if you live in a metropolitan area such as Los Angeles, you are probably familiar with urban smog-the dark yellow or brown haze that builds up in a large stagnant air mass and hangs over populated areas on calm hot summer day. smog is made up mostly of ground-level ozone (), but it also contains numerous other chemicals, including carbon monoxide (), particulate matter such as soot and dust, volatile organic compounds() such as benzene, butane, and other hydrocarbons. the harmful ground-level ozone should not be confused with the useful ozone layer high in the stratosphere that earth from the sun's harmful ultraviolet rays. ozone at ground level is a pollutant with several adverse health effects.