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."