An El Nino weather pattern this year has extended the annual dry spell, aggravating the impact of the fires, with the number of hot spots at the highest since devastating blazes in 2015.

Here are some facts about the fires, and the haze affecting Indonesia, Malaysia, Singapore and other countries this year:

1. Biomass burning, such as the slash-and-burn clearing of land in Indonesia, sends partially burned carbon aloft into the atmosphere, resulting in a smoky haze. This organic carbon contains a substantial portion of fine particles known as PM2.5, which can have a major detrimental impact on health and climate.

2. Fires in partially decayed vegetation called peat are of particular concern because they can be difficult to extinguish due to the depth and organic density of the soil. Some peat fires can burn for months or even years.

Peaty soil is particularly flammable when dry, often causing fires to spread beyond intended areas. The fires are set to burn off vegetation but end up scorching roots and the soil itself.

3. Compared to the last regional haze crisis in September 2015, there appear to be more widespread PM2.5 particles in the atmosphere this year. In 2015, smaller pockets of PM2.5 were present in the air, according to the data.

4. The air pollution index in Palangkaraya, the capital of Central Kalimantan province on Borneo island, has for days been over 500, the level denoted as "dangerous", data from Indonesia's Environment and Forestry Ministry showed.

The air quality in other areas, such as Jambi and Riau provinces in Sumatra, has hit "unhealthy" levels, with an index of 100 or more.

5. Indonesia has deployed more than 29,000 military, police and disaster agency personnel to douse the fires, having declared an emergency in six provinces in Sumatra and Borneo, according to a tweet by the Indonesian disaster management board on Wednesday.

6. Haze conditions have also badly affected Malaysia and Singapore, with unhealthy levels registered in both countries over the last week. Authorities in Malaysia have closed thousands of schools, begun cloud seeding to combat the situation and distributed face masks.

7. Acute respiratory infections have affected 11,758 people in Palangkaraya, 15,346 in Riau and 15,047 in Jambi, according to data from Indonesia's Health Ministry released on Sunday.

REUTERS