In the United States, the Interagency Monitoring of Protected Visual Environments (IMPROVE) program was developed as a collaborative effort between the US EPA and the National Park Service in order to establish the chemical composition of haze in National Parks and establish air pollution control measures in order to restore the visibility to pre-industrial levels. Additionally, the Clean Air Act requires that any current visibility problems be remedied, and future visibility problems be prevented, in 156 Class I Federal areas located throughout the United States. A full list of these areas is available on EPA's website.