Nitric oxide (NO) is an odourless, colourless gas which is produced during high temperature burning of fuel in, for example, cars and other road vehicles, heaters and cookers. Once it is mixed with air it quickly combines with oxygen, forming nitrogen dioxide (NO2). Most nitrogen dioxide in the atmosphere is formed from the oxidation of nitric oxide (NO) in this way, although some is release directly from source. It is also present in tobacco smoke. It is a reddish brown, nonflammable, gas with a detectable smell. In significant concentrations it is highly toxic, causing serious lung damage with a delayed effect. Other health effects of exposure to nitrogen dioxide include shortness of breath and chest pains. Nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone or smog.
Since nitrogen dioxide is a traffic-related pollutant, emissions are generally highest in urban rather than rural areas. Annual mean concentrations of nitrogen dioxide in urban areas are generally in the range 10-45 ppb, and lower in rural areas. Levels vary significantly throughout the day, with peaks generally occurring twice daily as a consequence of rush hour traffic. Concentrations can be as high as 200 ppb.