Forests in the United States occupy 33 percent of the nation's land area and exist on some lands in all fifty states. They cover approximately 299 million hectares and are rich in essential resources. Overall, U.S. forests are highly productive. However, in recent decades, several cases of forest decline on a regional scale have occurred which are not easily explained by normal natural causes. Atmospheric deposition of air pollutants is suspected to play a casual role in many of these cases. The evidence, however, is largely circumstantial in that forest areas with decline symptoms are frequently located where the deposition of pollutants is high. Proof of cause and effect has been illusive. Ozone at concentrations elevated by human activities has been proven to cause declines in some area, but many other wide-spread declines remain unresolved.