Image copyright Bjorn Vaughn Image caption Many parts of Indonesia are burning

Almost every year, a smoky haze blankets the South East Asian region - signalling the return of forest fires in Indonesia.

For many in this region, grey skies and a lingering acrid smell are not unfamiliar, but 2019 has already brought with it some of the worst haze levels in years.

But what causes these fires - and why do Indonesia's forests burn each year?

What's causing the haze?

According to Indonesia's national disaster agency, there were 328,724 hectares of land burnt this year from January to August alone.

Among the most affected regions were Central, West and South Kalimantan, Riau, Jambi and South Sumatra.

But Indonesia's not the only culprit. There have also been cases of open burning in neighbouring Malaysia, though it pales in comparison to Indonesia.

As of 14 September, there were 10 hotspots in the Malaysian states of Sabah and Sarawak, compared to 627 in Kalimantan, according to the ASEAN specialised meteorological centre.

The burning usually peaks from July to October during Indonesia's dry season.

Many farmers take advantage of the conditions to clear vegetation for palm oil, pulp and paper plantations using the slash-and-burn method.

They often spin out of control and spread into protected forested areas.

Image copyright Getty Images Image caption One of many forests in Riau province that are ablaze

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.

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.

In Malaysia, hundreds of schools have been forced to close after the haze reached "very unhealthy levels" of 208 on the Air Pollutants Index (API) in several districts.

Media playback is unsupported on your device Media caption 'I'm worried about my baby': Hear from Indonesians living with the haze

On 14 September, Pollutants Standards Index (PSI) levels in Singapore went beyond the 100 mark for the first time in three years, though it's yet to reach the hazardous levels of 2015.

In 2015, the PSI level in Singapore was at 341 - schools were forced to close and several fast-food chains suspended their delivery services.

On both indices, a reading above 100 is classified as unhealthy and anything above 300 is hazardous.

Image copyright Reuters Image caption Many in Singapore are wearing special masks to combat the haze

But it's in Indonesia where the impact is most felt.

In Palangkaraya, the capital of central Kalimantan, the Air Quality Index (AQI) reached 2000 on Sunday, according to Greenpeace Indonesia.

Anything between 301-500 is considered hazardous.

For many, its a reminder of 2015, the country's last major haze crisis.

Image copyright Bjorn Vaughn Image caption Not everyone in Palangkaraya can afford a high quality mask

The 2015 crisis cost the country $16bn (£12bn) and caused more than 500,000 people to suffer from respiratory ailments - a state of emergency was declared.

"It's just like 2015," llham, a Palangkaraya resident told BBC Indonesia. "[It caused] my wife's face to turn red... [from skin irritation]. When she was taken to the emergency room, it turned out to be because of the smoke."

"I haven't opened the windows and doors for two weeks," said Lilis Alice, another resident. "In the morning, it's dark. If I'm inside the house I have to turn on the lights. It's so dark."

Image copyright Bjorn Vaughn Image caption In Palangkaraya, visibility is extremely poor

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.

What impact has it had on nature?

Image copyright Getty Images Image caption Borneo orangutans have suffered as their homes are burnt

Over the years, forest fires have destroyed much of the natural habitat of Indonesia's orangutans and released large amounts of carbon into the atmosphere.

Kalimantan is home to many of the region's orangutans. The Bornean orangutan is critically endangered, according to the International Union for Conservation of Nature.

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 fire fighters.

The country 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.

Media playback is unsupported on your device Media caption 'Like breathing in a barbecue': What it's like living in the haze

In September 2015, Mr Widodo told the BBC his country needed at least three years to tackle the haze as it was "not a problem you can solve quickly".

Almost four years later, the forests in Indonesia continue to burn.

Why has it been so difficult to stop?

Indonesia has long struggled to police the vast rural expanse in Sumatra and Kalimantan.

The slash-and-burn technique employed by many in the region is arguably the easiest way for farmers to clear their land and helps them get rid of any disease that may have affected their crops.

But it's not just small-scale farmers at work here. The concern is many of these fires are started by big corporations that want to plant oil palm plantations.

Indonesia is the world's biggest producer of palm oil and the demand for the commodity has been rising. This means there is need for extra land for palm oil plantations.

Image copyright Getty Images Image caption Palm oil plantations are big money in Indonesia

But Indonesia and environment rights activists also say the country is not entirely to blame, as some of the big corporations accused of illegal burning have Malaysian and Singaporean investors.

Slash-and-burn is illegal in Indonesia but has been allowed to continue for years, with some saying corruption and weak governance have contributed to the situation.

Greenpeace International has said some companies in Indonesia appeared "to operate outside the law for years with little sanction".

In the meantime Indonesian authorities 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.