factors. In order to fully understand the relationship between indoor and outdoor NO2 concentrations, which is important for assessing human exposure to NO2, we reviewed and summarized the measured and modeled indoor/outdoor (I/O) NO2 concentration ratio based on a literature search of 218 publications in Web of Science and PubMed. The results showed that the I/O NO2 concentration ratio reported in the literature differed between countries and regions, thus indicating that differences existed in the indoor NO2 source strength and ventilation. The highest I/O NO2 concentration ratios were found for residential buildings, with cooking, smoking, and indoor combustion heating being the main indoor emission sources of NO2. Offices were associated with relatively few indoor sources, and schools had the least number of sources. For the same building type, seasonal differences in the indoor and outdoor NO2 concentrations were mainly due to the air change rate, which was highest during the summer. In this review, we also revisited physical-chemical principles based models for NO2 and summarized the input parameters. The results of this study will facilitate more accurate estimations of human exposure to NO2, and consequently, better assessments of the health effects caused by NO2.