An aerosol expert says people can be infected with the novel coronavirus disease or Covid-19 through inhalation as the virus can stay aloft for hours in tiny droplets in stagnant air inside buses, airplanes, or trains like the one in the photo. (Photo by Benjie Cuaresma)
By Tracy Cabrera
ACCORDING to a Virginia Tech aerosol expert, people can be infected with the novel coronavirus disease or Covid-19 through inhalation as virus can stay aloft for hours in tiny droplets in stagnant air.
With the help of some 200 other experts, Linsey Marr outlined the mounting scientific evidence showing the threat of airborne coronavirus that is highest in crowded indoor spaces with poor ventilation. This helps explain super spreading events reported in meatpacking plants, churches and restaurants.
It’s unclear, however, how often the virus is spread via these tiny droplets, or aerosols, compared with larger droplets that are expelled when a sick person coughs or sneezes, or transmitted through contact with contaminated surfaces, she added.
Aerosols are released even when a person without symptoms exhales, talks or sings.
What is clear, Marr said, is that people should consider minimizing time indoors with people outside their families. Schools, nursing homes and businesses should consider adding powerful new air filters and ultraviolet lights that can kill airborne viruses.
For a virus to be airborne means that it can be carried through the air in a viable form. For most pathogens, this is a yes-no scenario. The human immune-deficiency virus (HIV), too delicate to survive outside the body, is not airborne. Measles is airborne, and dangerously so: It can survive in the air for up to two hours.
For the coronavirus, the definition has been more complicated. Experts agree that the virus does not travel long distances or remain viable outdoors. But evidence suggests it can traverse the length of a room and, in one set of experimental conditions, remain viable for perhaps three hours.
Since aerosols are considered also as droplets and vice versa, both are only differentiated by size. Scientists refer to droplets fewer than 5 microns in diameter as aerosols. (By comparison, a red blood cell is about 5 microns in diameter; a human hair is about 50 microns wide.)
From the start of the pandemic, the World Health Organization and other public health organizations have focused on the virus’s ability to spread through large droplets that are expelled when a symptomatic person coughs or sneezes.
These droplets are heavy, relatively speaking, and fall quickly to the floor or onto a surface that others might touch. This is why public health agencies have recommended maintaining a distance of at least 6 feet from others, and frequent hand washing.
But some experts have said for months that infected people also are releasing aerosols when they cough and sneeze. More important, they expel aerosols even when they breathe, talk or sing, especially with some exertion.
Scientists know now that people can spread the virus even in the absence of symptoms—without coughing or sneezing—and aerosols might explain that phenomenon.
Because aerosols are smaller, they contain much less virus than droplets do. But because they are lighter, they can linger in the air for hours, especially in the absence of fresh air. In a crowded indoor space, a single infected person can release enough aerosolized virus over time to infect many people, perhaps seeding a super spreader event.
For droplets to be responsible for that kind of spread, a single person would have to be within a few feet of all the other people, or to have contaminated an object that everyone else touched. All that seems unlikely to many experts.
In this case, physical distancing is still very important. The closer you are to an infected person, the more aerosols and droplets you may be exposed to. Washing your hands often is still a good idea.
What’s new is that those two things may not be enough.
Health care workers may all need to wear N95 masks, which filter out most aerosols. At the moment, they are advised to do so only when engaged in certain medical procedures that are thought to produce aerosols.
For the rest of us, cloth face masks will still greatly reduce risk, as long as most people wear them. At home, when you’re with your own family or with roommates you know to be careful, masks are still not necessary. But it is a good idea to wear them in other indoor spaces, experts said.
As for how long is safe, that is frustratingly tough to answer. A lot depends on whether the room is too crowded to allow for a safe distance from others and whether there is fresh air circulating through the room.
What are some things I can do to minimize the risks?
Do as much as you can outdoors. Despite the many photos of people at beaches, even a somewhat crowded beach, especially on a breezy day, is likely to be safer than a pub or an indoor restaurant with recycled air.
But even outdoors, wear a mask if you are likely to be close to others for an extended period.
When indoors, one simple thing people can do is to “open their windows and doors whenever possible,” Marr said. You can also upgrade the filters in your home air-conditioning systems, or adjust the settings to use more outdoor air rather than re-circulated air.
Public buildings and businesses may want to invest in air purifiers and ultraviolet lights that can kill the virus. Despite their reputation, elevators may not be a big risk, Milton said, compared with public bathrooms or offices with stagnant air where you may spend a long time.
If none of those things are possible, try to minimize the time you spend in an indoor space, especially without a mask. The longer you spend inside, the greater the dose of virus you might inhale.