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(CNN) At the age of 13, Tan Yi Han could not see the edge of his schoolyard. It was 1998 in Singapore, the wealthy city-state known for its tidy streets and clean, green image. But for much of that particular school year, clouds of smoke shrouded the skyline. The record-setting air pollution, which had begun in 1997 and lasted for months, caused a 30% spike in hospital visits. It would later be remembered as one of southeast Asia's worst-ever "haze episodes."

Haze episodes have occurred in southeast Asia nearly every year since. Back in 1998, and for years afterwards, Tan didn't think too deeply about them. Yet at some point in his late 20s, he began to wonder: where did the haze come from? And why did it keep coming back?

Dirty air

Air pollution kills around 7 million people every year, according to the World Health Organization (WHO), accounting for one in eight deaths worldwide in 2012. The main causes of death were stroke and heart disease, followed by chronic obstructive pulmonary disease (COPD), lung cancer, and respiratory infections among children.

It is especially bad in the Asia-Pacific region, which has a population of over 4.2 billion and high population density. China and India alone, with a combined population of around 2.7 billion, are both enormous sources and victims of air pollution.

In 2010, 40% of the world's premature deaths caused by air pollution were in China, the world's largest emitter of carbon dioxide, according to a survey published in the Lancet . The University of Hong Kong's School of Public Health reported more than 3,000 premature deaths in the city in 2013, and the situation in many mainland Chinese cities is reckoned to be far worse. A poll by the U.S. Pew Research Center found that 47% of Chinese citizens thought air pollution to be a "very big" problem in 2013 (up from 31% in 2008) . It is now a central focus for many Chinese environmental groups and a growing source of anxiety for the country's leadership.

Similar health concerns are building in India, where air pollution is now the fifth-leading cause of death. Between 2000 and 2010, the annual number of premature deaths linked to air pollution across India rose six-fold to 620,000 , according to the Center for Science and Environment, a public-interest research and advocacy group in New Delhi. In May 2014, the WHO said that New Delhi had the worst air of 1,600 cities surveyed worldwide and that rising air pollution had increased the risk of strokes, cancers and heart disease. Another 2014 study has linked a significant drop in India's wheat and rice crop yields to rising levels of two air pollutants -- black carbon from rural cooking stoves and ground-level ozone formed from motor vehicle exhausts, industrial emissions, and chemical solvents -- between 1980 and 2010.

In both China and India, air pollution is one consequence of a massive exodus from farm to city that has occurred in recent decades. The change has contributed to rising emissions from both vehicles and factories, especially coal-fired power plants, and an emerging middle class that increasingly desires a range of consumer goods that are common in Europe and the United States.

Southeast Asia has encountered similar problems in recent decades as its economies and populations have boomed. In fact, according to the WHO, nearly one million of the 3.7 million people who died from ambient air pollution in 2012 lived in southeast Asia.

But on top of smokestacks and tailpipes, the region faces an added burden: smoke haze produced in Indonesia that is a by-product of the world's $50 billion palm-oil industry.

Palm oil

In the summer of 2013, a plane carried Tan Yi Han over the Straits of Malacca to Pekanbaru, the capital of Riau province, the largest palm-oil production region in Indonesia. Tan, then a 28-year-old financial consultant, was volunteering with the Global Environment Center, a Malaysian group that has worked for years to prevent and mitigate haze. He traveled to the heart of neighboring Indonesia, just after a record-breaking haze episode hit peninsular Malaysia.

On a driving tour in Riau, he saw endless acres of burned-out landscapes. Fires had turned swampy peat bogs, the area's natural vegetation, into land whose parched surface resembled charcoal. These fires are to dry out the peatlands for agricultural uses, mainly the cultivation of oil palms. But in some villages, fires had even destroyed existing oil palm trees that belonged to multinational companies or local farmers.

A resident attempts to put out a fire spreading through a plantation in Dumai district, Riau province, Sumatra, March 3 2014.

