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Kirk Smith, towering figure in environmental science, 73

NEW YORK — When Kirk R. Smith began his research career in the 1970s, he was studying the health risks posed by nuclear power. But after a trip to rural parts of Asia, he detected an even bigger threat, affecting more people: toxic fumes being spewed from the solid fuels that heat the humble chulha, a small indoor cooking stove made of mud and clay and used by more than 40 percent of the world’s population.

He then began to focus on what he called household air pollution. When asked why he was turning away from nuclear power, he would reply, “The risks are too small.”

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This led to his mantra and became the advice he gave to his students: “Follow the risk.”

In doing just that, Mr. Smith was among the first scientists to identify the health hazards caused by cookstoves in developing nations. With scores of studies and meticulous measurements, he defined the field of household air pollution and established such pollution as one of the leading causes of disease and death in the developing world.

He also raised an early voice of warning about the impact of climate change on public health.

Mr. Smith died on June 15 at his home in Berkeley, Calif. He was 73.

His wife, Joan Diamond, said the cause was cardiac arrest following a stroke.

Mr. Smith was a preeminent global health researcher at the University of California, Berkeley, where he taught in the School of Public Health. He was best known for recognizing and quantifying how toxic emissions from burning firewood, charcoal, and cow dung to heat stoves threatened the health of people in underdeveloped rural areas.

As part of that work, he made the startling discovery that on average, cookstoves release the smoke equivalent of 400 cigarettes per hour.

About 3 billion people cook with and heat their homes with these dirty fuels, according to the World Health Organization, which has said that the effects of household pollution kill 4 million people a year. They die of strokes, heart disease, lung cancer, pneumonia, and other ailments.

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Mr. Smith saw that the fumes from cookstoves threatened not only the people who work around the stoves all day — mostly poor women and children — but the entire planet, with toxic emissions contributing to outdoor air pollution and exacerbating the effects of climate change.

“We’ve realized that pollution may start in the kitchen, but it doesn’t stay there,” he said in an interview last year with Berkeley News, the university’s news website. “It goes outside, it goes next door, it goes down the street, and it becomes part of the general outdoor air pollution.”

He found that at least one-fourth of the diseases in the world are caused by environmental factors like air pollution, water pollution, lack of sanitation, and chemicals in the workplace — all factors that could be controlled.

“Kirk led the world to greater understanding of the environment’s outsized role in health, not the least of which was due to the burning of dirty household fuels, on which Kirk was the world’s top scholar,” Justin Remais, chair of the division of environmental health sciences at the Berkeley School of Public Health, said in an e-mail.

In addition to identifying problems, Mr. Smith sought solutions.

He initially thought the answer to household air pollution was better cookstoves. But he came to see that replacing old stoves with new ones would take decades, particularly on the massive scale needed, and that many lives would be lost before such a transformation could take place.

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Moreover, he realized, the new stoves, at least those that were affordable, would not dramatically improve health.

“He spent a lot of time thinking about and learning from the experiences of the people he was trying to help,” his daughter, Nadia Diamond-Smith, who works in global maternal and reproductive health in India and Nepal, said in an interview. He understood, she said, that if the new stoves were only marginally better, people wouldn’t use them, and that only something with more obvious benefits would make a difference.

In a major pivot, unusual for such a prominent scientist, Mr. Smith reset his goal and campaigned instead for cleaner fuels like liquefied petroleum gas, or LPG.

In India, where 700 million people relied on the old stoves, he spent years collaborating with colleagues and building relationships. He was finally able to help persuade local governments to make LPG more widely available.

In 2016, Mr. Smith said, India instituted a national program to distribute clean-burning stoves and propane to 80 million impoverished households, or about 500 million people. And over the last few decades, several other countries have moved to cleaner cooking fuels.

Mr. Smith’s advances were widely recognized. He was elected to the National Academy of Sciences in 1997 and won numerous awards, including the 2012 Tyler Prize for Environmental Achievement, often called the Nobel Prize of environment.

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As it happened, Mr. Smith was also a recipient of an actual Nobel. As part of a team of scientists, he shared the Nobel Peace Prize in 2007 with former Vice President Al Gore. The team — the Intergovernmental Panel on Climate Change, a UN body — was cited by the Nobel committee for creating “an ever-broader informed consensus about the connection between human activities and global warming.”

Mr. Smith grew up in and around Berkeley and Oakland until the family moved to Marin County in the late ’50s. His new next-door neighbor, Joan Diamond, became his wife in 1977.

She and their daughter survive him, as do two grandchildren and three half brothers, Scott Nisbet and Mark and Thaddeus Smith.