Classroom Air Quality in a Randomized Crossover Trial with Portable HEPA Air Cleaners

This block-randomized crossover trial evaluated whether adding portable HEPA filter air cleaners to Los Angeles Unified School District (LAUSD) classrooms—already equipped with MERV-13 HVAC filtration—could further reduce particle exposure for students in environmentally burdened communities. From July 2022 to June 2023 the team monitored PM1, PM2.5 and PM10 in 186 classrooms (99 with HEPA units, 87 with visually identical non-HEPA “sham” units) across 17 elementary schools near ports, refineries and major freeways. Intention-to-treat analysis showed HEPA rooms averaged 39.9 % lower PM2.5 (0.89 µg/m³ vs 1.47 µg/m³, p < 0.001) over the school year. Outdoor–indoor infiltration ratios confirmed filtration benefits: HEPA rooms cut penetration of ambient PM2.5 by 14 – 82 %, varying with building type and proximity to sources. Mixed-effects modelling (ICC = 0.15) indicated reductions were consistent across schools despite heterogeneity between portable and permanent buildings. Results demonstrate that portable HEPA units provide a statistically and practically meaningful IAQ improvement even when central HVAC meets pandemic-era MERV-13 standards—an actionable strategy for districts unable to overhaul legacy systems quickly. Authors note greatest potential impact in high-poverty, high-pollution neighborhoods targeted by California’s AB 617 incentives.