More than 100,000 people in Indonesia, Malaysia, and Singapore may have died prematurely last fall because of smoke haze from agricultural fires, according to a study by a Harvard University research team.

Fires started by Indonesian farmers, especially those working in palm oil and timber for wood pulp and paper, are mostly to blame, researchers said. “There is a lot of timber burning in those regions,” ThinkProgress reports. “Indonesia, which has a long history of slash and burn techniques to clear land for palm oil and wood pulp producers, is the sixth-largest emitter in the world,” and 63% of the country’s carbon footprint traces back to land use, including peat and forest fires.

Indonesia saw its worst forest fires in nearly two decades last fall, as El Niño aggravated local weather conditions. “The fires, which largely happened in the coastal peat forests and likely stemmed from slash and burn [forestry techniques], destroyed thousands of acres of peat forest — one of the world’s most effective carbon sinks — releasing tons of carbon, and massive quantities of small particulate matter called PM 2.5,” writes correspondent Alejandro Dávila Fragoso.

In addition to damaging soil, crops, and wildlife, “PM 2.5 can also reach deep into the lungs and bloodstream of humans, causing a long list of ailments such as decreased lung function, heart attack, aggravated asthma, and premature death,” he explains.