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Photochemical smog is a type of air pollution produced when sunlight acts upon waste gases such as from motor vehicle exhausts or power plants to form harmful substances such as ozone (O3), aldehydes and peroxyacetylnitrate (PAN). Ozone causes breathing difficulties, headaches, fatigue and can trigger respiratory problems. The peroxyacetylnitrate in photochemical smog can irritate the eyes, causing them to water and sting. In some cases it has also been seen as fatal. It is considered to consist of two things: Primary and secondary pollutants. Primary pollutants are when pollutants are released straight into the atmosphere, be it from factories, buses or power stations. It is then seen as a secondary pollutant if that primary pollutant is subject to another reaction. The everyday use of a car provides both primary and secondary pollutants. The following are a list of just what is released. At the high temperatures of the car's cylinder, nitrogen and oxygen from the air react to form nitric oxide (NO): N2(g) + O2(g) -----> 2NO(g) Some of the nitric oxide (NO) reacts with oxygen to form nitrogen dioxide (NO2): 2NO(g) + O2(g) -----> 2NO2(g) The mixture of nitric oxide (NO) and nitrogen dioxide (NO2) is sometimes referred to as NOx.
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