Joseph G. Allen is an assistant professor, director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health and co-author of “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.” Richard Corsi is dean at Portland State University’s Maseeh College of Engineering and Computer Science.

We have limited time and funds to get students and teachers back to school safely, but we can — and must — do it. Here’s how.

Start with the fact, as 239 scientists recently wrote to the World Health Organization (WHO), that airborne transmission of the novel coronavirus is happening. This is not to be feared; it just requires adding some new strategies to our arsenal in addition to hand-washing, distancing and other measures to keep community spread to a minimum. (Just because we reopen schools doesn’t mean we should reopen elsewhere.)

The public health science is quite simple: The amount of virus that kids and adults are exposed to is a function of how many viral particles are emitted into the air and how many are removed. Both sides of that equation can be controlled. To do it, we need to be school SMART:

S: Stay home when sick

The best way to eliminate emissions is to not have sick people in the school. People are most infectious two days before they start showing symptoms, and infectiousness ends five to seven days after. We won’t be able to keep people from going to school when they aren’t showing symptoms, but we certainly can when they do.

This can be encouraged in several ways: Provide paid sick leave for teachers and administrators; call up recent retirees and waive rules for new graduates to fill in vacancies; and set up parallel remote learning tracks for those kids who must stay home. Schools should also screen for the disease daily through self-certification checks and visual observation. (Rapid saliva tests — as soon as they are approved and deployed — will be the ultimate screening tool.)

M: Mask up

To control asymptomatic emissions and capture particles at the source, schools must require mask-wearing. Many people may assume that the typical three-ply surgical masks recommended by the WHO are not effective for preventing airborne viruses. In reality, multi-layer masks, when worn by everyone correctly, can be very effective. Consider that if two people wear masks, particles must pass through their material twice. Even if a mask filters out 60 percent of the particles, two of them together filter out 84 percent. Not quite as good as an N95, but not far off, either.

A: Air cleaner in every classroom

Portable air cleaners, also known as air purifiers, may be the fastest way to clean the air quickly indoors. A portable air purifier with a HEPA filter that is correctly sized for the room can deliver three air changes per hour of clean air, meaning all of the air in the room is cleaned every 20 minutes.

Some back-of-the-envelope estimates: U.S. schools have an average of 528 total students and 20 kids per classroom. With 131,000 schools in the country, that puts the need at about 3.4 million purifiers. In 2019, manufacturers produced a total of 2.9 million portable purifiers. Can they ramp up production to meet demand? Depends on the incentives and our country’s willingness to mobilize for schools. At roughly $300 for the base machine and replacement filter, this investment would be around $1 billion. Seems like a good price to pay to keep our kids and teachers safer and to get parents back to work.

R: Refresh indoor air

Every effort should be made to determine how much more outdoor air can be brought into schools, but there are limitations. In summer and winter months, the amount of air that can be brought in from outside will be limited by the cooling and heating capacity of existing HVAC systems. While bringing in twice as much as the minimum ventilation standard would be an excellent strategy, there may not be enough time or money to fix all of these school ventilation problems in the next 30 days before kids come back to school. That doesn’t mean we shouldn’t try.

Schools should also upgrade recirculated air filters to MERV13 or higher. If schools rely on natural ventilation, get those windows open and use simple box fans to pull in outdoor air.

T: Temporary classrooms

It’s time get creative and re-imagine classrooms. We don’t need to think about ventilation rates if we hold classrooms outdoors. Yes, there will be inclement weather — kids and teachers will have to wear hats and gloves when it gets cold, and papers will occasionally get blown around. But this is still far superior to learning via Zoom. A massive mobilization of tents for schools, maybe by the National Guard, could get us there. Think this is impossible? We’ve done it before, during the tuberculosis epidemic.

Let’s re-imagine schools, too. New York turned the Javits Convention Center into a hospital and put hospital field tents in Central Park. Is there anything stopping us from turning convention centers in every city into schools? Or turning city and town parks into tented classrooms?

We teach our kids to have a growth mind-set — to embrace challenges and persist in the face of obstacles. With reopening schools, we’re mostly hearing about roadblocks and “that can’t happen” thinking. Let’s get creative and live by what we teach.

Read more: