It may be the biggest success story in environmental health in modern America.

Children in the Los Angeles region have substantially healthier lungs than they did just 20 years ago, thanks largely to multibillion-dollar efforts to clean up southern California's infamous smog and soot.

In landmark research published Wednesday, University of Southern California scientists found that kids in the region are breathing better than they did in 1994, and the percentage of kids with abnormally poor lung function dropped by more than half.

The scientists reached a dramatic conclusion that they hope reverberates globally: Reducing air pollution improves people's health.

"It's remarkable; this is one of the biggest turnarounds I've ever seen," says lead researcher W. James Gauderman, with USC's Keck School of Medicine.

Frank Gilliland, with USC's Programs in Biomedical and Biological Sciences, says the new findings "show scientifically that targeting pollutants actually makes kids healthier. It's a very important message, especially for the developing world: These problems are fixable, and you can see big benefits."

In the study published in the New England Journal of Medicine, researchers followed 2,000 kids from five southern California cities with some of the worst air, including Long Beach, Riverside, San Dimas, Upland, and Mira Loma. They focused on kids ages 11 to 15, whose lungs are growing the most.

While other studies have compared kids from polluted neighborhoods to those living with cleaner air, the USC team tracked children from the same communities over 20 years, and correlated their findings with pollution data from local air monitors. That allowed them to more clearly weed out other potential factors.

Regardless of race, exposure to cigarette smoke, or factors like education and pets, kids tested between 2007 and 2011 had healthier lungs than kids the same ages tested between 1994 and 1998.

Average lung capacity for kids in all the communities increased substantially, but the lungs of those with asthma improved roughly twice as much as the lungs of other children.

The percentage of children in the study with abnormally poor lung function at age 15 dropped from nearly 8 percent for the 1994-98 group to 3.6 percent for kids between 2007 and 2011, according to the USC researchers.

And kids' lungs grew faster as air quality improved: Lung growth from age 11 to 15 was more than 10 percent greater in the later group.

Parents Notice Cleaner Air

"It's pretty amazing," says Long Beach resident Maria Gugerty, whose 17-year-old son, Will, is representative of millennials in the study who grew up with the more polluted air. "It demonstrates that even in a huge city in a huge state with big pollution problems, if people put their mind to it, you can make significant changes."

Gugerty, who has lived in Long Beach since the early 1990s, recalls regularly spotting soot on her windows and seeing a smog-blanketed horizon while driving. When her son was in elementary school, he struggled with asthma and complained frequently about his breathing. But she hasn't heard him complain in years.

"Often you could see this thick layer of gunk in the air, but I don't see that much anymore," Gugerty says.

Claudia Copley, who was a high school athlete in the smoggy city of West Covina in the 1980s, remembers sports practices being canceled because air quality was so hazardous. Copley says she's grateful that her two children, Ian, 18, and Piper, 11, haven't had to experience that. The air is even cleaner now than it was when they were born; smog alerts now are extremely rare.

"I'm so glad somebody did something about it," Copley says. "Because that's the whole goal, right? To make things better for our kids."

California's Groundbreaking Pollution Standards

Over the past several decades, California officials set groundbreaking standards that phased out many inefficient car and truck engines and some of the dirtiest fuels for everything from jet skis and lawnmowers to school buses and heavy-duty trucks. Local smog-fighters in the Los Angeles basin forced cleanup of oil refineries, manufacturing plants, and consumer products such as paints and solvents. Other local and state programs offered incentives for replacing old trucks and buses.

The result: Some of the most problematic pollutants-smog-forming nitrogen dioxide and fine particles created by diesel-engine exhaust and other fossil fuels-declined in the worst neighborhoods by up to 50 percent in 20 years. Maritime pollution, particularly in neighborhoods near the massive ports of Los Angeles and Long Beach, also has dropped substantially.

The cleaner air came as virtually every type of pollution source in the region grew—numbers of people, cars, trucks, ships, aircraft, and vehicle miles traveled.

Nevertheless, the four-county Los Angeles basin still has some of the nation's worst levels of ozone, a key ingredient of smog, and fine particles, which have been linked to asthma attacks and heart problems. The region still violates federal health standards for ozone and particles.

"In this day and age that's important to communicate: Reducing pollution has been a major boon to children growing up in southern California," Gilliland says. "But there's still too much of it. More reductions are needed."