HEPA purifiers alone may not be enough to reduce viral exposure in schools

School is in session, and viral illness is on the rise. A new study suggests that lowering exposure to respiratory viruses in classrooms isn’t as simple as adding high-efficiency particulate air (HEPA) purifiers to the room.

In a secondary analysis of a randomized clinical trial, investigators from Mass General Brigham and their colleagues found that exposure to respiratory viruses in the air was high, even in classrooms with HEPA purifiers.

Their results are published in JAMA Network Open.

“We found that air samples from classrooms included in our study had an average of three different respiratory viruses per classroom, with some having as many as 13, including pathogens capable of causing serious disease such as respiratory syncytial virus (RSV) and influenza virus,” said corresponding author Peggy S. Lai, MD, MPH, a physician scientist in the Pulmonary and Critical Care Division in the Mass General Brigham Department of Medicine. “Air purifiers did not reduce overall viral load in classrooms, suggesting that additional interventions may be needed in schools.”

HEPA filters are designed to reduce fine and particulate matter from the air, but there is limited real-world data on their effectiveness at filtering respiratory viruses transmitted via aerosols and droplets. In the School Inner-City Asthma Intervention Study (SICAS-2), researchers enrolled elementary school students diagnosed with asthma and attending public schools in the Northeastern U.S. between September 2015 and June 2020.

The original trial examined whether HEPA purifiers in classrooms could reduce asthma symptoms. The students’ 200 classrooms were randomly assigned to receive portable HEPA purifiers or “sham” purifiers (which didn’t have a filter in them). In this secondary analysis, the researchers looked at the concentrations of 19 respiratory viruses in the air, assessing high viral exposure as well as viral diversity.

The research team detected viruses in 98.4% of air samples from the classrooms. Although they did not detect a decrease in respiratory viral exposure overall for the classrooms with HEPA purifiers, the researchers did see a modest decrease (32.8%) in viral diversity. But this decrease in viral diversity wasn’t associated with a reduction in school absences. The researchers did see a connection between lower humidity and high viral exposure, suggesting that controlling humidity may be an important factor for lowering exposure to certain viruses.

“School-aged children and teachers face increased risks from respiratory viruses, which can impact their health, lead to missed school days, and create challenges for families,” said Lai.

“While our study did not find that HEPA air purifiers reduced high viral exposure in classrooms, it’s important to note that the trial was originally designed to measure other outcomes. In addition to air filtration or better ventilation, maintaining classroom humidity between 40% and 60% may help lower viral exposures and improve comfort for students and teachers.”