SINGAPORE - WHY is the haze so bad? Climate scientist Matthias Roth, an associate professor of geography at the National University of Singapore, explained that the dry season in Sumatra has come relatively early.
1. Early dry spell: The dryness makes it easier to clear land by burning forests, and there is no rain to rid the air of pollution.
2. More hot spots: The number of hot spots - defined as a fire covering a hectare of land or more - has been on the rise.
3. Monsoon winds: Winds from the west and south-west during the south-west monsoon season push the smoke towards Singapore.
4. Weather conditions: Singapore's current weather - light winds and an absence of rain - means the haze persists without being removed from the atmosphere. June is typically the third driest month of the year, after July and February.
At night, the PSI often creeps up, said Associate Professor Roth, as pollutants can be trapped near the surface of the earth when the sun is not present to heat the air and cause it to mix.
But is it likely to get worse later in the year?
"As always, this is hard to predict," Prof Roth said. "If dry conditions persist or return later in the year, and burning activity continues at the current or even higher rate, worse conditions are possible."
Indonesia has pledged to control fires in other parts of Sumatra, which include several in North Sumatra province that have raged for a few days.
Extinguishing the fires has also caused some smoke, said Mr Raffles Panjaitan, director of security and investigation at the Forestry Ministry.
But in the long term, said Ms Syamsidar, a World Wide Fund for Nature activist based in Riau, the most important thing is prevention. "This would work if law enforcement is strong enough to deter people," she said.
Over the years, oil palm and timber plantation companies have expanded aggressively in Indonesia, while enforcement against the illegal burning of land has been lax.
Indonesia is the world's largest palm oil exporter and one of the world's largest paper producers.
These fires, which have become a regular affair for several months every year, have triggered protests from Indonesia's neighbours since haze started to envelop the region 16 years ago.
ASEAN members signed an agreement on transboundary haze pollution in June 2002, but Indonesia has yet to ratify the agreement.
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