One of the most basic forms of air pollution - haze - degrades visibility in many American cities and scenic areas. Haze is caused when sunlight encounters tiny pollution particles in the air, which reduce the clarity and color of what we see, especially during humid conditions. Since 1988 the federal government has been monitoring visibility in national parks and wilderness areas, for example, in the Great Smoky Mountains National Park. In 1999, EPA announced a major effort to improve air quality in national parks and wilderness areas.