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Experts say improvements to HVAC systems and better air filters can help slow the spread of the novel coronavirus indoors. Getty Images
  • Researchers have pinpointed how the novel coronavirus spreads through indoor places such as schools and stores.
  • They say revamping HVAC systems can help slow the spread.
  • Less expensive solutions include opening windows and installing better air filters.
  • Experts advise people to wear facial coverings regardless of how efficient a building’s air circulation system is.

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The World Health Organization has now officially recognized that the novel coronavirus can be transmitted through the air via tiny aerosol particles emitted when we talk, cough, or breathe.

This proclamation has led to more questions about airborne transmission of the virus that causes COVID-19.

Chief among them is how to stop that airborne transmission, particularly indoors.

This will be an even more pressing question if state and local governments go through with plans to reopen schools this fall.

A key part of reducing that indoor spread of the virus could be air circulation. That includes using windows as well as the circulation systems already installed in buildings.

However, a new study concludes that those systems may have a limited ability to remove the virus from the air of rooms and buildings.

The study’s researchers also emphasize that how those systems are set up is a key factor.

There are some measures that experts say may help — without costing too much time or money. These include increasing the amount of fresh air indoors, upgrading filters, and properly wearing masks.

In the new study, researchers measured how the virus travels through the air indoors by studying the movement of aerosols from eight people with COVID-19 who were asymptomatic — and how ventilation systems and spacing affected that flow.

The researchers found that good ventilation filtered out some of the airborne virus, but the majority of virus-carrying particles were left behind on surfaces.

In a classroom, ventilation filtered out just 10 percent of the aerosols emitted by an asymptomatic teacher talking for 50 minutes.

Part of the issue was that the ventilation formed vortexes where the aerosols were caught in a spinning cycle. This made it difficult for those aerosols to reach the vent to exit the room.

Reducing these “hot spots” where virus-carrying aerosols accumulate requires thinking about how the ventilation is set up, the researchers said.

“What we found from the classroom simulation is actually the placement of the ventilation is very important,” Jiarong Hong, PhD, MS, an associate professor of mechanical engineering at the University of Minnesota and one of the authors of the new study, told Healthline.

“The potential hot spots are where air is coming in or out,” he added.

In general, though, the more ventilation, the better.

That could be by opening windows or, Hong said, adding vents in ceiling panels.

“To reduce risk, you have to increase the ventilation sites whenever possible,” he noted.

Hong said an air filtration system would ideally be placed as close as possible to the person who is producing the most aerosols — in a classroom, typically the instructor.

If it’s possible to have additional air filtration units they could be placed in the middle of tables, he suggested.

How possible it will be for a school or business to add vents or filtration units — or even open windows — is an open question.

Many schools were struggling with tight budgets and funding cuts before the pandemic began.

In May, superintendents across the country warned of “significant revenue shortfalls” and “dark clouds… on the educational horizon” due to current revenue decreases and expected future declines as cities and states struggle with their own revenue shortfalls due to the disruption of economic activity.

The story probably isn’t much different for many businesses. Even if they were doing fine pre-pandemic, it seems unlikely many would have much budget capability for ventilation or filtration upgrades at this point.

“Schools will need to figure out what’s possible,” Rajat Mittal, PhD, MS, a professor of mechanical engineering at Johns Hopkins University in Maryland, told Healthline.

Mittal, who wasn’t involved in the new study, has researched how the novel coronavirus moves indoors. Some of his findings were presented in a July paper on the physics of how the virus moves through the air.

“It’s a very complicated thing. Ventilation is not something that can be changed that easily in an existing building,” he said.

Current systems, he noted, are largely designed for comfort and temperature control, not to stop the spread of viruses.

“The chances we can go back and completely redo HVAC systems is close to zero. … That’s kind of the reality of the situation,” Mittal said.

There are a few things that can be done with a minimal amount of effort and cost.

One option is providing and increasing the amount of fresh air.

The simplest way to replace indoor air with fresh air is, of course, opening windows.

But that’s not always possible. Maybe you’re in Dallas in August, or Minneapolis in January, or just in a big school building where not all classrooms have windows.

There are other ways to get fresh air though. Think of the button on your car’s air-conditioning/heating panel that controls whether it pulls in outside air or recirculates the air in the car.

Building AC systems recirculate air, Mittal said. The air is pulled through a filter and then put back into the room or building, moving it back and forth but not really replacing that air with fresh air.

However, Mittal said doing so “would, on average, help the situation.”

“There’s at least a possibility of doing that. There are ways [it] can be done,” Mittal said. “It might require some hardware changes, but it’s easier than changing ducts in every room.”

In terms of air systems, though, it would also increase costs, he noted. Cooling down already-cooled, recirculating air is cheaper than cooling fresh, hot air from outside.

A second option is just to replace current air filters with better ones, although research is still ongoing to determine how effective current filter options are against the virus.

An overview of potential coronavirus-related building upgrades released by McKinsey last month noted that most homes and commercial buildings have HVAC systems that use a type of filter that can catch particles up to 1 to 3 microns in size.

The COVID-19 particle is about 0.1 microns, although the aerosols that carry them can range in size, possibly from larger than 2 microns to smaller than 1 micron.

The McKinsey paper notes that HEPA filters can catch particles 0.3 microns in size, making them potentially the most effective against viruses.

But, the research paper notes, “only some air conditioners can accommodate HEPA filters, and technicians must configure them properly and replace them regularly. Upgrading HVAC systems by incorporating higher-grade filters can be very expensive and is not always feasible.”

Moreover, “even a HEPA filter will not eliminate all concerns about airborne transmission. Although a [2016] NASA study documented that HEPA filters can stop particles as small as 0.1 microns — the approximate size of the coronavirus — other direct research is limited, and the official U.S. ratings system specifies their efficacy only for particles of 0.3 microns.”

Then there’s masks.

Studies have shown facial coverings are effective at reducing how many aerosols the wearer spreads into the air.

But researchers are still looking into just how much they might reduce the risk of inhaling the aerosols that are emitted.

Linsey Marr, PhD, a Virginia Tech University civil and environmental engineering professor, wrote in a New York Times op-ed that her lab has found a loosely tied bandana blocked at least half of aerosols larger than 2 microns.

“We also found that especially with very small aerosols — smaller than 1 micron — it is more effective to use a softer fabric (which is easier to fit tightly over the face) than a stiffer fabric (which, even if it is a better filter, tends to sit more awkwardly, creating gaps),” Mann wrote.

She also noted a few other measures, including opening windows and doors, adjusting and upgrading filters in HVAC systems, adding portable air cleaners, or installing germicidal ultraviolet technologies.

But, for masks, fit and proper use appears to be the key variable.

“It’s going to be effective, but how effective it is depends on how rigorous the individual is” in wearing it and wearing it properly, said Hong.