According to a new research the reduction in air pollution in U.S. cities over the last two decades has resulted in the addition of five months to the life of the average American.

The study carried out between Brigham Young University and Harvard School of Public Health was published in the New England Journal of Medicine. The researchers used advanced statistical models to co -relate statistics on air pollution and life expectancy from 51 cities between 1980 and 2000.

In the research they focused on "PM 2.5" pollution - which measured levels of tiny particles with a diameter one-twentieth of the width of a human hair. These can travel into the lungs and are linked to asthma and heart disease.

The researchers used data on life expectancy, economic, demographic and pollution and they reported that in cities which made the transition from polluted to clean air the result was an average increase of 10 months longer life span. The researchers noted that for every decrease of 10 micrograms per cubic meter of particulate pollution, life expectancy increased by more than seven months.

In some previously heaviest-polluted cities such as Buffalo, New York and Pittsburgh where fine-particle counts dropped by 13 to 14 micrograms people typically started living about 10 months longer. The basic principle was the bigger the decline the longer people’s lifespan became.

The reduction in air pollution is due to reduction of emissions of fine particles given off by automobiles, diesel engines, steel mills and coal-fired power plants the researchers wrote. Study leader Arden Pope, an epidemiologist at Brigham Young University in Utah said one of the biggest reasons for the increased lifespan is the changes in smoking habits, followed by improved socio-economic conditions but the improved air quality played a major role. "It's stunning that the air pollution effect seems to be as robust as it is after controlling for these other things," Pope said. "We find that we're getting a substantial return on our investments in improving our air quality. Not only are we getting cleaner air that improves our environment, but it is improving our public health."

Professor Jonathan Ayres, a specialist in the medical effects of air pollution based at the University of Birmingham said the life span estimates seemed high.  "There's no doubt there are differences in the way that people in the US respond to air pollution compared to people in the UK. However, the research is a strong justification for the efforts that have been made to reduce pollution over the past couple of decades."

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." (Additional reporting by Harkiran)