Other research has found an association between ventilation and learning, as well. In one study, the scientists had students take tests in rooms whose ventilation they controlled. They found that students in better ventilated spaces had better scores. “There’s pretty strong evidence that improved ventilation will get you improved student performance,” Chan says. There is also evidence, albeit more mixed, that better ventilation reduces absences.

The science has yet to penetrate into practice, though. In one study, coauthored by Chan, 85 percent of the California classrooms included failed to meet the minimum standard of 15 CFM per person. Across the country, many classrooms have unit ventilators, big metal boxes that sit against exterior walls and push air out and suck fresh air in. (Allen often sees them covered with books or potted plants.) Some schools have centralized systems with vents in the ceiling. But even in schools with upgraded systems, Chan and colleagues at LBL and UC Davis found that about half of classrooms still didn’t get enough fresh air, because controls were not set to adequate ventilation, or the systems weren’t maintained or installed correctly.

People are starting to realize this is an issue, even on the level of the federal government, Chan says. A Government Accountability Office report from before the pandemic found that 40 percent of schools needed to upgrade their HVAC systems. But, when the novel coronavirus pandemic hit, not much had been done.

Outside that Boston-area school this summer, Allen and his team found an unobtrusive vent in the white-painted brick wall and wrapped it in blue fabric. In this way, the air coming out of the classroom on the other side of the wall was funneled into a small black box called a balometer. On the device’s screen, a number popped up, revealing just how much air was moving out of the classroom.

This was the first step in what Allen says is a pretty simple way to reduce the likelihood that SARS-CoV-2 will spread in schools.

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Allen estimates that for reducing Covid-19 risk, the air in the room should be completely replaced at least five times an hour. In that Boston school, the balometer registered about 400 cfm of fresh air coming in through the unit ventilator in one classroom. The room measured 1,010 square feet and had 9.5 foot ceilings: It had 9,595 cubic feet of air. Multiply 400 cubic feet per minute by 60 minutes, divide it by the volume, and you find that the air only gets turned over 2.5 times an hour. That’s subpar by Allen’s standards.

Opening the windows and doors could boost that number past five, sometimes far higher. Allen and his team confirmed that finding this summer, when they placed dry ice, which produces carbon dioxide, in classrooms to mimic a room full of people. With a cheap carbon dioxide sensor, they could see how quickly the CO2 dissipated. “In one, when the windows and doors were open, we were at 17 to 20 air changes per hour,” Allen says.

When windows and doors can’t be opened—which will become more of an issue as cold weather arrives—installing a filter in the unit ventilator or the central ventilation system can help. While the air itself isn’t always coming from out of doors, it’s been pushed through a filter. Viruses travel in tiny specks of fluid, and these specks are large enough that they can be captured by filters of a certain grade. MERV13, which pulls out particles as small at 0.3 microns, is the rating that Allen suggests for this situation. When that’s not an option, the next best choice is to add in a portable air cleaner, which, again, is just a HEPA filter and a fan.