A review of indoor nitrous acid (HONO) pollution: Measurement techniques, pollution characteristics, sources, and sinks

Indoor air quality is of major concern for human health and well-being. Nitrous acid (HONO) is an emerging indoor pollutant, and its indoor mixing ratios are usually higher than outdoor levels, ranging from a few to tens of parts per billion (ppb). HONO exhibits adverse effects to human health due to its respiratory toxicity and mutagenicity. Additionally, HONO can easily undergo photodissociation by ultraviolet light to produce hydroxyl radicals (OH•), which in turn trigger a series of further photochemical oxidation reactions of primary or secondary pollutants. The accumulation of indoor HONO can be attributed to both direct emissions from combustion sources, such as cooking, and secondary formation resulting from enhanced heterogeneous reactions of NOx on indoor surfaces. During the day, the primary sink of indoor HONO is photolysis to OH• and NO. Moreover, adsorption and/or reaction on indoor surfaces, and diffusion to the outside atmosphere contribute to HONO loss both during the day and at night. The level of indoor HONO is also affected by human occupancy, which can influence household factors such as temperature, humidity, light irradiation, and indoor surfaces. This comprehensive review article summarized the research progress on indoor HONO pollution based on indoor air measurements, laboratory studies, and model simulations. The environmental and health effects were highlighted, measurement techniques were summarized, pollution levels, sources and sinks, and household influencing factors were discussed, and the prospects in the future were proposed.