• Posted October 15, 2025

HEPA Air Purifiers Don't Get Rid Of Airborne Viruses In Classrooms, Study Says

School is back in session and with it, the beginning of the cold and flu season.

Unfortunately, air purifiers aren’t likely to reduce the risk of children’s exposure to respiratory viruses in the classroom, researchers reported Oct. 10 in JAMA Network Open.

Even high-efficiency particulate air (HEPA) filters didn’t stop the spread of airborne viruses, the study found.

“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,” senior researcher Dr. Peggy Lai said in a news release. She’s a physician scientist in the Pulmonary and Critical Care Division at the Mass General Brigham Department of Medicine in Charlestown, Mass.

“Air purifiers did not reduce overall viral load in classrooms, suggesting that additional interventions may be needed in schools,” Lai said.

For the study, her team analyzed data from a previous clinical trial that ran from September 2015 to June 2020, in which 200 classrooms from 39 public schools in the Northeastern U.S. participated.

The classrooms were randomly assigned to get an active HEPA air purifier or an identical sham purifier that didn’t run air through a filter.

The team looked at concentrations of 19 respiratory viruses found in the air of the classrooms.

Overall, more than 98% of the air samples contained viruses, results showed.

HEPA purifiers did cause a modest decrease (33%) in the diversity of viruses found in classrooms.

But this decrease did not appear to reduce kids’ absences from school due to illness, researchers reported.

They did see a connection between lower humidity and high viral exposure, indicating that controlling a classroom’s humidity might help prevent the spread of viruses.

“Model-based studies on relative humidity suggest that lower relative humidity levels below 40% could lead to longer persistence in the air of certain viruses, such as influenza and SARS-CoV-2, by promoting viral stability and suspension in aerosols,” researchers wrote.

Lai said more research is needed on ways to prevent transmission of disease-causing viruses in the classroom.

“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,” Lai said.

“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,” Lai said. “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.”

More information

The U.S. Centers for Disease Control and Prevention has more on everyday ways for schools to prevent infections.

SOURCES: Mass General Brigham, news release, Oct. 10, 2025; JAMA Network Open, Oct. 10, 2025