A new study in the New England Journal of Medicine stated that cleaner air is providing health benefits for children. Researchers found that children who breathed cleaner air in recent years had stronger lungs than those studied two decades before.

University of Southern California researchers looked at 2,120 children. They focused on three groups of children between the ages of 11 and 15, a critical time for lung function. James Gauderman, lead author and professor of preventive medicine at the Keck School of Medicine at USC, stated that lung function declines slowly and steadily after kids hit the age of 18.

The three groups of children were studied between 1994 and 2011. During that time, pollution levels dropped nearly 40 percent. The study states that this was partially due to heavy emission standards for cars, according to USA Today. Their lung function was tested using an instrument that measured the amount of air they could exhale.

At the start of the long-running study, roughly 8 percent of children had abnormally low lung function. In other words, their lungs were functioning 80 percent less than the normal level for their age, according to Gauderman. When the last group of children turned 15, that number had dropped to 3.6 percent.

Gauderman said that children are breathing better because the air is cleaner, and this is the first study to provide “sound, scientific research” on the link between clean air and the health of children. He added that the cleaner air is impressive considering the number of cars on the road has only increased.

Although the study focused on Southern California, it suggests that other dirty cities may experience health benefits with some clean up. Gauderman said that reducing pollutants, such as nitrogen dioxide and particulate matter, should “lead to improved health for children” residing in any city. He called the story “an incredible environmental success.”

The American Lung Association’s Janice Nolen also stated that this study is the first to discover a huge benefit for children. She said that keeping high standards for clean air will help to maintain this kind of achievement.