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Though visual air pollution is not a direct measure of the level of air quality, it can give the viewer a clue  to the healthfulness of ambient air. Many of the air pollutants that contribute to the formation of the harmful, but invisible, ozone also impact clarity of the air.

Smog, a grayish red haze, forms in the air during hot summer days. Smog forms when nitrogen dioxide, hydrocarbons from gasoline vapors, automobile exhaust, small dust particles, carbon particles and other industrial pollutants mix in stagnant air. Smog reduces visibility and because of its chemical activity often produces ozone in unhealthful concentrations.

High levels of ozone are known to increase breathing difficulty for people that suffer from asthma, heart disease and emphysema. Ozone also increases the number of cases of bronchitis in children and senior citizens. Long term exposure can even cause healthy young adults to experience breathing difficulty especially those that exercise or work for long periods outdoors.