By Caitlin McCabe

Add this to the Covid-19 prevention toolbox: strong ventilation.

After urging steps like handwashing, masking and social distancing, researchers say proper ventilation indoors should join the list of necessary measures. Health scientists and mechanical engineers have started issuing recommendations to schools and businesses that wish to reopen for how often indoor air needs to be replaced, as well as guidelines for the fans, filters and other equipment needed to meet the goals.

“We didn’t focus on it enough initially,” said Abraar Karan, a doctor at Brigham and Women’s Hospital in Boston who treated Covid-19 patients. “We told everyone to stay home. We weren’t thinking about people congregating in public spaces.”

Driving the thinking is mounting evidence that the new coronavirus is transmitted through the air among people with prolonged exposure to the pathogen. Especially troublesome, epidemiologists and other scientists say, is evidence from numerous indoor outbreaks suggesting the virus’s ability to spread to others even when close contact is avoided.

The precise role that airborne transmission plays is still being debated by parts of the scientific community. Yet proponents of aerosol transmission say the evidence so far argues for the need to keep clean air flowing in indoor spaces where people gather.

Ideally, they say, public spaces like a standard classroom should aim to have air replaced with clean air between four to six times an hour to dilute Covid-19 particles that might accumulate.

That can be done, aerosol scientists and building engineers say, through strategies that introduce outdoor air and filter indoor contaminants. Those include opening windows and doors, installing window fans, using portable air purifiers with high-efficiency particulate air, or HEPA, filters and upgrading heating, ventilation and air-conditioning systems to meet certain standards.

Some businesses have begun taking such steps, including malls and gyms in New York, where reopening guidelines list enhanced air filtration as mandatory for the spaces. But in aging schools nationwide, strengthening ventilation may be difficult.

About 41% of U.S. public-school districts need to update or replace their HVAC systems in at least half their schools, representing about 36,000 schools nationwide, according to a report published in June by the U.S. Government Accountability Office, a federal watchdog.

Repairs can be costly. Denver Public Schools plans to spend nearly $5 million before students return to improve HVAC systems across roughly 185 buildings, including upgrading filters, repairing broken parts and increasing the amount of outdoor air in the systems, it said.

Public health officials and scientists studying the virus are still working to better understand how Covid-19 is transmitted. Some argue it is predominantly spread by large droplets, transmitted by coughing, talking or sneezing, which people nearby can inhale. The droplets, which tend to fall to the ground quickly, can also splatter on surfaces that bystanders touch and transfer to their mouths, noses and eyes.

Other scientists argue a significant role is played by smaller particles invisible to the naked eye, called aerosols, that linger in the air and travel. A recent study—which found that particles extracted almost 16 feet from hospitalized Covid-19 patients could infect cells in a lab—suggests aerosols as a potential source of spread, aerosol scientists say. The study, posted on the preprint server medRxiv, hasn’t been peer reviewed.

“Based on the evidence we have on hand, it seems wildly irresponsible to me not to recommend strategies” for ensuring strong ventilation, said Joseph Allen, director of Harvard University’s Healthy Buildings program, which studies how buildings affect human health.

He and other health-science and mechanical engineering experts recently released a tool to help schools determine how to attain several air changes an hour. Currently, he said, some schools may only be achieving one or two.

Poor ventilation may have played an important role in several indoor Covid-19 cases, researchers said.

Five people sitting at tables adjacent to an infected but presymptomatic diner inside a Guangzhou, China, restaurant in January later tested positive for Covid-19, despite video that indicates the separate parties didn’t have close contact, according to an April study published on a preprint server. Some patrons who contracted the virus, the authors said, were seated as far as 15 feet away from the infected diner.

The third floor of the restaurant, where the patrons were seated, had no outdoor air supply, exhaust fans in the walls weren’t running and ventilation was mostly provided by an occasionally opened door, the study found.

Likewise, a coronavirus outbreak at a choir rehearsal in Washington state was likely exacerbated by poor ventilation, a study showed. Some 53 of 61 attendees were confirmed or strongly suspected to have Covid-19, including two who died.

Doors were closed at the church’s fellowship hall where the March rehearsal took place, the study, published on a preprint server in June, found. One of the authors said the research team believes that a furnace wasn’t likely operating for most of the rehearsal, providing no filtration or outdoor air supply through the system when it was off.

The studies demonstrate the importance of introducing outdoor air and having HVAC systems equipped with filters that can remove viral particles, according to scientists who have examined indoor outbreaks of Covid-19. Mechanical engineers recommend increasing a HVAC system’s outdoor air supply and installing a MERV13 filterif the system can handle it. That filter, they say, can trap and remove a substantial number of small and large particles before recirculating air back into a room.

Absent a well-functioning HVAC system—or if no HVAC system exists—researchers who have studied ventilation say there are relatively inexpensive strategies that schools and businesses can implement to dilute virus-laden air, such as opening windows and doors.

Installing fans in windows, so long as they aren’t positioned to directly blow air from one person to another, can also increase airflow.

Both strategies, however, may be difficult to rely on during the winter. As a result, researchers proposed using portable air purifiers with HEPA filters.

HEPA filters, which trap contaminants pulled in by purifiers before pushing clean air back out, are almost 100% efficient at capturing all airborne particles, including the very smallest sizes.