Tan had a memorable encounter in the village of Rantau Bais. A couple there plied him with tea and snacks, then quietly asked if he could spare any of his own food for them. Their daughter had developed a respiratory problem because of the haze. The surprise medical bill, coupled with the fire destroying their oil palm crops, had left the family penniless and hungry.

Until that moment, he had mostly thought of peat blazes as "forest fires," as they are often called in media reports. But here was a visceral reminder that the fires affect working land and real people. "It really touched me," said Tan. "I made a promise to myself that I'd do my best to prevent them from suffering from fires again."

It was an issue, he felt, that required far more public discussion -- and when the time was right, action. "I must get more people involved," he had thought, "and turn this into a movement."

Peat fires

Hazy skies may all look similar, but the emissions from any particular source are unique. A factory smokestack in Beijing releases a different mix of chemical compounds into the atmosphere than an automobile tailpipe in New Delhi does. And the extent of pollution in a given city will depend on how carefully emissions are controlled, and how easily they can be dispersed.

Vehicle and factory emissions have been analyzed for decades in high-income countries, but haze smoke, and its impact on health, is not well understood. "Not many people have investigated it even though it's a very important phenomenon," said Mikinori Kuwata, an atmospheric chemist at Singapore's Nanyang Technological University.

Unlike factory and vehicle emissions, wildfire smoke is not regulated by tailpipe scrubbers, catalytic converters or other pollution-mitigating applications. The composition of the smoke also varies widely according to the type of material that is burning. Peatlands, for example, typically take a longer time to burn than drier matter -- just as a damp piece of wood takes longer to burn in a campfire. According to the U.S. Environmental Protection Agency, peat fires burn at lower temperatures and produce smoke that is more harmful, and in larger quantities, than the average forest fire or grassland fire does.

The emissions from a given peat fire will largely depend on the peat's composition, its burning temperature and how far below the ground the fire occurs. But such details aren't yet available in Indonesia, whose peatlands cover an area roughly the size of the United Kingdom. As a result, Kuwata told me, "We do not have a very reliable inventory" of the country's peatland fire data. Kuwata burns Indonesian peat in his Singapore laboratory to study its chemical properties, but his work is limited, he said, because he can never be sure whether his experiments mirror reality.

Indonesia has an enormous repository of tropical peatlands -- and, for a generation, areas of these have been burned to prepare the land for the cultivation of oil palms. Peat smoke now contributes around 40% of Indonesia's overall greenhouse gas emissions . Palm oil is an ingredient in a range of consumer products, from lipstick to ice cream. Yet it has also helped to give its source country the dubious distinction of being the world's third-largest greenhouse gas emitter after China and the United States -- as well as a leading source of hazardous smoke haze.

Blame game

On a summer afternoon, the skies were a milky white in Riau, the Indonesian province that produces about a quarter of Indonesia's palm oil. My first stop was the headquarters of WALHI, an NGO in the city of Pekanbaru that lobbies the Indonesian government for action on haze and other environmental problems.

I arrived at WALHI's headquarters, a low-rise residential building near the Pekanbaru airport, just as a group of farmers and environmental activists were discussing haze, over coffee and cigarettes, with Sri Nurhayati Qodriyatun, a researcher for the Secretary General of Indonesia's parliament.

Qodriyatun said her boss had dispatched her to Riau to compile a report on haze. At the meeting, she explained that, according to government estimates, forest fires were generally not occurring in areas owned by large plantations.

The crowd stirred.

"Government statements about haze are false!" shouted an activist from a local NGO, Forest Rescue Riau Network. "And there's no coordination between ministers -- they just pass the blame around!"

Indonesian firefighters put out a fire on smoldering peatland in Siak Regency located in Riau province on Indonesia's Sumatra island, March 1, 2014.

The exchange underscored the long-running debate across southeast Asia about who, exactly, is responsible for Indonesia's peat fires. Farmers and environmental groups often accuse companies, many of which are headquartered in Singapore or Malaysia, of malfeasance. But many companies say such criticism is overblown, and that they have largely reformed their destructive land-clearing practices in recent years through voluntary reform initiatives like the Roundtable on Sustainable Palm Oil, an industry-led consortium.

