Nitrogen dioxide is a toxic gas with the chemical formula NO2. It is one of several nitrogen oxides (including nitric oxide or nitrogen monoxide (NO) and nitrous oxide (N2O). NO2 is formed at high temperatures when fuel is not completely burned or when NO reacts with certain gases in the atmosphere. NO2 exists in the atmosphere at an average concentration of 0.3 parts per million (ppm), but concentrations can be much higher in dense urban areas as vehicular traffic is the biggest outdoor source of nitrogen dioxide. The main indoor sources include tobacco smoke and combustion appliances for cooking and/or heating. Nitrogen dioxide that originated outdoors can also migration indoors. Exposure to NO2 can inflame the airways in the lungs and affect how well they work, increasing susceptibility to respiratory infections and to allergens, and/or exacerbating the symptoms of those already suffering from lung conditions such as asthma or heart conditions. NO2 is also a greenhouse gas that affects the environment and the production of ozone. The AIR QUALITY STANDARDS REGULATIONS (2010) require that the annual mean concentration of NO2 must not exceed 40 µg/m3 and that there should be no more than 18 exceedances of the hourly mean limit value (concentrations above 200 µg/m3) in a single year.

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