Editor’s note: This segment was rebroadcast on April 11, 2022. Find that audio here.

Among the key questions for those returning this fall to offices — as well as gyms, theatres, restaurants and more — is whether their space is COVID-19 safe.

Experts agree that proper ventilation can help prevent COVID-19 spread since the virus is airborne.

Virus transmission is “happening indoors in under-ventilated places. We’ve known this for a long time now,” says Joseph Allen, director of Harvard University’s Healthy Buildings Program and author of “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.”

But experts warn that not all the ventilation technologies available are useful or even safe.

Efficacy data is limited on products that claim to reduce the amount of virus in the air, Allen says, and some of these products — such as plexiglass, deep cleaners and a variety of disinfecting agents — can generate secondary pollutants.

Plexiglass, for example, is OK in front of a cashier at a supermarket since they check out hundreds of people per day, he says. But some studies have shown widespread overuse of plexiglass in places such as schools can impede airflow and trap infectious respiratory aerosols, Allen says.

“It doesn’t stop what we really need to stop: respiratory aerosols that can carry the virus from deep in our lungs,” he says.

To effectively reduce the amount of tiny droplets and aerosol particles lingering in a room, Allen first recommends bringing in more outdoor air by opening up one, two or as many windows as possible.

Second, if you have a mechanical ventilation system, he says to update the filter to a MERV 13 filter. Typical buildings have MERV 8 filters, which only capture up to 20% of particles, whereas the MERV 13 filter can capture 80%, he says.

If you can’t open a window or switch out to a MERV 13 filter, Allen says to get a portable air cleaner with a HEPA filter. Purchase a device with a clean air delivery rate (CADR) of 300 for every 500 square feet of room, he says. Avoid portable air purifiers with ionizers or UV light.

Most offices are heated and cooled with HVAC, systems that have been designed to use minimum amounts of outdoor air.

“We’ve kind of stopped letting our buildings breathe, but we need to let them breathe again,” Allen says. “We’ve been designing our buildings for the past 40 years without human health in mind.”

Interview Highlights

On increasing outdoor air into buildings

“It depends on the outdoor weather. But any building manager knows that the amount of outdoor air that’s coming in depends on outdoor weather conditions and the heating and cooling capacity of the system. So they constantly are modulating or adjusting these outdoor air dampers, essentially changing how much outdoor air comes in. The problem is, if you’re recirculating that air with these low-grade filters like a MERV 8 filter, you’re not capturing much of that virus.

“So it’s really these two in combination that work really well: You want to open up those [HVAC] dampers, bring in as much outdoor air as you can, and then make sure it’s going through these better filters. And in the event you can’t do those — it’s too hot, it’s too cold, you can’t bring out enough outdoor air — ventilation and filtration have to be thought of as a team and they’re working together. So if it’s an area with that outdoor air pollution or high wildfires and wildfire smoke, well, then you want to bring in less outdoor air. But that’s where your filtration game has to pick up. And when it’s temperate [and the] weather’s nice, bring in as much outdoor air as you can. That’s a great strategy.”

On air changes per hour (ACH)

“The amount of air in the room or building that gets changed every hour — air changes per hour is ACH — we’ve been recommending four to six air changes per hour through any combination of ventilation and filtration, turning over the air very frequently.”

On carbon dioxide monitors to measure ventilation

“At these lower costs, CO2 monitors are just terrific for helping to understand the amount of outdoor air you’re getting in. I have two of these on my desk right now. If there are high levels of CO2, it’s telling you you have an under-ventilated place. And for years, our field has used these as a quick indicator of this ventilation rate, using about a thousand parts per million of CO2 on the monitor to indicate a quote-unquote, acceptable level of ventilation.

“But therein lies a major problem. The ventilation standards that are set for buildings are based on a standard called the standard for acceptable indoor air quality. But it’s not a standard that works during a pandemic and for control of respiratory viruses. You want to see numbers down at about 800 parts per million of CO2.”

On whether it’s useful to use carbon dioxide monitors to gauge returning to the office

“Yeah, it is used in this way to set the ventilation rate for what we call demand control ventilation, which if you have higher CO2, it tells the system there’s more people in this space [and] you need to deliver more air. We’ve actually recommended disabling this demand control ventilation function. We want air delivered all the time, regardless if it’s two people in a room, three people in a room or 10 people in a room. But you’re right in that organizations can set occupancy limits based on the ability of that room or building to provide these targeted air change per hour metrics. And so if you’re not hitting these, then one approach is to de-densify or limit the number of people in the space.”

On types of airflow — one famous study showed that an air conditioner in a restaurant infected everyone in its path

“The airflow definitely matters. And you don’t want to put a fan or anything that’s blowing right across somebody’s breathing zone, right onto somebody else. And that example from the restaurant is interesting because it’s not so much that it’s the air conditioner that was the problem. It’s that the air was blowing across all of the patrons and the air was being recirculated through the air conditioner with no filtration on the recirculated air. Had that system had a good filter in it, it’s very likely that no one would have been sick in that space because any of the air that was circulated back through the system, all the virus in there or the majority of it would have been captured.

“My team published a study just last week looking at office workers in buildings around the world and showing that higher ventilation rates and higher filtration led to improved cognitive function performance. So when we think about these healthy building strategies that are good for COVID, these are strategies that should have been in place before COVID and they should stay with us after we get COVID under control as well.”

On inquiring about the MERV filter or ventilation systems at your office or school

“What’s really interesting is that there’s been this shift happening where before the pandemic, if I asked people on the street, you know, what’s a filter, very few people are going to be able to answer that. But now the healthy building literacy is just so high and people are asking about this and it’s great to see.”

Karyn Miller-Medzon produced and edited this interview for broadcast with Todd Mundt. Serena McMahon adapted it for the web.

This article was originally published on WBUR.org.

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