Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant, may exacerbate asthma and possibly increase susceptibility to infections. In addition, nitrogen oxides have a lifetime of approximately 1 day with respect to conversion to nitric acid. Ozone can irritate the eyes and air passages causing breathing difficulties and may increase susceptibility to infection. It is a highly reactive chemical, capable of attacking surfaces, fabrics and rubber materials.
Ozone is also toxic to some crops, vegetation and trees.
In the presence of nitrogen oxides these free radicals catalyse the oxidation of hydrocarbons to carbon dioxide and water vapour. Partially oxidised organic species such as aldehydes, ketones and carbon monoxide are intermediate products, with ozone being generated as a by-product. Because of the time required for chemical processing, ozone formation tends to be downwind of pollution centres. The resulting ozone pollution or summertime smog may persist for several days and be transported over long distances. VOCs are released in vehicle exhaust gases either as unburned fuels or as combustion products, and are also emitted by the evaporation of solvents and motor fuels.