The link between air quality and respiratory health is an obvious one. Various studies have shown adverse health outcomes associated with air pollution include cardiovascular disease, autism, heart attack and stroke. Now, new research published in JAMA finds that reduced smog can result in significantly fewer respiratory problems.

The study, conducted by researchers at the University of Southern California, followed more than 4,600 local children aged five to 18 from 1993 to 2012. The authors found a 47 percent reduction in ambient air pollution during that period was associated with a 32 percent decline in bronchitis symptoms among 10-year-olds with asthma. Children without asthma reaped benefits too: They were also 21 percent less likely to experience these respiratory problems.

Bronchitis is a common occurrence in children, and yet it's an “underappreciated health issue,” according to the study. Symptoms include a persistent cough, production of mucus, fatigue chest discomfort, and shortness of breath.

"Chronic respiratory symptoms in children are of significant public health and clinical concern and detrimental effects of air pollution on children's respiratory health are now well established, as manifested in an increased diagnosis of asthma, more respiratory symptoms, and impaired lung development," Kiros Berhane, lead author of the study, told MedPage Today. "This evidence is based on several studies from around the world, including our own research over the last 20-plus years."

In addition to improved symptoms, Berhane and his team found reduced pollution levels offset the side effects of nitrogen dioxide, which finds its way into the air through road traffic and energy production. The chemical can reduce resistance to respiratory infections, but in clean air, the likelihood of this happening dropped by 47 percent. Better air quality was also associated with a 21 percent decrease in bronchitis symptoms among children with asthma and a 16 percent reduction in kids without asthma.

"It is important to note that while reductions in bronchitis 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 in a statement.

The prevalence of asthma in children is an issue, with about one in 10 children in the U.S. suffering from the disease in 2014, according to the Centers for Disease Control and Prevention. The study authors noted that expenses associated with the disease rose to about $50 billion in 2007, costing the nation about $3,300 per person each year.

"Changes in children's respiratory health have a ripple effect," Berhane said. "A child may stay home because of severe bronchitis symptoms. That could mean parents have to call in sick or arrange for a caregiver. Beyond quality of life, childhood asthma and bronchitis symptoms take a toll on children's school attendance, parental productivity and society in general."

The findings build on a previous study conducted by the same research team that found kids’ lungs were stronger when pollution levels in the Los Angeles-area declined.

Source: Berhane K, Chang C, McConnell R, et al. Association of Changes in Air Quality With Bronchitis Symptoms in Children in California, 1993-2012. JAMA. 2016.