If you’re waiting for a break from the bad news about COVID, don’t hold your breath or actually do hold your breath.
Experts have understood for months that the new coronavirus, which causes COVID-19, can be transmitted by hooking a vehicle into large respiratory droplets that are expelled when someone coughs, sneezes, or speaks. But there is increasing evidence that smaller particles in the air called aerosols can also carry and spread the virus.
Methods to prevent the spread of respiratory drops are familiar: social distancing, frequent handwashing, and wearing a mask. But the methods for stopping aerosols, which are lightweight and can remain airborne for hours, are less obvious and often depend on technology. But can something like an air filter really stop an apparently unstoppable pandemic?
Let’s start with the basics. The largest respiratory droplets known to spread the coronavirus are larger fragments of saliva and mucus that a person pushes out when exhaling forcefully. If the person is infectious, these drops can be enriched with virus particles that, if they can reach their nose, mouth or eyes, can enter their body and cause infection. The drops are between 5 and 10 microns wide (a human hair is, on average, approximately 70 microns in diameter), and are heavy, as far as body secretions are concerned, so they quickly fall to the floor, or close surface , or a close facial hole. But when we talk, laugh, sing, or even breathe, we also produce smaller, lighter drops (less than 5 microns) that evaporate before gravity can do its thing, causing dry debris to remain in the air like a microscopic pen.
At the start of the pandemic, experts weren’t sure if these sprays could contain enough virus or stay long enough to infect someone. A person cannot become infected by inhaling one or two viruses: they must be exposed to a certain concentration of the virus before it can be established, although experts are not yet sure what this threshold, called infectious dose, is. . But there is now increasing evidence that, yes, aerosols carry the virus and, in a high enough concentration, can cause infection.
“It is a kind of process of elimination. You get to the point where you say, ‘It looks like some kind of aerosol transmission,’ ”said Lisa M. Brosseau, research consultant at the Center for Infectious Disease Research and Policy at the University of Minnesota. “In the midst of a pandemic, that’s what you have. You take the evidence you have, what you can observe, and you draw conclusions from that. “
When people are outside, the transmission of aerosols is less of a concern because in open spaces, these particles quickly disperse and dilute, making it difficult to accumulate an infectious concentration, Brosseau said. It is like throwing a little dye in a river. But indoors, especially if ventilation is poor, it’s much easier for aerosols to accumulate, more like putting food coloring in a birdbath.
Sometimes, though, you can’t help but take a dip in those murky waters. The good news is that there are ways to dilute and clean aerosols from interior spaces. But it requires more effort.
Opening a window can help because it forces fresh air in and some of the polluted air will go outside. “There is no doubt that, over time, on average, the concentration of aerosols will decrease if you open the windows,” said Rajat Mittal, a professor of mechanical engineering who studies aerodynamics at Johns Hopkins University.
But if, for example, you’re sitting in a classroom in Minnesota in January or Arizona in September, opening windows might not be the most practical solution.
Instead, school boards and offices should consider their heating, ventilation, and air conditioning (HVAC) systems. But you can’t just mix the air around the building, as it could spread the virus to more places. Instead, you have to replace the air itself.
“You can achieve an air change in two ways,” said David Krause, a certified industrial hygienist and owner of HealthCare Consulting and Contracting. The first is “through the sudden change of air, bringing in outside air and exhaust air from the room. Or you can do this using high-efficiency filters that effectively remove virus-containing particles from the air.”
Filters can capture virus-laden particles through various methods, including physically trapping them with a fine enough filter and using electrostatic attraction to charge particles and force them to settle in the air.
To trap the small aerosols that could be carrying the coronavirus, filters need a high enough MERV, yes, MERV. Surely you’ve heard of the minimum value of efficiency reports before, right? A filter with a MERV rating of 1, for example, will capture less than 20 percent of particles 3 to 10 microns wide. As the MERV rating increases, so does the amount of particulate it captures, along with the amount of force an HVAC system needs to push air through the fine filter.
To effectively neutralize indoor transmission of the new coronavirus, you would need a MERV of at least 13, according to the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE).
“We do not recommend major modifications to most air conditioning systems. Many of them can simply swap the efficiency of one filter for another, “said William Bahnfleth, professor of architectural engineering at Pennsylvania State University and chair of the ASHRAE epidemic task force.” I have MERV-13 filters in the air conditioner. from my house”.
But changing the filter alone might not be enough. Krause noted the guidelines from the Centers for Disease Control and Prevention, published before the COVID-19 pandemic, which describe exactly what standards buildings must have to achieve “airborne infection isolation,” which It means stopping the spread of aerosols less than 5 microns. At a minimum, buildings must achieve 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 another with an air filtration system.
“The other four and a half or five air changes per hour, you really have to rely on separate HEPA air filters in the room,” said Krause, referring to the high-efficiency particulate air filters. “We should be seeing this if we have the illusion of sending the children back to school.”
However, all that filtering can be expensive. A commercial-size MERV-13 filter can cost three to four times more than a lower-level filter, and portable air filters can cost up to $ 1,000 each, according to Krause, so the cost for offices and schools is not it is small. And the use of a filtration system creates a new risk for anyone who has to change virus-soaked filters.
Still, HVAC systems cannot guarantee 100 percent security. The idea behind all these air and filter changes is to dilute and clean the air so that aerosols do not accumulate in high concentrations. But it is impossible to guarantee that a spray is absorbed by a filter or ventilation system before a person breathes it in, especially if many people are together for long periods of time.
Ultraviolet light filters might help, too: UV light can kill microorganisms, including viruses, but the experts I spoke to raised concerns about this method. On the one hand, killing the virus in this way requires continual exposure to ultraviolet light for several minutes; if you just float by a light, like one in a vent, that might not be enough. On the other hand, UV radiation has other potential risks, including skin and eye damage.
Regardless of what the building managers choose, their efforts will be debatable if people do not take steps to prevent the spread of COVID-19 through respiratory drops by wearing masks, washing their hands frequently, and practicing social distancing. All the open windows of the world cannot help if you are swallowing virus-laden drops of saliva.
