To examine whether reducing fine particles of 2.5 µm or smaller in diameter (PM2.5), a risk factor for asthma exacerbation, improves small airway physiology among children with asthma.

Forty-three children with mild to moderate asthma (5–13 years old) were recruited from an outpatient clinic affiliated with Shanghai General Hospital.

Data were collected from a randomized, double-blind, cross-over study, and participants had 2 intervention sessions between February 14–April 24, 2017. During the true filtration session, a portable air purifier, that removed PM2.5, coarse particles, and gases, was placed in the child’s room. During the sham filtration, the air purifier was modified by removing the HEPA filter that removed PM2.5 and the carbon filter for gases, while keeping the external device appearance identical. Each session was for 2 weeks with a 2-week washout period between the 2 sessions. The primary outcome of interest was fractional exhaled nitric oxide (FeNO). Other secondary measurements included airway mechanics (ie, types of airway resistance, reactance, and airway inflammation), spirometry measures, peak expiratory flow (PEF), and PM2.5 and ozone concentrations in the rooms and outdoor of the child’s home.

A 27.6% (95% CI: 8.9% to 42.4%) reduction in FeNO was observed in true versus sham filtration. Improvements in airway mechanics were observed in true filtration, compared with sham, and mean PEF during true filtration was significantly higher by 1.6% (95% CI: 0.8% to 2.5%). True filtration resulted in mean (SD) reductions of 63.4% (35.9%) in bedroom PM2.5 concentrations compared with sham filtration. No significant differences were observed for changes in spirometry outcomes (eg, FEV1). Post-hoc stratified analyses found that children with higher baseline FeNO levels had no significant improvements in any outcomes, and children with higher baseline eosinophil counts only had significant improvements in PEF.

The findings suggest indoor reduction of PM2.5 concentration is associated with improved airway mechanics and reduced airway inflammation, particularly in the small airways, the site for early and critical changes in asthma worsening. Large airway changes, such as FEV1 or PEF, were less impressive with no or small improvements. Improvements in FeNO were consistent with prior studies using HEPA filtration.

In this small but well-designed, observational study, clinicians are provided evidence that removal of PM2.5 through bedroom airway filtration may help improve lung pathophysiologic changes among children with asthma. As the authors note, a large-scale clinical effectiveness trial is still needed to establish causality.