BOSTON (Reuters) - Dramatic improvements in U.S. air quality over the last two decades have added 21 weeks to the life of the average American, researchers reported on Wednesday.

The Statue of Liberty and the New York City Skyline are seen in this photograph taken from a tour boat in New York Harbor, June 30, 2008. REUTERS/Mike Segar

Reducing fine particles given off by automobiles, diesel engines, steel mills and coal-fired power plants have added as much as 15 percent of the 2.72 years of extra longevity seen in the United States since the early 1980s, they wrote in the New England Journal of Medicine.

Changes in smoking habits are the biggest reason why Americans are living longer, said Arden Pope, an epidemiologist at Brigham Young University in Utah who led the study.

Improved socioeconomic conditions, judged partly by the proportion of high school graduates living in an area, rank next. But cleaner air was a big factor.

“It’s stunning that the air pollution effect seems to be as robust as it is after controlling for these other things,” Pope said in a telephone interview.

Using life expectancy, economic, demographic and pollution data from 51 metropolitan areas, Pope and his colleagues found when fine-particle air pollution dropped by 10 micrograms per cubic meter, life expectancy rose by 31 weeks.

Areas such a Akron, Ohio, and Philadelphia showed that kind of drop in air pollution.

The bigger the decline, the longer people began living.

In some areas where fine-particle counts dropped by 13 to 14 micrograms -- such as Buffalo, New York and Pittsburgh -- people typically started living about 43 weeks longer.

The findings show there has been a real dividend from the efforts since the 1970s to improve air quality, said Pope.

In a commentary, Daniel Krewski of the University of Ottawa said the study “provides direct confirmation of the population health benefits of mitigating air pollution and greatly strengthens the foundation of the argument for air-quality management.”

Based on earlier research, the World Health Organization has estimated that 1.4 percent of all deaths around the world are caused by air pollution particles.