A new study by American and Chinese scientists, published in the journal JAMA Pediatrics, in April 2020, shows that a simple bedroom filter can help asthmatic children breathe better.
Fine particulate matter smaller than 2.5 micrometers (PM2.5) is an air pollutant that is present almost everywhere. It comes from the burning of fossil fuels, emissions from vehicles running on gasoline or diesel, wildfires, burning of any biomass including tobacco, and industrial emissions. These particles are smaller in diameter than a human hair. They are thus easily inhaled.
Following inhalation, they travel deep into the lower airways, "the airways with inner diameters smaller than 2 mm", where they trigger the characteristic inflammation and bronchoconstriction that produces the wheezing of asthma. They also worsen the symptoms in people who are already wheezing. In this situation, inhalers that deliver local medication to the lungs cannot relieve the wheezing because they can only deliver the drug to the upper airways.
How was the study done?
This is the first study to analyze the effect of reduced exposure to PM2.5 on the small airway function in asthmatic individuals.
The researchers did the double-blind crossover study in a suburb of Shanghai in 2017, in a period that witnessed moderate levels of air pollution with PM2.5. The study involved 43 children who suffered mild to moderate asthma, who had a history of at least one asthma exacerbation over the past year. 72% were on asthma medications.
They were given two air filters to use for two weeks each, in whichever order they chose, with a 2-week gap in between. They (and their parents) were trained to perform self-administered peak expiratory flow (PEF) measurement by a physician. They were instructed to measure PEF twice a day, in the morning and at night, with a handheld meter.
One of the filters was a high-efficiency particulate air (HEPA) filter that can trap PM2.5 and remove it. The other was a sham filter. Only the researchers knew which was which. The children were first tested for IgE allergies by a blood test. The children were also measured two times (within the 24 hours before the filtration began, and within 24 hours after it ended, respectively), for fractional exhaled nitric oxide (FeNO), by spirometry for lung function, and by impulse oscillometry (IOS) for airway mechanics.
The average daily concentrations of outdoor PM2.5 ranged from 28.6 to 69.8 micrograms/m3.
The blood tests suggested that 35 of the children had atopic asthma, 27 had dust mite allergy, and 9 had mold allergy. Among the children in the study, 26%, or over a quarter, had an exacerbation during the period of study.
The investigators found that both filters reduced the level of PM2.5 by 80% and 36%, respectively. The use of HEPA filters reduced the concentrations of PM2.5 inside the children's bedrooms by a third to two-thirds, compared to when the sham filters were in use. This decrease agreed with marked increases in the ease of air inflow and outflow from the children's smaller airways and lungs.
For instance, the total airway resistance went down by almost a quarter, on average. Small airway resistance decreased by well over 40%, and airway elasticity went up by 73% compared to the sham filter value. Again, the exhaled nitric oxide went down by 28% on average. Nitric oxide is a molecule that signifies the presence of lung inflammation. The findings exceeded the limit for clinical importance in children with asthma. Larger airways were less significantly impacted. And finally, children with eosinophilic inflammation of the airways at baseline also improved, but less so than those without this condition.
The benefits were linked to the actual presence of the filters. However, the researchers think that with continued long-term use, the benefits will be more apparent as well as more permanent.
What do the results mean for children with asthma?
Small airway impedance is the earliest and most significant change, which leads to the progression and exacerbation of asthma. Thus, the findings that reducing exposure to PM2.5 improves airway function and airflow, especially in the small airways, are compelling.
The usefulness of indoor filtration for PM2.5 should be confirmed, as the current study strongly suggests it enhances the flow of air through the lungs of people with asthma. The effects are due to less inflammation, better airway performance, and enhanced lung function. Of course, the study did not measure what allergens were removed as well. However, this doesn't detract from the validity of the associations.
The researchers say, "This interventional (observational) study provides compelling data to support a larger-scale clinical trial to evaluate the efficacy and effectiveness of residential air filtration" in children and potentially adults with asthma. In case the trial succeeds, the HEPA filters could be a good way to prevent many cases of asthma in children from becoming worse, in an outdoor or indoor setting, in places that experience moderate to heavy pollution all over the world. And in places affected by nearby wildfires, they could literally save the lives of vulnerable children.
Also, the differential response in children with and without baseline eosinophilic inflammation of the lungs could shape a precision approach to preventing asthma exacerbations in children.
Researcher Junfeng Zhang points out, "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."
Cui, X. et al. (2020). Association Between Bedroom Particulate Matter Filtration and Changes in Airway Pathophysiology in Children with Asthma. JAMA Pediatrics. doi:10.1001/jamapediatrics.2020.0140.
Cui X, Li Z, Teng Y, et al. Association Between Bedroom Particulate Matter Filtration and Changes in Airway Pathophysiology in Children With Asthma. JAMA Pediatr. Published online April 06, 2020. doi:10.1001/jamapediatrics.2020.0140