What causes South East Asia's haze?
Forest fires in Indonesia have resulted in a smoky haze blanketing the South East Asian region for months.
Both the haze and the controversy around it have intensified in recent years. But what causes it, and what makes it such a contentious issue?
What's causing the haze?
Every year Indonesia sees agriculture fires in Riau province in East Sumatra, South Sumatra, and parts of Kalimantan on Indonesian Borneo.
The fires are said to be caused by corporations as well as small-scale farmers who use the slash-and-burn method to clear vegetation for palm oil, pulp and paper plantations.
The fires often spin out of control and spread into protected forested areas.
The problem has accelerated in recent years as more land has been cleared for expanding plantations for the lucrative palm oil trade.
The burnt land also becomes drier, which makes it more likely to catch fire the next time there are slash-and-burn clearings.
This year has seen one of the worst and most prolonged periods of haze, thanks to unusually dry weather in Indonesia caused by the El Nino climate phenomenon.
Why is it an issue?
The haze usually measures hundreds of kilometres across. It has spread to Malaysia, Singapore, the south of Thailand and the Philippines, causing a significant deterioration in air quality.
This year it has been blamed for deaths in Indonesia and respiratory illnesses in around 500,000 people, according to the government.
Elsewhere it has prompted school closures, flight cancellations and virtual shutdowns of towns and cities.
Singapore and Indonesia use the Pollutants Standards Index (PSI) to measure air quality, while Malaysia uses the similar Air Pollutants Index (API). On both indices, a reading that is above 100 is classified as unhealthy and anything above 300 is hazardous.
At the forest fire epicentres in Kalimantan and Sumatra, PSI readings have exceeded 2,000, prompting the government in late October to prepare ships to evacuate children in several provinces.
What makes it so dangerous?
Besides irritating the respiratory tract and the eyes, pollutants in the haze can cause serious long-term damage to health.
The indices used to measure air quality in the region usually measure particulate matter (PM10), fine particulate matter (PM2.5), sulphur dioxide, carbon monoxide, nitrogen dioxide and ozone.
PM2.5 is considered the most dangerous as it can enter deeper into the lungs. It has been associated with causing respiratory illnesses and lung damage.
The forest fires have also destroyed much of the natural habitat of Indonesia's orangutans and released large amounts of damaging carbon into the atmosphere.
US-based environmental research organisation World Resource Institute said in October that the daily carbon emissions were surpassing the average emissions by the United States.
What is being done to stop it?
Indonesia has been dumping millions of litres of water in affected areas and has sent in the army to help firefighters put out the fires.
It has also accepted help in the form of firefighting teams and military equipment from several countries including Russia and Singapore.
Indonesia has for years promised to step up enforcement. Under President Joko Widodo, it has named 10 corporations as suspects this year, and said it is investigating more than 100 individuals.
But at the end of September, Mr Widodo told the BBC that his country needed at least three years to tackle the haze as it was "not a problem that you can solve quickly".
In 2002, all 10 South East Asian countries signed an agreement to combat the issue through greater monitoring and encouragement of sustainable development, but efforts have been limited.
Why has it been so difficult to stop?
Indonesia has long struggled to police the vast rural expanse in Sumatra and Kalimantan.
But Indonesia and environment rights activists also say it is not entirely to blame, as some of the corporations accused of illegal burning have Malaysian and Singaporean investors.
Singapore in 2014 passed a set of laws that allow it to prosecute individuals and companies that contribute to the haze, and has begun taking legal action against several firms.
There have also been name-and-shame campaigns and calls to boycott the products of the companies said to be contributing to the haze.
In the meantime Indonesian authorities continue to struggle to put out the fires, many of which have flared up on flammable and dry peat-rich areas.
A peat fire is difficult to put out as it can burn underground for months, and requires a lot of water to extinguish. Fires can spread underground and spring up in other places later.