Amid the stimulus checks and unemployment extension in the $1.9 trillion relief package signed into law last week, is $130 billion to help schools reopen safely — including for personal protective equipment, reducing class sizes and, importantly, improving ventilation.

School safety measures call to mind images of masked students spread out among plexiglass-enclosed desks. But one of the most important things schools can do might be something you can’t see said Donald Milton, a professor of environmental health at the University of Maryland.

“Ventilation is critical because respiratory viruses can be transmitted by inhalation,” he said. “And the way to cut down on what people are inhaling, is to remove it from the air.”

But until recent weeks, the Centers for Disease Control and Prevention had offered little guidance on improving ventilation in schools, and many had turned to unproven measures said professor Joseph Allen, director of the Healthy Buildings program at Harvard.

“This is where the hygiene theater has come into play,” he said. “You see someone in gloves wiping down every surface — walls, and things that just don’t make sense.”

Much more important, he said, is ensuring the air in rooms is completely refreshed, known as air exchange, about five times per hour. 

That can be achieved by adjusting HVAC systems, something that might be tough for the many schools with old and failing air conditioning. 

“So it’s not as simple as put a filter in, you’re going to have to upgrade the whole system to be able to do that,” said engineer Maria Lehman, treasurer of the American Society of Civil Engineers. The group gave U.S. public schools a D+ in its last infrastructure report, which found more than 40% of schools had HVAC systems in need of repair.

“It’s of concern particularly because of evidence that we’re starting to build, that air pollution is harmful to student learning,” said Claudia Persico, a professor of public policy at American University.

Millions of students in the U.S. attend schools in close proximity to freeways or other sources of pollution, she said, so opening windows to improve airflow isn’t always a great option.

Though having a window to open would be a luxury for New York special education teacher Annie Tan.

“Right before COVID hit, I was in a windowless classroom,” she said. “I bought an air purifier for my class because there was a smell my classroom.”

In fact portable HEPA air purifiers, the kind some people have in their homes, are an effective way to clear viruses from the air. 

By one analysis from Richard Corsi, an indoor air expert at Portland State University, the devices could be installed in every classroom in America for about a billion dollars.