text: Since 1991, haze has been a particularly acute problem in Southeast Asia. The main source of the haze has been smoke from fires occurring in Sumatra and Borneo which dispersed over a wide area. In response to the 1997 Southeast Asian haze, the ASEAN countries agreed on a Regional Haze Action Plan (1997) as an attempt to reduce haze. In 2002, all ASEAN countries signed the Agreement on Transboundary Haze Pollution, but the pollution is still a problem there today. Under the agreement, the ASEAN secretariat hosts a co-ordination and support unit. During the 2013 Southeast Asian haze, Singapore experienced a record high pollution level, with the 3-hour Pollutant Standards Index reaching a record high of 401.
  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 of the air to pre-industrial levels.  Additionally, the Clean Air Act requires that any current visibility problems be addressed and 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.