By Sara Schonhardt

A study by two U.S. universities estimated that more than 100,000 people in parts of Southeast Asia died prematurely last year from breathing the noxious haze related to fires set to clear land for agriculture.

Researchers from Harvard and Columbia compared the likely health impact of the smoke that spread across much of the region a year ago to 2006, another exceptionally bad year for fires.

Using a complex modeling system, they put the number of smoke-related “excess deaths” from July 2015 through October at 100,300—mostly in Indonesia, the main source of the fires, but also Singapore and Malaysia. That was more than twice the estimate for the same four-month period in 2006.

The study, published Monday in the journal Environmental Research Letters, attributed much of the worsening haze to more fires set to clear peatland—swampy soil that stores carbon and becomes highly combustible when drained to develop lucrative palm-oil and wood-pulp plantations.

In both years, the study found Indonesia’s South Sumatra region contributed more than half of the regionwide haze. Last year, 72% of the fire activity on the island was on peatlands, up from 44% in 2006, it said.

Indonesia’s official death toll from last year’s haze was 19. An estimated half a million people sought medical care for respiratory illnesses on Sumatra and neighboring Kalimantan island alone, out of more than 40 million there exposed to the toxic haze.

Shannon Koplitz, lead author of the study from Harvard’s department of earth and planetary sciences, said the goal of the research was to influence strategies for managing fires and land use to reduce smoke exposure.

“The study tells Indonesia where to focus their efforts to prevent illegal clearing by fires, in order to save lives,” said Joel Schwartz, a researcher from Harvard’s T.H. Chan School of Public Health who was involved in the study.

Indonesia has been under domestic and international pressure to tackle the illegal fires and resulting haze, which last year resulted in billions of dollars in economic losses and made it one of the world’s biggest greenhouse-gas emitters.

President Joko Widodo has formed an agency to restore degraded peat and put a moratorium on new peatland development, said Hadi Daryanto, director general of social forestry at the Ministry of Environment and Forestry.

The government has also launched integrated patrols and deployed thousands of firefighters to put out blazes. It has vowed to prosecute those responsible.

“Efforts to handle the forest fires show a decreasing number of hot spots nationally,” Mr. Daryanto said.

The research doesn’t include official data on mortality or actual deaths. It estimated premature death in adults attributed to breathing high levels of carbon-based particulate matter, but didn’t include other hazardous particles, such as cyanide, nitrogen dioxide or other toxic gases.

It also didn’t include children, who doctors say are more vulnerable to the effects of haze, or the long-term impact of repeated exposure.

PHOTO: ASSOCIATED PRESS