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During the summer, ozone can be unhealthy for several days in a row.Because it is an invisible gas, ozone does not directly affect visibility.When this happens, relative humidity will be near 100 percent and precipitation may be evident from the photo or indicated in the visibility information panel. It occurs naturally in the upper atmosphere, where it absorbs harmful ultraviolet rays.But at ground level, it forms as result of air pollution from cars, trucks, busses, power plants, fuel and paint vapors, and other sources. Concentrations may reach unhealthy levels on warm, sunny days.They also come from the evaporation of fuels and household and industrial solvents.This variety of gasses and direct particle emissions results in a mixture of fine particles with different sizes, chemical properties, and health and environmental impacts.If black carbon concentrations are high when visibility is poor, then the visibility impairment is probably due to air pollution.

To learn more about fine particles and visibility, see the Causes of Poor Visibility.They can be emitted directly into the air during any process involving burning or combustion, including activities around the home (e.g.cooking, smoking, space heating, and open burning) and those involving motor vehicles, various engines, power plants, and other such sources.Most haze is caused by coal-fired emissions from power plants.Medium and high levels of fine particle concentrations are a strong indication that poor visibility is due in large part to pollution.