Whoever is right, said Qodriyatun, the fires have damaged Indonesia's international reputation, and the Indonesian government pays little attention to their health implications in Riau and beyond.

"Personally, I don't think the government is managing this well," she told me after the meeting. "Usually they just react after the fires start, but they should think more about prevention."

Peat fires, though, are notoriously hard to predict and extinguish. They start and spread easily, and sometimes uncontrollably, depending on conditions like wind speed, the depth of the soil and the dryness of the air.

"It's really hard to know how bad a fire will be when it starts," said Dedy Tarsedi, a farmer in the Riau village of Bungaraya. We were sitting in a roadside café flanked by oil palm trees. Tarsedi told me that oil palm is the crop of choice for Bungaraya farmers because it is more valuable than paddy rice. A hectare of oil palm, he said, typically earns a farmer around 48 million Indonesian rupiah (nearly $4,000) per year. Paddy rice, by contrast, brings in just 40 million rupiah.

But as oil palm cultivation has increased in the village, so have fires. And they affect both corporate plantations and smallholder farmers.

"If a fire happens and we can't control it, we'll report it," said Maman, a Bungaraya farmer. But sometimes, even helicopters are powerless to stop the burning, he added. "And during the really bad fires, a lot of the kids cough and end up at the clinic with health problems."

In 2009, Indonesia passed a law banning fires on peat plantations. Farmers in Bungaraya told me that, as a result, they had started to clear peat bogs manually, without using fire. But Tarsedi said manual clearance is more labor-intensive and requires extra fertilisers. And that, he said, requires extra time and money that most farmers don't want to part with.

Where there's smoke ...

When the wind blows from the west, smoke can whip east across the Straits of Malacca and into both Singapore and Kuala Lumpur (the capital of nearby Malaysia) -- collectively home to about 7 million people. South-east Asia is not the only place where the burning of vegetation occurs over large areas; most of the world's fires occur in Africa and South America. But southeast Asia's fires are unique, says Miriam Marlier, an atmospheric researcher at Columbia University, because they occur so close to dense urban centers.

There are no comprehensive studies on how repeated exposure to peat smoke affects human health over the long term, much less how peat smoke's chemical properties differ from other kinds of biomass smoke. Yet emerging research offers early clues.

U.S. researchers have found that peat fires in the southern states during the summer of 2008 caused a spike in emergency room visits for heart failure and asthma-related respiratory complications. In a follow-up study, published in June 2014, they burned semi-charred peat from the fires in the vicinity of lab mice. Subsequent pulmonary problems in the mice were mainly linked to coarser-grained smoke particles and cardiac problems to finer-grained particles.

A primary concern from a health perspective is that peat fires tend to generate larger amounts of fine-grained particulate matter, called PM2.5, than normal forest fires. That is worrying mainly because finer-grained particles are thought to penetrate further into the bloodstream than coarser ones do, posing a potentially higher risk to the heart and other internal organs. Finer-grained particles are also harder to block with the simple surgical masks that many people in Asian cities have traditionally worn as protection against air pollution.

widely cited 2012 study , published in the journal Environmental Health Perspectives, estimated that about 339,000 deaths between 1997 and 2006 were associated with landscape fires. About four in five deaths were linked to chronic, rather than sporadic, exposure. Sub-Saharan Africa and southeast Asia accounted for 157,000 and 110,000 deaths, respectively, and the rate of mortality spiked during years dominated by the El Nino weather phenomenon, which typically correlates with drier conditions in southeast Asia. "Reducing population level exposure to air pollution from landscape fires is a worthwhile endeavor that is likely to have immediate and measurable health benefits," the researchers concluded.

Another 2012 study, by Miriam Marlier and other scientists from American and British institutions, found that between 1 and 11% of South-east Asia's population was repeatedly exposed to pollution above the WHO's recommended air quality levels during sporadic haze episodes between 1997 and 2006. Elevated exposure during El Nino years caused around 15,000 cardiovascular-linked adult deaths per year, the researchers wrote. Roughly two-thirds of those were linked to fine-grained PM2.5 particles, while the other third were linked to levels of ozone. However, there was not enough evidence available to determine exactly how the toxicity of PM2.5 in peat fires differed from that of PM2.5 emissions in American cities.

