Fires in forests and former forestlands occur in Indonesia in the dry season every year, particularly in the provinces of Riau and Jambi on the island of Sumatra, and West Kalimantan and Central Kalimantan on the island of Borneo. The haze that spreads to other countries is mostly caused by smoldering (flameless) fires on peatland. Most fires are deliberately lit. Some then escalate and get out of control; some may smolder in peatlands for months or even years. The high-impact air pollution event of June 2013 over Singapore was caused by fires of limited extent and duration burning in recently deforested peat soil areas of Riau province. The 2013 fires were not as extreme as the 1997 and 2006 El Niño-induced fires, but rather a high-impact event preceded by a brief dry period.
