But ozone also exists at ground level and is the main component of smog. Disproportionate increases of man-made emissions of the pollutants that contribute to ozone formation have resulted in a serious increase in smog levels in recent years. NO x are compounds containing nitrogen and oxygen. VOCs are both an indoor and outdoor pollutant. Fuel combustion is another source, such as benzene from car exhaust.
Forests are the main source of naturally occurring VOCs.
Because sunlight is a key factor in the formation of ground-level ozone, ozone levels are highest on hot summer days. Stagnant air masses can trap pollutants over a region for several days so that ground-level ozone accumulates and concentrations can exceed established health guidelines. For example, in Canada, high concentrations of ozone frequently occur in the Windsor-Quebec corridor, the Lower Fraser Valley and the South Atlantic region. Ozone is a powerful and irritating pollutant. Short-term exposures can irritate the nose and throat, and produce symptoms like coughing and difficult or painful breathing.