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Experts say some classrooms may need an air purifier — and offer advice on sizing

An empty classroom in Boston.
An empty classroom in Boston.David L. Ryan/Globe Staff

Experts from Harvard University and the University of Colorado have developed a downloadable calculator to determine how powerful an air purifier is necessary to help keep kids in classrooms safe from the coronavirus.

Portable air purifiers are just one part of a multi-layered strategy recommended to keep the air in school buildings safe developed by experts from the Healthy Buildings Program at Harvard’s T.H. Chan School of Public Health.

Memo Cedeno Laurent, associate director of the program and one of the creators of the calculator, said the first steps to prevent airborne spread of the virus are to look into a building’s ventilation system to make sure it is drawing enough fresh air and is outfitted with filters that can screen out the virus, and to look at simply opening windows, which can provide a good supply of fresh, virus-free outside air.

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If those two steps don’t provide enough clean air, portable HEPA (high-efficiency particulate air) air purifiers can be used to remove virus particles from the air. The experts say HEPA filters capture 99.97 percent of airborne particles.

“This would be in case they need supplemental provision of clean air,” he said. “Not every classroom needs one of these.”

The tool, which is a downloadable spreadsheet, allows the user to fill in such variables as the square footage of the room, the height of the ceilings, the building’s current ventilation rate, and the capacity of the air purifier being considered. It then shows whether the air purifier will be able to meet the standard that the experts recommend, five or more air changes in the room in an hour.

The experts advised people to look at the performance of an air purifier in terms of its clean air delivery rate (CADR), which is how many cubic feet of air per minute it can clean of particles. CADRs differ for different substance. The experts said to look at the CADR for smoke or dust.

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The spreadsheet also includes a page showing the actual air purifiers considered by one district and how they performed.

The tool can also be used to calculate what size air purifiers someone should get for a room in a home or business.

The experts did not recommend any specific models but said that people should opt for HEPA air purifiers, and avoid any add-ons such as ionizers. If one isn’t enough, they suggest buying two.

Laurent said in many cases, the simplest answer may be the best: opening the windows, which can provide a surprising amount of air to a room. But he also said windows that don’t open are a common problem.

In a perfect world, the experts say, holding class outside would provide the freshest air and most effective dilution of the virus.

How will you know how much ventilation a classroom is getting? Laurent said researchers are working on a set of simple procedures that will allow people to assess the ventilation rates in a given room.

Joseph Allen, the director of the Healthy Buildings Program, who also worked on the calculator, said, “There’s always something you can do. ... You can turn any building into a healthy building with just a little attention.”

The use of portable air purifiers as part of a strategy to provide schools with clean air is just one of a host of recommendations detailed in a Healthy Buildings Program report issued in June, Risk Reduction Strategies for Opening Schools.

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“We tried to give a roadmap for schools to take” to protect students, teachers, and staff, he said.

The other recommendations range from everything from asking people in schools to wear masks and clean surfaces, to installing plexiglass barriers in some settings, to “reimagining” music and theater classes, to changing arrival, departure, and transition times in hallways, to forming a COVID-19 response team.

The Harvard healthy buildings experts have also developed a document suggesting 20 questions parents should ask before sending their children back to school.

“We should expect things to be different at schools this fall. It cannot be schools as usual,” said Allen, who noted that parents might be aware of the need for students to wear masks and wash their hands frequently, but unaware of other steps that can be taken. “It’s a guide to help parents know what questions to ask, and know what answers to look for.”

About 70 percent of Massachusetts school systems plan to bring students back to the classroom at least part time this fall, even as teachers unions have been aggressively pushing to keep buildings closed, the Globe reports. The other 30 percent of districts will offer remote-only instruction. Boston has not yet released its reopening plan.

Shelly Miller, a professor at the University of Colorado at Boulder, also worked on the calculator project.

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Martin finucane can be reached at martin.finucane@globe.com.