Since 1991, haze has been a particularly acute problem in Southeast Asia. The main source of the haze has been fires occurring in Sumatra and Borneo. In response to the 1997 Southeast Asian haze, the ASEAN countries agreed on a Regional Haze Action Plan (1997). In 2002, all ASEAN countries signed the Agreement on Transboundary Haze Pollution, but the pollution is still a problem 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 Pollution Standards Index reaching a record high of 401.