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. The effects of ground-level ozone can be worsened by outdoor exercise, since more air is inhaled during exercise. The more activity we undertake and the higher the ozone level, the more ozone we breathe in. Scientific studies indicate that respiratory symptoms subside after a few days of repetitive exposure, and the discomfort disappears. It is important to note, however, that although the outward symptoms disappear, the damage may continue to occur deep in the lungs.