Early evidence that air filters could improve education outcomes started to emerge years before the COVID-19 pandemic and came from a surprising source: a natural gas leak in Aliso Canyon, California. In late 2015 the gas company, facing public pressure, installed commercial-grade air purifiers in every classroom within a five-mile radius. Michael Gilraine, an economist studying the incident, recognized a rare opportunity: Schools just inside the five-mile boundary received air filters, while demographically similar schools just outside the artificial boundary did not.
Gilraine found that students in schools with air filters saw their test scores jump: Math scores increased by about three months of learning, and English scores were close behind. The gains persisted and even grew over time. To put this effect size into context, students in the most prominent class-size study, the Tennessee STAR experiment — who were randomly placed in much smaller classes averaging 15 students instead of 22 — experienced roughly similar gains.