Nitrogen Dioxide gas is a highly reactive gas formed via the oxidation of nitric oxide. Although it can be produced naturally, the majority of nitrogen dioxide found in urban areas has been caused by human activities. Nitrogen dioxide is not only a toxic gas but it is also a precursor to several harmful secondary air pollutants such as ozone and particulate matter. It also plays a role in the formation of acid rain and photochemical smog.

The major source of nitrogen dioxide gas in urban environments is the burning of fossil fuels. In urban areas this is most commonly associated with motor vehicle exhaust. Areas with high density road networks close to large populations such as in towns and cities are most at risk of over, exposure. Industrial sites will also produce high concentrations of nitrogen dioxide. These include any industry that use combustion processes such as power plants, electric utilities and industrial boilers. Long term exposure of nitrogen dioxide gas can lead to respiratory problems such as asthma and bronchitis.