The venerable magazine The Economist posed an interesting observation recently that any attempt to put an economic value on fresh air, clean water or tropical rainforests can offend the delicate sensibilities of those who argue that the conservation of nature is a moral duty.

Yet although the best things in life appear to be free, that does not mean they are without financial value. It simply means that nobody asks you to pay when, for example, you watch a beautiful sunset over the hills or draw breathe of fresh air. But putting a financial value on the environment, notes the Economist, may be the most important thing that people can do.

A case in point is evident from the results of a new study by researchers at Brigham Young University (BYU) and Harvard School of Public Health that shows average life expectancy in 51 U.S. cities increased nearly three years over recent decades, and approximately five months of that increase came thanks to cleaner air.

“Such a significant increase in life expectancy attributable to reducing air pollution is remarkable,” said C. Arden Pope III, a BYU epidemiologist and lead author on the study in the Jan. 22 issue of the New England Journal of Medicine. “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.”

The research matched two sets of data from 51 cities across the nation: changes in air pollution between about 1980 and about 2000; and residents’ life expectancies during those years. The scientists applied advanced statistical models to account for other factors that could affect average life spans, such as changes in population, income, education, migration, demographics and cigarette smoking.

In cities that had previously been the most polluted and cleaned up the most, the cleaner air added approximately 10 months to the average resident’s life.

On average, Americans were living 2.72 years longer at the end of the two-decade study period; up to five months, or 15 percent, of that increase came because of reduced air pollution. Other studies show that these gains are likely coming from reductions in the cardiovascular and cardiopulmonary disease that typically accompany air pollution.

“There is an important positive message here that the efforts to reduce particulate air pollution concentrations in the United States over the past 20 years have led to substantial and measurable improvements in life expectancy,” said study co-author Douglas Dockery, chair of the Department of Environmental Health at Harvard School of Public Health.

Pope and Dockery have teamed with other researchers on landmark studies published in the early 1990s that revealed the negative health effects of particulate air pollution known as “PM2.5” - tiny pollutants smaller than 2.5 microns in diameter, smaller than 4/100 the width of a human hair. The Environmental Protection Agency used those and related studies as the basis for tightening air pollution standards in 1997.

The latest study evaluated the impact of resulting decreases in particulate pollution on average life spans in cities for which air pollution data were available. In fact, researchers had to build life expectancy data for the 214 counties that are part of the study’s 51 metropolitan areas.

“Life expectancy is the single most comprehensive summary of how people’s longevity is affected by factors like air pollution that cause early death,” said co-author Majid Ezzati, associate professor of international health at Harvard School of Public Health. “We were able to use routine mortality statistics to track longevity in all cities over a long period of time and analyze how it has been influenced by changes in air pollution.”

The analysis found that for every decrease of 10 micrograms per cubic meter of particulate pollution in a city, its residents’ average life expectancy increased by more than seven months. During the 1980s and 1990s the average PM2.5 levels in the 51 U.S. cities studied dropped from 21 to 14 micrograms per cubic meter. In cities such as Pittsburgh and Buffalo, the decrease was closer to 14 micrograms per cubic meter.

The research also observed gains in life expectancy even in cities that initially had relatively clean air but had further improvements in air quality, suggesting the continuing benefits to ongoing efforts to reduce air pollution.

The researchers emphasized that there are other important and often overlapping factors that influence life expectancy, but this study demonstrated that improvements in air quality can contribute to significant and measurable improvements in life expectancy.

Measuring a tangible value on other dimensions of the environment may be more difficult than comparing life expectancies in cities by isolating the key independent variable – clean air. But as noted by The Economist, the absence of proper green accounting contributes to the unnecessary degradation of the natural environment.

“If a government is unaware of the value of a landscape to its tourism, or of a swamp to its fishing industry—and thus its foreign-exchange income—then it will invest too little in managing these resources. Worse, if the true value of a forest or swamp is hidden, governments may destroy it by subsidising the conversion of the land to agriculture. The costs are unknown for now, but may appear eventually as the price of building a filtration plant to remove the sediment from the water that the forest once took care of, or the price of importing food when fish vanish.”

It’s a powerful argument, one that suggests more attention should be placed on measuring the true financial and health benefits of a clean environment.