A large-scale study in elementary school classrooms has found that portable HEPA air purifiers, when used as a standalone measure, do not significantly decrease the concentration of airborne respiratory viruses. Researchers from Mass General Brigham discovered that even in rooms equipped with multiple active HEPA units, the overall viral load remained high, challenging a key strategy widely adopted by school systems to improve air quality and reduce disease transmission.
The findings, published in JAMA Network Open, suggest that ensuring safer air for students and teachers requires a more complex, multi-layered approach than simply plugging in an air cleaner. While HEPA filters are known to capture fine particulate matter, this real-world evidence indicates they are insufficient on their own to combat the high density and variety of viruses circulating in schools. The study highlights other environmental factors, particularly humidity, as critical components in a comprehensive strategy to mitigate viral exposure.
Secondary Analysis of an Asthma Study
The new insights into viral transmission emerged from a secondary analysis of a different clinical trial, the School Inner-City Asthma Intervention Study, known as SICAS-2. The original research, which ran from September 2015 to June 2020, was designed to determine if HEPA purifiers could reduce asthma symptoms among elementary school students in 200 classrooms in the Northeastern U.S. This setup provided a unique opportunity for scientists to later investigate a different question using the collected data.
In the SICAS-2 trial, classrooms were randomly assigned to one of two groups. One group received four portable HEPA purifiers, while the control group received four “sham” units that were visually identical but contained no filters. School staff and the investigators were blinded to which classrooms had the active or sham devices. For the subsequent viral analysis, researchers collected 532 bioaerosol samples from these classrooms and tested them for the presence of 19 common respiratory viruses.
Ubiquitous Viral Load in Classroom Air
The analysis revealed a near-constant presence of viruses in the classroom environment. A striking 98.5% of all air samples collected contained at least one respiratory virus. On average, each classroom sample contained three different types of viruses, with some samples showing as many as 13 distinct viruses simultaneously. This demonstrates that school settings are significant reservoirs for a wide array of pathogens.
The most frequently detected virus was rhinovirus, which appeared in nearly 90% of samples. However, the researchers also commonly found pathogens capable of causing more severe illness. Respiratory syncytial virus (RSV) and influenza A and B were also prevalent, with influenza A found in almost 18% of samples and RSV B in nearly 24%. According to corresponding author Dr. Peggy S. Lai, these findings underscore the significant risks faced by students and educators.
Minimal Effect on Overall Virus Levels
The study’s central finding was the limited effectiveness of the air purifiers on the total amount of virus in the air. The HEPA intervention was not associated with a statistically significant reduction in high viral exposure. “Air purifiers did not reduce overall viral load in classrooms,” Dr. Lai stated, indicating that these devices alone could not overcome the high rate of viral shedding and circulation within the enclosed spaces.
However, the purifiers did have one measurable effect: they were associated with a modest reduction in viral diversity, which is the number of different virus types detected in a sample. Classrooms with active HEPA units saw a nearly 33% decrease in the variety of viruses compared to the control group. Despite this, the reduction in viral diversity did not lead to a tangible health benefit, as it was not associated with a decrease in school absences for the students.
Humidity Emerges as a Key Factor
In searching for other variables that influence viral concentrations, the research team identified a strong connection between the environment and viral exposure. The analysis showed that lower relative humidity was a key risk factor for higher viral loads. This suggests that controlling the moisture content in the air could be a crucial tool for managing the spread of airborne viruses in schools.
This finding aligns with previous research showing that many respiratory viruses survive and travel more effectively in dry air. Based on this connection, the researchers suggested that maintaining classroom humidity levels within a specific range could be an important intervention. Dr. Lai recommended that, in addition to ventilation, keeping indoor humidity between 40% and 60% may help lower viral exposure while also improving overall comfort.
Future Requires Layered Interventions
The study’s conclusion is that schools must move beyond single-solution approaches to air quality. The authors emphasize the need for multicomponent interventions to effectively reduce the risk of respiratory virus transmission. A layered strategy, they argue, would be more robust and effective than relying solely on portable air purifiers.
Such a strategy would combine several elements. Enhanced ventilation, whether through mechanical systems or simply opening windows, remains a fundamental recommendation. The study’s data adds humidity control as another critical component. These environmental controls, combined with established public health measures, could work together to create a safer indoor environment. Dr. Lai noted that while the trial was not originally designed to measure viral outcomes, its results clearly point toward the need for more comprehensive solutions to protect the health of students and staff.