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WASHINGTON — More people are being killed by pollution from cars, trucks and other sources than by traffic crashes, researchers estimate in a report that says cleaning up would prolong the lives of thousands of people. 

The researchers, in a study in the journal Science, said that cutting greenhouse gases in just four major cities — Sao Paulo, Brazil; Mexico City; Santiago, Chile and New York City — could save 64,000 lives over the next 20 years. 

Greenhouse gases, principally carbon dioxide or ozone, are those pollutants that tend to trap the sun’s heat in the atmosphere or to affect solar radiation. 

The gases have been blamed for causing global warming, but the study’s lead author, Devra Lee Davis, a professor at Carnegie Mellon University’s Heinz School in Pittsburgh, said the effects are not just long-term. 

“The message in our study is that there are real and immediate health benefits” in reducing greenhouse gases, she said. 

She said that burning of fossils fuels, such as gasoline in cars or coal in power plants, can create air pollutants such as ozone, airborne particles small enough to be inhaled, carbon dioxide and other gases. The pollutants, said Davis, can cause people to die prematurely from asthma, breathing disorders and heart disease. 

“It is our best estimate that more people are being killed by air pollution ... than from traffic crashes,” said Davis. “There are more than a thousand studies from 20 countries all showing that you can predict a certain death rate based on the amount of pollution.” 

Some experts, however, say that the direct connection between air pollution and death is not that clear, even in cities. 

Dr. Russell V. Luepker, a cardiologist and professor at the University of Minnesota, said that air pollution is not recognized as a significant cause of heart disease in the United States. 

“It is not a major factor in developing heart disease, but it does play a role in acute episodes that can kill you,” said Luepker, an expert designated by the American Heart Association as a spokesman. “More people either come to emergency rooms or die of heart disease during pollution episodes,” but the pollution did not start the disease, he said. 

Dr. Marian Frieri, a professor of medicine and an asthma expert at State University of New York at Stony Brook, said that air pollution can contribute to asthma inflammation but is only one factor on top of another condition. 

Davis and four co-authors said that adopting greenhouse-gas abatement technologies now available could prevent thousands of cases of chronic bronchitis and save millions of days of restricted or lost work. 

“We’re not talking about Buck Rogers-like, futuristic technologies,” Davis said. 

She said although the study concentrated on just four cities that have a combined population of 45 million, the conclusions probably could be applied to cities worldwide. The data are consistent with a World Health Organization study that estimated that air pollution would cause about 8 million deaths worldwide by 2020, she said. 

Dr. Jonathan Patz of the Johns Hopkins Bloomberg School of Public Health said the study by Davis and her co-authors draws “an important conclusion.” 

“It shows that there are significant health benefits to be had from reducing emissions from the burning of fossil fuels,” he said. 

Carbon dioxide and other gases from the burning of coal and oil have been blamed by many researchers for warming of the climate. Some have predicted long-term and varied global effects, including such phenomena as melting glaciers, rising sea levels and recurring weather extremes. 

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