Cleaner air has for the first time been linked to bigger and stronger lungs among school-age children, according to findings released Wednesday from a two-decade study in Southern California.
The research by USC scientists, published in the New England Journal of Medicine, found the region's steep decline in air pollution since the mid-1990s is strongly associated with "statistically and clinically significant improvements" in children's lung function and growth.
The analysis, which studied more than 2,000 children in five cities over the years, provides the strongest evidence yet that years of government regulations to reduce air pollution in California and across the nation are paying off with measurable improvements in children's health.
Scientists knew from previous research that air pollution stunts the growth of children's lungs, permanently reducing their ability to breathe.
"We can now turn that around and say that improving air quality leads to better health," said W. James Gauderman, professor of preventive medicine at USC's Keck School of Medicine and lead author of the study with eight other researchers. "This is one of the first good news studies ever about air pollution."
The Children's Health Study tracked the lung development of children recruited from public schools in five of the most polluted communities in Southern California: Long Beach, Mira Loma, Riverside, San Dimas and Upland. Field teams took repeated measurements of students' ability to breathe as they grew from age 11 to 15. While air quality monitoring stations logged pollution levels at each location, researchers compared students growing up from 2007 to 2011 to two previous groups of children whose lungs were tested from 1994 to 1998 and from 1997 to 2001.
As air quality improved, the number of children with abnormally low lung function — less than 80% of the lung capacity expected for their age -- dropped by more than 4 percentage points, from 7.9% in the mid-1990s to 3.6% in 2011. The children's four-year lung growth improved by more than 10% over the same period.
Technicians tested the children's lung capacity and strength by asking them to blow into a spirometer, a device that measures the amount of air they can exhale and how quickly.
The experiment took place as pollution declined sharply throughout the Los Angeles basin. Over the course of the study, levels of nitrogen dioxide dropped 33% across the five sites and fine particle pollution fell by 47%.
Improved lung function was most strongly associated with lower levels of nitrogen dioxide and particulate matter. Consistent with previous studies, researchers did not find that reductions in ozone, a key ingredient in Southern California smog, affected lung function.
The association was seen in boys and girls, across racial backgrounds and in children with asthma and without it, "which suggests that all children have the potential to benefit from improvements in air quality," the study said.
The researchers focused on adolescents between 11 and 15 because that is when lungs grow most rapidly. That development stops by the end of their teens, meaning the improvements detected among children in the study are likely to stay with them for life, reducing the risk of a wide variety of health problems, including premature death.
Cathleen Imbroane, principal of Elizabeth Hudson K-8 in Long Beach said, "Everybody is working to improve the air quality here and I think we are reaping the benefits."
"When our kids are outside, the air is not smelly like it used to be," said Imbroane, who has noticed the improvement in her five years of running the school next to the port complex and diesel truck-choked Terminal Island Freeway.
"I rarely see a kid come into the office with respiratory distress," she said, "and there's a big difference in the number of kids reporting asthma-like symptoms and coming in to get an inhaler."
Previous research, including the long-running Harvard Six Cities Study, has shown that reducing air pollution increases life expectancy and brings other health benefits to adults. That study laid the foundation for federal air pollution standards for fine particulate matter in the 1990s.
The USC analysis is the first to demonstrate health improvements among children as air pollution eases.
The findings have national implications, because levels of the pollutants tracked in the study have plummeted in U.S. cities in recent decades after years of tightening emissions standards on vehicles and industry under the 1970 Clean Air Act.
"We should expect that these benefits could also be seen in other parts of the United States," said Douglas Dockery, a professor of environmental epidemiology at the Harvard T.H. Chan School of Public Health, who wrote an editorial that accompanies the findings in the New England Journal of Medicine.
Previous publications resulting from the USC Children's Health Study gave early some indications about the health benefits of cleaner air. A 2001 study found that when children from polluted communities moved to cleaner areas, their lung function improved.
Once notorious for smog, Southern California's air has cleared up dramatically in response to years of strict pollution-control rules targeting cars, diesel trucks, power plants, seaports, consumer products and factories.
"This study shows that California's tough diesel and other air quality regulations protect children," state Air Resources Board Chairman Mary D. Nichols said in an emailed statement.
Yet significant challenges remain. Southern California still has some of the worst air pollution in the nation. A growing population, more vehicles on the road and projections of growth in truck, train and ship traffic through the ports of Los Angeles and Long Beach will require even stricter rules to keep emissions trending downward.
Environmental health advocates said the findings should not be a reason for complacency.
"I hope this will put pressure on policymakers to do more," said Penny Newman, who heads the Riverside County-based Center for Community Action and Environmental Justice near two of the communities studied. "If we know that the more we clean up the air the better the health is for these kids, well that's a really powerful motivation."
USC's Children's Health Study, which has cost more than $30 million over two decades, was paid for largely by the National Institute of Environmental Health Sciences and the state Air Resources Board. Additional funding came from the Hastings Foundation and the Health Effects Institute, a nonprofit organization sponsored by the auto industry and the federal government.