Clean air doesn't just give Americans a pleasant, smog-free view. It's added an average of five months to our lives.

In a study of three decades of health data from 51 U.S. cities, researchers found that people are living about three years longer than they did before. Controlling for changes in income, education, demographics and smoking, about five months of that can be chalked up to air improvements.

"Our efforts in the past 20 years to reduce air pollution through better technology and regulation have actually worked," said Majid Ezzati, an international health expert at the Harvard School of Public Health. "People are living longer as a result of it."

Along with Harvard environmental epidemiologist Douglas Dockery and Brigham Young University economist C. Arden Pope, Ezzati tracked the drop of what are known as PM2.5s — pollutant particles with a diameter of 2.5 microns, or 1/25 the diameter of a human hair.

During the 1980s and 1990s, average PM2.5 levels in the 51 cities dropped from 21 to 14 micrograms per cubic meter. A 10