Updated: Aug 25, 2020
If you’re waiting for a break from bad news about COVID, don’t hold your breath — or, actually, do hold your breath.
Experts have understood for months that the novel coronavirus, which causes COVID-19, can be spread by hitching a ride inside large respiratory droplets that are expelled when someone coughs, sneezes or talks. But there’s growing evidence that smaller airborne particles called aerosols can carry and spread the virus, too.
The methods for preventing spread from respiratory droplets are familiar: social distancing, frequent hand-washing and wearing a mask. But the methods for stopping aerosols, which are lightweight and may be able to hang in the air for hours, are less obvious and often rely on technology. But can something like an air filter really stop a seemingly unstoppable pandemic?
Let’s start with the basics. The larger respiratory droplets that are known to spread the coronavirus are larger bits of spit and mucus that a person propels outward when they forcefully exhale. If the person is infectious, these droplets can be spiked with virus particles which, if they can find their way to your nose, mouth or eyes, can enter your body and cause infection. The droplets are between 5 and 10 microns across (a human hair is, on average, roughly 70 microns in diameter), and they’re heavy, as far as bodily secretions go, so they quickly fall to the floor — or nearby surface, or nearby facial orifice. But when we talk or laugh or sing or even just breathe, we also produce smaller, lighter droplets (less than 5 microns) that evaporate before gravity can do its thing, causing the dried remnants to stay aloft like a microscopic feather.
At the start of the pandemic, experts weren’t sure if these aerosols could contain enough virus or hang around long enough to actually infect anybody. A person can’t be infected by inhaling one or two viruses — they need to be exposed to a certain concentration of the virus before it can gain a foothold, though experts still aren’t sure what this threshold, called the infectious dose, is. But now there’s growing evidence that, yes, the aerosols carry the virus and, in a high enough concentration, can cause infection.
“It’s sort of a process of elimination. You get to the point where you say, ‘It sort of looks like aerosol transmission,’” said Lisa M. Brosseau, a research consultant at the University of Minnesota’s Center for Infectious Disease Research and Policy. “In the midst of a pandemic, that’s what you have. You take what evidence you have, what you can observe, and you draw conclusions from that.”
When people are outside, aerosol transmission is less of a concern because in wide-open spaces, these particles are quickly dispersed and diluted, making it difficult for an infectious concentration to accumulate, Brosseau said. It’s like dropping some food coloring into a river. But indoors, especially if the ventilation is poor, it’s much easier for aerosols to accumulate — more like dropping food coloring into a birdbath.
Sometimes, though, you can’t avoid taking a dip in those murky waters. The good news is there are ways to dilute and clear out aerosols from indoor spaces. But it requires more effort.
Opening a window can help because it forces fresh air in, and some of the contaminated air will make its way outside. “There is no doubt that, over time, on average, the concentration of aerosols is going to go down if you open up the windows,” said Rajat Mittal, a mechanical engineering professor who studies aerodynamics at Johns Hopkins University.
But if you’re, say, sitting in a classroom in Minnesota in January, or Arizona in September, opening up the windows might not be the most practical solution.
Instead, school boards and offices should look to their heating, ventilation and air conditioning (HVAC) systems. But you can’t just shuffle air around the building — that could spread the virus to even more places. Instead, you have to replace the air itself.
“You can achieve an air change by one of two ways,” said David Krause, a certified industrial hygienist and the owner of HealthCare Consulting and Contracting. The first is “through the gross changeout of air, bringing in outside air and exhausting air from the room. Or you can achieve it by using high-efficiency filters that effectively remove virus-containing particles from the air.”
Filters can capture virus-laden particles through a number of methods, including physically trapping them with a fine enough filter and using electrostatic attraction to charge particles and force them to settle out of the air.
To catch the tiny aerosols that might be carrying the coronavirus, filters need a high enough MERV — yes, MERV. Surely you’ve heard of the minimum efficiency reporting value before, no? A filter with a MERV rating of 1, for example, will capture less than 20 percent of particles that are 3 to 10 microns across. As the MERV rating increases, so does the number of particles it captures, along with the amount of force an HVAC system needs to push the air through the fine filter.
To effectively neutralize indoor transmission of the novel coronavirus, you’d need a MERV of at least 13, according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). “We’re not recommending major modifications to most air conditioning systems. Many of them can simply change out one efficiency of filter for another,” said William Bahnfleth, a professor of architectural engineering at Pennsylvania State University and the chair of ASHRAE’s epidemic task force. “I have MERV-13 filters in my home air conditioner.”
But changing out the filter alone might not be enough. Krause pointed to guidelines from the Centers for Disease Control and Prevention, which were published before the COVID-19 pandemic, that outline exactly what standards buildings need to have to achieve “airborne infection isolation,” which means stopping the spread of aerosols smaller than 5 microns. At a minimum, buildings need to be reaching six air changes per hour, according to these CDC guidelines. Krause said the average commercial building now only performs one or two air changes per hour, and could squeeze in another with an air filtration system.
“The other four and a half or five air changes per hour, you’re really going to have to rely on in-room, standalone, HEPA filter air cleaners,” Krause said, referring to high efficiency particulate air filters. “We should be looking at these if we have any illusions of sending kids back to school.”
All that filtration can be expensive, though. A commercial-sized MERV-13 filter can cost three to four times more than a lower-standard filter, and portable air cleaners can be up to $1,000 each, according to Krause, so the cost for offices and schools is not small. And using a filtration system creates a new risk for whoever has to change out the virus-soaked filters.
Still, HVAC systems can’t guarantee 100 percent safety. The idea behind all these air changes and filters is to dilute and clean the air so that aerosols can’t build up into high concentrations. But it’s impossible to guarantee that an aerosol will get sucked up by a filter or a ventilation system before it gets sucked up by a person, especially if a lot of people are close together for long periods of time.
Ultraviolet light filters could also help: UV light can kill microorganisms, including viruses, but the experts I spoke to raised concerns about this method. For one, killing the virus this way requires continually exposing it to UV light for several minutes — if it simply floats by a light, such as one in a vent, that might not be enough. For another, UV has other potential risks, including skin and eye damage.
Regardless of what building managers choose to do, their efforts will be moot if people aren’t also taking measures to prevent the spread of COVID-19 through respiratory droplets by wearing masks, washing their hands frequently and practicing social distancing. All the open windows in the world can’t help if you’re swallowing globs of virus-laden spit.
PHOTO ILLUSTRATION BY FIVETHIRTYEIGHT / GETTY IMAGES