It’s the first study to document that physiological improvements occur in children’s airways when air filters are in use, suggesting that with consistent use filters may help prevent, not just alleviate, asthmatic flare-ups.

While using the filters daily for two weeks, children in the study experienced decreased airway resistance and lung inflammation and increased airway elasticity, among other benefits.

“Pharmaceutical companies have spent large amounts to develop drugs that can work on lower airways, but they are very expensive. Our results show that using an air purifier to reduce the exposure of lower airways to pollutants could help asthmatic children breathe easier without those costly drugs,” said Zhang, professor of global and environmental health. 

“This warrants a clinical trial to confirm findings,” he said. 

Thirty times smaller in diameter than a human hair, fine particulate matter (PM2.5) is a ubiquitous air pollutant. It originates from fossil fuel emissions, industrial sources and wildfires. The particles penetrate deep into the lower airways where they can trigger or exacerbate asthma symptoms. Inhalers don’t help, since they are only designed to open the upper airways. 

Zhang and his colleagues published their paper in JAMA Pediatrics, a journal of the American Medical Association. 

They conducted the study in Shanghai during a period of moderately high PM2.5 pollution in 2017. Forty-three children with mild to moderate asthma were given two air filters to use in their bedrooms. One was a high-efficiency particulate air filter capable of removing PM2.5; the other was a sham filter. Each filter was used for two weeks with a two-week interval in between. Neither the children nor their families knew which filter was which. 

Results showed that PM2.5 concentrations inside the children’s bedrooms were up to two-thirds lower when the real air filters were in use than when the sham ones were being used. 

This drop coincided with significant improvements in how easily air flowed in and out of the children’s small airways and lungs. These improvements included a 24% average reduction in total airway resistance, a 43.5% average reduction in small airway resistance, a 73.1% average increase in airway elasticity, and a 27.6% average reduction in exhaled nitric oxide, a biomarker of lung inflammation. 

Although the benefits lasted only as long as the air filters were in use, the filters could serve as a practical preventive measure for asthma management in polluted outdoor or indoor environments worldwide, Zhang said.

They could also be lifesavers in areas near wildfires.  

“Look at the high PM2.5 pollution levels that occurred in San Francisco last year as a result of smoke from the California wildfires, and at the air-quality problems happening this year from the bushfires in Australia,” he said. “People should really consider using one of these devices during wildfires.” Xiaoxing Cui, a 2018 doctoral graduate of Duke, led the study with Zhang.