Smog-covered mountains, gritty sidewalks, smelly fumes from traffic-choked freeways. The Los Angeles area was a tough place to breathe several decades ago. 

Now a study shows how much that has changed, especially for the region's youngest residents.

Children in recent years breathed cleaner air and had stronger lungs compared to those who were studied two decades earlier, researchers found. The improved health coincided with drastic reductions in pollution in the Los Angeles basin and surrounding areas as air quality regulators cracked down on emissions from tailpipes and smokestacks.

While the research focused on Southern California, the results suggest that other cities with dirty air may see a health boost with a cleanup effort.

Since pollutants such as particulate matter and nitrogen dioxide tend to be higher in cities, reducing "those pollutants should lead to improved health for children living in any urban environment," said lead researcher Jim Gauderman, a professor of preventive medicine at the University of Southern California's Keck School of Medicine.

Smog was first documented in Los Angeles during World War II when a thick haze reduced visibility to three city blocks and irritated residents' eyes.

For decades, the city struggled with air pollution. As LA's population grew, so did the number of vehicles on the road. In the 1970s, unhealthy air was recorded more than 200 days a year. The air was so bad that it blotted out the San Gabriel Mountains and children didn't play outside when health advisories were in effect.

Air quality has since improved thanks to stricter emissions rules for cars, trucks, ships and factories.

In the study, researchers tracked the lung development of more than 2,000 children in five Southern California communities beginning in the mid-90s. Teams traveled to schools and asked children to blow into a device that measures how strong their lungs are and how much air they can exhale in one second.

As pollution declined over the years so did the percentage of children who had weak lung function, from about 8 percent during 1994 to 1998 to about 4 percent in 2007-2011.

Researchers also found that health improved with cleaner air regardless of gender, ethnicity, education and other factors.

The findings appear in Thursday's New England Journal of Medicine. In an accompanying editorial, researchers not connected with the study said there's an incentive to continue cutting air pollution.

"The current report and other studies suggest that further improvement in air quality may have beneficial public health effects," wrote Douglas Dockery and James Ware of Harvard University.