Since the mid-1980s, when Indonesia first began to clear its bountiful forests on an industrial scale in favour of lucrative palm-oil plantations, “haze” has become an almost annual occurrence in South-East Asia. The cheapest way to clear logged woodland is to burn it, producing an acrid cloud of foul white smoke that, carried by the wind, can cover hundreds, or even thousands, of square miles.

The intervening decades have seen the passage of numerous national and international regulations to stop the fires, but all, it seems, to no avail. The past two weeks have seen some of the worst smog ever, taking a severe toll not only on peoples’ lungs, throats and tempers, but also on diplomatic relations and Indonesia’s attempts to improve its environmental image. Worse still, despite the outcry, it is hard to see how matters are going to improve over the next few years.

Most of the burning, which starts every dry season, is concentrated this year in Riau province on the east coast of Sumatra. Indonesia is the world’s biggest palm-oil producer and Riau its most productive province. Sadly for Singapore and Malaysia, it lies just across the Strait of Malacca from them. From June 16th Singapore and large parts of Malaysia were smothered in smog from this year’s fires.

In Singapore the pollution was the worst ever, pummelling the previous records set in 1997, when the haze affected six countries and perhaps 70m people. Then, the Pollutants Standard Index (PSI) in Singapore, a measure of air quality, hit a panic-inducing 226, defined as “very unhealthy”. On June 19th, by contrast (the day of the satellite picture above), the PSI climbed to over 300, defined as “hazardous”, before peaking at 401 on June 21st. The government issued face masks and almost everyone took its advice to stay indoors. Malaysia declared a state of emergency in parts of its southern state of Johor when the Air Pollution Index, only slightly different from Singapore’s PSI, exceeded 500; it reached 750 on June 23rd. Kuala Lumpur, the capital, and coastal cities were also badly affected, as was Riau province itself, where hundreds were evacuated.

Fraternal relations within the Association of South-East Asian Nations (ASEAN), the regional political grouping, quickly dissolved into acrimonious finger-pointing. Agung Laksono, the minister in charge of Indonesia’s response to the crisis, said that Singaporeans were behaving “like children, in such a tizzy”. Singaporeans and Malaysians pointed out that Indonesia was the only ASEAN member not to have ratified a 2002 Agreement on Transboundary Haze Pollution. It was only on June 24th, when the damage was done, that its president, Susilo Bambang Yudhoyono, apologised to his irate neighbours.

At least three laws in Indonesia prohibit the burning and clearance of forests, and in particular Sumatra’s extensive peat wetlands. But environmental campaigners argue that the government has never seriously enforced these laws. Despite the arrest in Sumatra this week of eight farmers, supposedly caught red-handed, hardly anyone has been successfully prosecuted over the years for lighting fires. Palm oil’s economic importance to Indonesia seems to afford the industry protection. Last year exports totalled $17.9 billion, second only to coal. Some 5m people live off the industry. These are big numbers in a relatively poor country.

About half of the vast amount of land on which the fires are burning in Sumatra belongs to big palm-oil conglomerates, many of them Malaysian-owned. They have been accused of setting illegal fires in the past, in order to clear more of their concessions for palm oil. Satellite imagery clearly shows fires burning on the land of some of them, and the Indonesian government has named eight companies that it wants to investigate. Even so, it is going to be very difficult to apportion blame. One company, Singapore-based Asia Pacific Resources International Limited, acknowledges that there have been three fires on its land, but claims these had “originally started outside of its concession area”.

Another perennial problem is corruption. This year’s disaster was preceded on June 14th by the arrest of Rusli Zainal, the governor of Riau since 2003. He was charged, among other crimes, with dishing out illegal logging permits to finance a forthcoming re-election campaign. Under the country’s political decentralisation in 2001, generally considered to be good for democracy, the power to regulate land use passed from Jakarta to regional and often district-level politicians. They have often abused this authority to raise money.

Much of the area now burning in Riau is peat wetland, almost all that’s left after years of rampant deforestation. Peat, which can go down to a depth of 30m in Sumatra, is highly combustible, even many metres down. A fire doused on the surface might smoulder underground long after. It is illegal to burn peat for commercial development. But as the past few weeks have proved, the law is not enough. And, ominously for those hoping for clear skies and clean air, a lot of peat is left.

South-East Asia’s smog: Unspontaneous combustion, Economist, June 29, 2013, at 39