A combination of fog and chemicals that come from automobile and factory emissions and is acted upon by the action of the sun. Nitrogen dioxide, in the presence of the sun and some hydrocarbons, is turned into nitric oxide and atomic oxygen. The atomic oxygen reacts with the oxygen molecules and other constituents of automobile exhaust fumes to form a variety of products including ozone. The ozone is harmful in itself and is also implicated in a highly complex series of continuing reactions. As long as there is ozone or nitrogen dioxide and sunlight present, other undesirable reactions will occur.