Bronchitic symptoms on the decline as pollution levels drop in Los Angeles region over the past two decades.

A USC study that tracked Southern California children over a 20-year period has found they now have significantly fewer respiratory symptoms as a result of improved air quality.

The finding expands on the landmark USC Children’s Health Study, which a year ago reported that kids’ lungs had grown stronger over the past 20 years as pollution levels in the Los Angeles Basin declined. In the current study, USC researchers examined a health issue that makes many parents anxious while pulling at their pocketbooks: bronchitic symptoms that could land otherwise healthy children in a doctor’s office or hospital.

To assess respiratory symptoms, USC scientists studied children in eight California communities and defined bronchitic symptoms over the preceding year as a daily cough for at least three consecutive months, congestion or phlegm not related to a cold, or inflammation of the mucous membranes, according to Kiros Berhane, lead author and a professor of preventive medicine at the Keck School of Medicine of USC.

“This is one of the few times that we have been able to report good news, and this is very likely a direct result of the science-based policies that have been put in place,” Berhane said. “The message that clean air leads to better health in children should be taken seriously because it has implications for how we live and how productive we become.”

The study, published April 12 in the Journal of the American Medical Association, followed 4,602 children in three cohorts as they aged from 5 to 18. During 1993 to 2012, children and their parents from Long Beach, San Dimas, Upland, Riverside, Mira Loma, Lake Elsinore, Alpine and Santa Maria answered questionnaires about children’s health. Air quality was continuously monitored in each community.

“Because of the wide variations in ambient pollution levels among the eight California communities we analyzed, these findings are applicable to other parts of the United States and maybe other parts of the world as well,” Berhane said, adding the results could help with asthma management and the overall respiratory health of children.

How much children’s respiratory health improved

Because bronchitic symptoms are usually about four times higher in children with asthma, the scientists examined associations of air pollution reduction with bronchitic symptoms separately for kids with and without asthma. Researchers also adjusted their analyses for age, gender, race or ethnicity, secondhand tobacco smoke and presence of cockroaches in the home.

“It is important to note that while reductions in bronchitic symptoms were larger in children with asthma, they were still substantial and significant in children without asthma as well — indicating that all children have benefited from the improvement in air quality over the past 20 years,” Berhane said.

The study found that tiny particles called particulate matter 2.5 (PM 2.5) — which can penetrate deep into lungs and cause serious health problems — dropped by 47 percent from 1992 to 2011 in the study region. USC researchers were able to associate cleaner air with improved children’s respiratory health. Kids with asthma were 32 percent less likely to suffer from bronchitic symptoms, and children without asthma experienced a 21 percent reduction in these respiratory problems.

Moreover, nitrogen dioxide, which can reduce resistance to respiratory infections, decreased by 49 percent in the same two decades. USC researchers linked the drop in nitrogen dioxide with a 21 percent decrease of bronchitic symptoms in children with asthma and a 16 percent decline of bronchitic symptoms in kids without asthma.

“This type of data is important for policymaking and for how clinicians would advise their patients,” Berhane said.

By Zen Vuong