In addition to limiting the distance that we can see, the scattering and absorption of light caused by air pollution can also degrade the color, clarity, and contrast of scenes. The same particles that are linked to serious health effects can also significantly affect our ability to see. Both primary emissions and secondary formation of particles contribute to visibility impairment. In the Eastern United States, reduced visibility is mainly attributable to secondarily-formed particles. While these secondarily-formed particles still account for a significant amount in the West, primary emissions from sources like wood smoke contribute a larger percentage of the total particulate loading than in the East.
Humidity can significantly increase the effect of pollution on visibility.
Some particles, such as sulfates, accumulate water and grow in size, becoming more efficient at scattering light and causing visibility impairment. Poor summer visibility in the eastern United States is primarily the result of high sulfate concentrations combined with high humidity levels. Visibility impairment has been analyzed using visual range data collected since 1960 at 280 monitoring stations located at airports across the country. The dark blue color represents less haze and red represents more haze. Overall, the maps show that visibility impairment in the Eastern United States increased greatly between 1970 and 1980 and decreased slightly between 1980 and 1990.