People wear masks as they ride through haze pollution on February 23, 2014 in Pontianak, Kalimantan, Indonesia.

Some scientists suggest that peat smoke's long-term effects on humans may be broadly similar to those of urban air pollution, which also includes PM2.5 particles. No one is sure because so little research has been done to test the theory.

Rajasekhar Balasubramanian, an American environmental engineer who studies haze at the National University of Singapore, speculates that long-term exposure to haze episodes could potentially make the population less healthy over time, even if people continue to live long lives. In a 2013 study, he and his colleagues found that the air above Singapore during a smoke haze episode contained arsenic, chromium, cadmium and other carcinogenic elements. They estimated that normal urban levels of PM2.5 pollution would cause about 12 out of every million Singaporeans to develop cancer over a lifetime, but if haze were to occur for 10 days per year over 70 straight years, the number of likely cancer cases would increase by nearly half.

Yet there still is no coordinated international effort to analyse haze in a truly interdisciplinary fashion. That is partly due to the sporadic and unpredictable nature of haze, Balasubramanian said: southeast Asia's highly variable weather makes it tricky to predict when haze will appear or where it could spread. He likens a particle of peat smoke to a grasshopper that jumps into the air, shoots along horizontally, then quickly zooms back to earth -- only to jump again.

Another problem, Balasubramanian said, is that the general public does not yet view haze as a serious health threat. "People view it as, 'Oh, yeah, it's a problem that occurs in Indonesia'," he told me one afternoon in his office at the National University of Singapore. For governments and funding bodies, "the priority's more mitigation: how to mitigate human exposure to this haze issue, rather than to study the problem itself."

Political problems

The task of mitigating pollution is also clouded by politics. Countries in southeast Asia have little control over what blows across their borders: unlike the European Union, the Association of Southeast Asian Nations (ASEAN) lacks the legal authority to force its members to act against their own interests.

A case in point is ASEAN's 2002 trans-boundary haze agreement, a non-binding document in which the group's 10 member states pledged to prevent and monitor peat fires. The agreement called for technology exchanges and other measures to improve regional dialogue and cooperation on haze. It was initially hailed as a landmark achievement, but until September 2014, Indonesia's parliament had refused to ratify it. Laode M Syarif, an environmental lawyer based in the Indonesian capital of Jakarta, said that was mainly because Indonesia has long tried to use the haze agreement as a way to leverage Singapore into overturning a refusal to extradite Indonesian citizens who are wanted for crimes in their homeland.

ASEAN tends to view economic development, national sovereignty and mutual non-interference as its highest priorities, said Helena Varkkey, a senior lecturer in the Department of International and Strategic Studies at the University of Malaya. In her view, ASEAN has taken a mild-mannered approach to haze fighting out of deference to powerful palm-oil companies, many of them based in Singapore or Malaysia.

Indeed, many analysts have said that Indonesia's land concessions -- areas allocated for commercial plantations -- are deeply entwined with corruption. A popular joke has it that, if Indonesia's overlapping concession maps were all counted as national territory, the country would grow in size. But companies and officials mostly refuse to share those maps with the public. "It's a mess," said Andika Putraditama, a research analyst at the Jakarta office of the World Resources Institute, a research organization headquartered in Washington, DC. It's also another reason why Indonesia's peatlands continue to burn.

Public pressure

Against this backdrop, Tan Yi Han, the Singaporean financial consultant and self-styled haze activist, is hoping to influence the regional debate on haze. In early 2014, he founded a citizens' organization called "People's Movement to Stop Haze," or PM Haze, to kick-start the discussion.

"My gut feeling is, we need influence," Tan said at a Sunday-evening PM Haze meeting. There was only one other participant: Putera Zenata, an Indonesian schoolteacher who had joined the group after finding Tan online. The venue was Zenata's modest apartment in a middle-class Singapore neighborhood.

