According to a Harvard University study published in Environmental Research Letters,
in excess of 100,000 deaths were caused across Indonesia, Malaysia, and Singapore in 2015 as a result of hazardous levels of smoke from agricultural and forest fires. These fires are started each year by farmers producing palm oil and timber for wood pulp producers in Kalimantan (Indonesia Borneo) and South Sumatra. The study reports that the hazardous levels of air pollution from the extreme smoke haze blanketed much of Equatorial Asia causing many school and business closures, planes to be grounded and tens of thousands to seek medical treatment for respiratory illness.
This region of Equatorial Asia has experienced several major smoke haze events over the past twenty years. The fires, largely in coastal peatlands, burn at relatively low temperatures and can smolder for weeks or even months before extinguishing. According to the study a similar smoke haze event in 2006 is estimated to have resulted in around 38,000 deaths. A significant element in the increased impact in terms of air pollution deaths in 2015 compared to 2006 was a significant increase in the intensity of fires and smoke from the South Sumatra Province of Indonesia, and the proximity of fires to population centers.
During periods of extreme dry weather caused by El Niño and a phenomenon called the positive Indian Ocean Dipole, smoke emissions are considerably higher — either because farmers are taking advantage of the dry weather to burn more land or because once burning, the fires are more difficult to control. Although many fires burn in remote areas of Indonesia, prevailing winds can carry the smoke hundreds of miles to densely populated cities like Palembang in Sumatra, and Singapore and Kuala Lumpur. In September–October 2015, El Niño and warmer sea surface temperatures in the western Indian Ocean relative to the east set the stage for the massive fires in Sumatra and Kalimantan, leading to persistently hazardous levels of smoke pollution across much of Equatorial Asia.