By Darren Wong, MD, and Nancy J. Selfridge, MD

Dr. Wong is a Clinical Skills Facilitator, Ross University School of Medicine, Barbados, West Indies.
Dr. Selfridge is Professor, Clinical Foundations Department, Ross University School of Medicine, Barbados, West Indies.

SYNOPSIS: Children with asthma showed improved small airway mechanics following indoor filtration of particulates (2.5 µm and greater) using high-efficiency particulate air filtration devices.

SOURCE: Xiaoxing C, Li Z, Teng Y, et al. Association between bedroom particulate matter filtration and changes in airway pathophysiology in children with asthma. JAMA Pediatr 2020;174:533-542.

In many environments, from industrial settings to indoor homes, pollution has an elevated amount of particulate matter 2.5 µm or smaller (PM2.5). It is widely believed that PM2.5 deposits in smaller pulmonary airways leads to asthma exacerbation. Specifically, PM2.5 has been associated with increased oxidative stress and pulmonary inflammation. Before this study, there was little conclusive information on whether the reduction of PM2.5 exposure improves small airway function in children with asthma.

This randomized, double-blind, crossover study of 43 children with mild to moderate asthma and ages ranging from 5 to 13 years took place over the course of 70 days in Shanghai, China, where the PM2.5 used ranged from less than the U.S. National Ambient Air Quality standard (35 µg/m3) to nearly double this level. PM2.5 was measured indoors and outdoors, and filtration of PM2.5 occurred in the children’s indoor bedrooms with high-efficiency particulate air (HEPA) filtration and activated carbon. The primary outcome of the study examined fractional exhaled nitric oxide (FeNO), while also examining the effects of PM2.5 on airway mechanics and function using impulse oscillometry (IOS) and spirometry, respectively. The authors also examined PM2.5 filtration on children with various baseline FeNO and eosinophil levels and its subsequent effect on airway physiology.

In comparison to no filtration, true filtration led to a reduction of PM2.5 by 79.6% and 63.4% in outdoor and bedroom concentrations, respectively. Furthermore, true filtration showed improved respiratory inflammation (FeNO 24.4%; 95% CI, 11.8%-37.1%), mean peaked expiratory flow (PEF) (1.6%; 95% CI, 0.8%-2.5%), and airway mechanics (reduction in resistance at 5 Hz [R5] and resistance at 5 Hz to 20 Hz [R5-R20] by 43.5% and 73.1%, respectively). Further analysis showed that for every 10 µg/m3 reduction in bedroom PM2.5 concentration, there was a significant improvement in airway mechanics (reduction in R5 and R5-R20 by 4.6% and 7.6%, respectively), small airway airflow, and inflammation (6.8% reduction in FeNO). Children with lower baseline FeNO or eosinophil count showed significant improvement in airway mechanics vs. children with higher baseline values. However, airway function did not show significant improvement, since overall small airway function (forced expiratory flow), forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and FEV1/FVC were not significant.


This study reinforces the evidence suggesting the negative effects of air pollution on the respiratory system. Prior studies have shown air particulates found in air pollution result in increased oxidative stress and airway inflammation in children with asthma.1-4 A study conducted in 2015 revealed that for every 10 µg/m3 increase in PM2.5, there was an increased relative risk of 1.021 for asthma-related hospital admission.5

The filtration of PM2.5 in this study revealed improved airway mechanics and reduction in small airway inflammation. Similar results were seen in other studies.2,3,6 However, no significant improvement was seen in airway function. This contrasts with a 2008 study that revealed a significant inverse association between PM2.5 and FEV1 and FVC in children with and without asthma.2 The discrepancy may be caused by the duration of the study/sample size or other risk factors that were not accounted for, such as other air pollutants like sulfur dioxide (SO2) or nitrogen dioxide (NO2).4,7,8 A study of children with asthma in inner U.S. cities showed higher five-day average concentrations of NO2, SO2, and PM2.5 were associated with significantly lower pulmonary function and increased asthma-related missed school days.4 Other factors that could influence airway mechanics and airway function include urbanization, age, and social economics.1,6,7 Studies have shown exposure to an increase in air pollution in minority children in the first year of life was associated with an odds ratio of 1.17 for physician-diagnosed asthma.7 Based on the study, I would agree that additional clinical trials are needed to evaluate whether filtrating PM2.5 is an effective tool to improve airway mechanics and prevent/reduce asthma symptoms. However, if financial costs are not an issue, I would suggest households include a HEPA filter to reduce PM2.5 levels, since the study reveals significant improvement in airway mechanics and reduction in small airway inflammation. 


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