Toxic haze that spread across Southeast Asia from Indonesian forest fires last year caused the deaths of about 100,000 people across the region, according to the first academic study of the health impact of the smog.
The death toll was concentrated in Indonesia, which had about 92,000 excess deaths from persistent haze that choked the region between July and October, according to researchers at Harvard and Columbia.
The deaths were mainly caused by cardiovascular disease, though there were also some deaths from respiratory disease, the researchers say.
The researchers focused on adult mortality, and are now working on a further study to examine the impact on children.
There was likely to have been an increase in child deaths from pneumonia in Indonesia due to the 2015 haze, according to Sam Myers, senior research scientist at Harvard School of Public Health and one of the study's authors.
The findings are likely to fuel political tensions over the annual blight on the region, which is driven by Indonesian smallholders or plantation owners clearing land for farming and palm oil or paper production.
Yuyun Indradi, a Greenpeace campaigner in Indonesia, said: "If nothing changes, this killer haze will carry on taking a terrible toll.
"Industry and government must take real action to stop forest clearing and peatland drainage for plantations."
Singapore has urged Indonesia to assist in collective action to tackle companies that clear land by burning, but this has met with a prickly response from Jakarta, which has accused its neighbour of meddling in domestic affairs.
Indonesia's president Joko Widodo said in April that palm oil growers should boost productivity rather than clearing further forest land.
Jakarta has already imposed a moratorium on clearing primary forest and converting peatland for other uses, but campaigners say enforcement of this restriction has been weak.
The researchers quantified emissions from the fires, then used weather data to track the spread of the haze. The scientists used data for mortality rates to determine how many of the deaths last year were due to exposure to fine particles known to be hazardous to health.
Dr Myers said: "Essentially, we asked how much mortality would have occurred that year without the fires and how much would occur with the fires and reported the difference."
While this is the first academic study of the 2015 Southeast Asian haze, previous research has shown the deadly consequences to human health of air pollution.
Globally, deaths from infectious diseases have declined in recent years as more people have access to clean water, improved sanitation, vaccines, mosquito nets and medicines.
However air pollution and other environmental factors remain a significant cause of death in poor countries in Southeast Asia and the western Pacific, according to the World Health Organisation.
Palm oil, a key ingredient in popular consumer products ranging from toothpaste to lipstick, has been linked to "slash and burn" forest clearance in Indonesia.
Producers of palm oil have come under intense pressure from corporate buyers, investors and environmental groups to ensure their supply chains are sustainable.
Last week, some of the world's biggest agricultural businesses — including major palm oil producers — came together in an alliance that aims to raise environmental standards in the sector.