Southeast Asia has been enveloped in choking haze from agricultural fires in Indonesia over the past fortnight, prompting flight cancellations, closing schools and raising fears this weekend's glitzy Formula One night race in Singapore could be affected.

The haze is a regular occurrence, with the region wheezing through outbreaks every year during the dry season. Here are some questions and answers about what causes the outbreaks and why they keep happening.

Why do the outbreaks happen and when did they start?

The main cause is illegal fires started in peatland and forest on Indonesia's Sumatra island and the Indonesian part of Borneo to quickly and cheaply clear land for palm oil and pulp and paper plantations. They increased in number as plantations expanded, in particular due to rising global demand for palm oil, a key ingredient in everyday goods such as shampoo and biscuits.

The outbreaks started in 1997, with what is still regarded as the most serious haze on record. It followed rapid expansion of plantations in the preceding years, and coincided with an El Nino weather system that made conditions drier than usual in Indonesia.

Who is affected by the haze?

The haze has a devastating impact on daily life every year on Sumatra and the Indonesian part of Borneo. This year, air quality has hit hazardous levels, tens of thousands have contracted respiratory illnesses, many flights have been cancelled and schools closed.

From Indonesia, the smog is blown over Southeast Asia, and fouls the air in neighbouring Singapore and Malaysia. Schools were closed in the Malaysian capital Kuala Lumpur and neighbouring states on Tuesday, while dense clouds have shrouded the skyline of financial hub Singapore.

How bad is it this year?

More than 2,000 fire "hotspots" -- either areas already on fire or very hot and likely to soon go up in flames -- were detected by satellites on Sumatra and Indonesian Borneo Tuesday, with the situation exacerbated by an El Nino weather system that has made conditions tinder-dry.

But the crisis eased in the past two days, and the haze has been much worse in the past. The most serious recent outbreak was in 2013, when air quality deteriorated to the worst level for years in Singapore and Malaysia.

Will the Singapore Grand Prix really be cancelled?

It's unlikely. Organisers of Formula One's only night race have acknowledged the situation is volatile but so far insist the event, which is coupled with pop concerts, will go on.

Still, Lewis Hamilton may find himself battling through smog in reduced visibility on his way to a third world title, as mask-wearing spectators watch him race along the narrow street circuit which snakes past iconic landmarks.

What is Indonesia doing to stop it?

Water-bombing helicopters and planes have been deployed to fight the fires, and aircraft are also "cloud-seeding", which involves using chemicals to induce rain.

About 3,000 extra military and police personnel have been sent to Sumatra to help fight the fires and catch those responsible. The national police say that 133 people and seven companies are being investigated over the illegal blazes. However, environmentalists note that few convictions have resulted from such probes in the past.

But why is it still happening after all these years?

Although starting fires to clear land is punishable by long jail terms and hefty fines in Indonesia, law enforcement is weak and corruption rife.

Major companies have "zero burn" policies, meaning they have vowed not to clear land using fires. But activists are sceptical that all firms are sticking to their pledges, and small landowners have also been blamed for starting fires to clear land.

Indonesia's neighbours -- in particular Singapore -- have been calling for years for more to be done to stop the outbreaks. Jakarta agreed this week to share information with the city-state on companies accused of starting fires, which could lead to prosecutions there.