Air Purifier Intervention for Respiratory Viral Exposure in Elementary Schools: A Secondary Analysis of a Randomized...

The magnitude of school respiratory virus exposure and the effectiveness of environmental mitigation measures remain unclear.

To evaluate whether portable high-efficiency particulate air (HEPA) purifiers are associated with reduced respiratory virus exposure in elementary school classrooms.

This ad hoc secondary analysis was performed between July 2023 and September 2024 and used data from a cluster-randomized, placebo-controlled trial (School Inner-City Asthma Intervention Study) of HEPA purifiers conducted from September 2015 to June 2020. A total of 200 classrooms from 39 public schools in Northeastern US were enrolled and randomized.

Classrooms were randomized 1:1 to receive either active HEPA purifiers or visually identical sham HEPA units (4 per classroom). School staff and investigators were blinded to intervention assignments.

The primary outcome was high viral exposure, identified by K-means clustering of individual viral concentrations. Secondary outcomes included viral diversity (defined as number of detected virus types) and individual viral concentrations. Week-long bioaerosol samples were collected 3 times during 1 school year, and concentrations of 19 respiratory viruses were quantified by digital droplet polymerase chain reaction.

Of the 200 enrolled classrooms (91 in the sham purifier group and 109 in the HEPA purifier group) analyzed, the median (IQR) class size was 19 (18-20) and the median (IQR) grade was 3 (2-5). A total of 532 bioaerosol samples were collected; viruses were detected in 524 samples (98.5%), with a median (IQR) of 3 (2-5) viruses per classroom. Rhinovirus was most prevalent (476 [89.5%]), while respiratory syncytial virus A and B (66 [12.4%] and 127 [23.9%]) as well as influenza A and B (94 [17.7%] and 76 [14.3%]) were also detected. High viral exposure was present in 118 samples (22.2%). The HEPA purifier intervention was not associated with lower odds of high viral exposure (odds ratio [OR], 0.50; 95% CI, 0.08-3.25; P = .46) but did correspond to a modest reduction in viral diversity (β = -1.02; 95% CI, -1.68 to -0.35; P = .003). Elastic net regression identified relative humidity, grade, winter season, and coarse particulate matter as the environmental risk factors for viral exposure.

In this secondary analysis, HEPA purifiers were not associated with a reduction in high viral exposure but were associated with a modest decrease in viral diversity. Multicomponent interventions may be needed to mitigate respiratory viral exposures in schools.