A New Calculator Shows What Size Air Purifier Is Needed For Classrooms

As schools across the country reopen, districts are working hard to ensure that students and faculty will be as safe as possible. One of the most effective ways to limit the spread of coronavirus is to manage indoor air. Experts are increasingly recommending the use of true HEPA air purifiers to keep air in the classrooms free of pollutants. To determine what size purifier is necessary, scientists at Harvard University and the University of Colorado have teamed up to develop a downloadable calculator to assess the strength of air purifier necessary for each classroom.

How it works

Users download a spreadsheet and fill out questions about 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.  The calculator uses that information to determine if the air purifier meets the standard that the experts recommend: five or more air changes in the room in an hour.

Experts also advised people to consider the clean air delivery rate (CADR) of the purifier. CADR is a rating system measures the performance of residential air purifiers. It measures how much air is filtered in an hour for a particular pollutant. The Harvard team recommends looking at the CADR for smoke or dust in determining the best purifier for classrooms. Generally speaking, the higher the CADR number, the faster the air purifier filters the air.

What other measures can schools take to lower transmission rates?

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

Open windows and air purifiers are both part of a long list of suggestions for safe school openings detailed in the Healthy Buildings Program report issued in June.  Other recommendations include masks, clean surfaces, plexiglass barriers, “reimagining” music and theater classes, changing arrival, departure, and transition times in hallways, and forming a COVID-19 response team. The Harvard team has also included 20 questions parents should ask before sending their children back to school.

While acknowledging the risks of opening schools, 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.”

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