Singapore's financial business district covered with smog, on September 15, 2014. Offcials said air pollution hit unhealthy levels due to smog from fires raging across rainforests in Indonesia's neigboring Sumatara island.

In June 2014, one of Tan's hometown newspapers, the "Independent," dubbed him "Singapore's intrepid haze fighter." But PM Haze, with 10 active members and no outside funding, is well behind many established advocacy and research groups that fight air pollution elsewhere in Asia. In New Delhi, the Center for Science and the Environment has proposed specific ways that the government could tackle air pollution -- for example, by cracking down on open fires. And in Beijing, the Institute of Public and Environmental Affairs is promoting a pollution-monitoring mobile phone app as a way of ramping up pressure on polluting companies.

By his own admission, Tan has very little experience in the non-profit sector. He told me that he has no plans to pressure the government or companies into action -- at least not yet. For the moment, he said, PM Haze is simply trying to learn about the problem, in all its complexity, and then communicate its findings to the Singaporean public. In early November 2014, the group developed the content for an informational "haze exhibition" in Singapore that drew an estimated 800 visitors. And in the longer term, Tan said, they would like to film a documentary in Indonesia.

"My personal goal is to stop haze by 2023," he added casually.

That could be a pipe dream. But according to Wilson Ang, assistant director for Sustainability at the Singapore Institute of International Affairs, the haze of June 2013 made the Singaporean public "much more involved" in the issue. Along with PM Haze, the episode spawned the creation of the Haze Elimination Action Team, another grassroots community group. Both groups have since gone on site visits to Indonesia, opened dialogues with palm-oil companies, and offered feedback or recommendations to Singaporean officials. "Such a ground-up approach is very much welcomed by the government," said Ang.

Taking action

Haze, however, is still a growing public health concern for many countries, especially lower-income ones. "We put a lot of legislation in place to control point sources, and still, when you add it up, ambient conditions don't get better," said Jacqueline McGlade, chief scientist at the United Nations Environment Program. Other challenges, she told me, are linking air pollution data with research on impacts and holding governments accountable for enforcing pollution laws.

More than ever, air pollution is a prominent target of policy reforms and public health interventions. Many lower-income countries, grappling with the environmental and health consequences of their booming populations, are tightening air pollution standards. International aid and development agencies are also rolling out projects to monitor or regulate particulate emissions.

In southeast Asia, haze has recently resurfaced on ASEAN's political radar. In early July 2014, officials from Riau province announced that they would conduct a large-scale "compliance audit" of local officials and agroforestry companies linked to peatlands. On August 5, Singapore's parliament passed a law that allows the government to fine both domestic and international companies up to $2 million Singapore ($1.5 million) for causing or contributing to haze. And on 16 September, Indonesia's parliament finally ratified ASEAN's 2002 trans-boundary haze agreement after 12 years of resistance.

Also that summer, a senior adviser to Joko "Jokowi" Widodo, then Indonesia's President-Elect, said the new administration planned to renew the 2009 Indonesian law that banned peat burning when it expires in 2015. Widodo himself said he planned to streamline land governance by creating a "one-map" forestry policy. "The haze is caused both by the people and also the companies," he told the Straits Times, a Singapore newspaper, in late August. "If we have good, tough law enforcement, then it can be resolved."

How significant are these developments? In conversations with several haze-watching analysts across south-east Asia, I heard a wide range of opinions. Some, like Helena Varkkey, aren't especially optimistic, mainly because Indonesia and ASEAN have so far made so little progress on the haze problem. Neither the Singaporean law nor the regional haze agreement, they pointed out, would be enforceable in Indonesian courts. And if climate change increases the number of droughts and wildfires around the world, as many scientists predict it will, the incidence of peatland fires may also rise -- and pose additional enforcement challenges.

But others said it is positive that the Indonesian and Singaporean governments are at least taking action -- the sort that could breathe new life into existing Indonesian laws designed to tackle haze. The recent political activity gives them hope that annual peat fires will not become southeast Asia's status quo for future generations.

"Jokowi did say that he aims to take action against the haze," said Tan Yi Han, the haze fighter. "That's just words, but it's better than nothing."