In transmission, size does matter

Why is airborne transmission such a concern now when it comes to COVID-19? NCSE Director of Teacher Support Lin Andrews explains that the key to deciphering the latest science behind airborne transmission is an understanding of scale.


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Today I’m going to be talking about scale. Specifically, about the important practical difference between tiny stuff and really, really tiny stuff. (Scientists, of course, have a better way of comparing the sizes of things than tiny vs. really, really tiny: the metric system. We’ll get to that.) If your students want to understand why masks are so important, they’re going to need to think about scale. A fun way to start is through this amazing interactive that I used when I was in the classroom — when you move the scale bar at the bottom, it zooms in so you can see a cell (tiny) compared to a bacterium (really tiny) to a virus (really, really tiny).

So why is understanding scale crucial to understanding the importance of masks during the coronavirus pandemic? The take-home message of my April 2020 article “Don’t jump the gun” three months ago was that when science is moving fast, early reports need to be greeted a bit skeptically, since understanding will change over time. In that article, I mentioned masks and—as predicted—our understanding of their utility has indeed changed.

In early April, public health authorities were saying that the public need not wear masks. They didn’t want people to hoard medical-grade masks when they were desperately needed by medical professionals. Furthermore, because of how the virus was believed to spread, masks were not thought to be a very important preventative measure. However, a new concern has recently emerged that has changed the conversation completely. A large group (239 people in 32 countries) of researchers and scientists have argued that COVID-19 is very likely to be spread by airborne transmission and urged the World Health Organization to take a stance on this issue immediately for the safety of all.

Is this information new? Didn’t we already know that COVID-19 could travel in droplets through the air? Well, yes. But the key is what do we mean by droplets, and this is where scale becomes really important.

If airborne transmission is a likely mode of COVID-19 spread, then we must rethink current prevention strategies for all indoor activities.

According to the Centers for Disease Control and Prevention (CDC), the major modes of transmission for the virus are currently believed to be direct (person-to-person/fomite) contact or indirect (droplet) transmission. Person-to-person means via intimate contact (including shaking hands) or by fomites (inanimate objects and surfaces to which viral particles cling—for example, doorknobs). Indirect transmission occurs when a person coughs, sneezes, talks, or sings, and small respiratory droplets (up to roughly a millimeter across) travel short distances, possibly landing in mouths or noses of people nearby or inhaled into the lungs when walking through the unseen “spray.”

There’s the metric system, as promised! A millimeter may seem pretty small—a dime is only 1.35 millimeters thick, for example—but a millimeter-diameter droplet is heavy enough to drop out of the air quickly, so these droplets were expected to fall to surfaces within a short amount of time and only travel over very short distances. Under these circumstances, you can see why masks wouldn’t seem so critical—unless you were right next to someone coughing or sneezing, the droplet would fall from the air before you could breathe it in.

Okay, so that’s droplet transmission. What scientists are saying now is that coronavirus spreads by airborne transmission, and the key word there is “airborne”—in other words, the opposite of falling to the ground. While larger droplets rarely travel beyond the social distancing standard of six feet, scientists now believe that coronavirus can stay in the air longer and travel farther in the form of aerosols, which are smaller than five micrometers (µm), or five thousandths of a millimeter, in diameter.

Note that one millimeter is 200 times longer than five micrometers. But the difference is even more pronounced when we’re considering volumes and not just diameters. A one-millimeter sphere has a volume of 0.5 cubic millimeters but a five-micrometer sphere has a volume of 0.00000000007 cubic millimeters. You could fit about eight million five-micrometer spheres into a one-millimeter sphere. Bottom line: they’re a lot smaller.

Aerosols are released when people breathe normally or talk softly. Because they’re so small and light, these aerosols can linger in the air (one study estimated up to eight minutes and another study in a laboratory setting showed potential suspension for up to three hours). Also, aerosols can travel much farther than six feet, as one super-spreader study from a restaurant in China shows. In that instance, the virus was found to have traveled up to 20 feet with no direct contact of any kind.

So why does this new information matter? Because if airborne transmission is a likely mode of COVID-19 spread, then we must rethink current prevention strategies for all indoor activities. Social distancing, surface disinfecting, and handwashing will no longer be enough. Mask wearing will need to become our first line of defense. We can understand why the World Health Organization is being so cautious with this recommendation given its enormous repercussions. But as always we hope they will follow the science.

The scientific community has learned a lot since April 2020 about the SARS-CoV-2 virus. Still, we are far from fully understanding how this virus operates. In the meantime, err on the side of caution and put on that mask for the good of all.

NCSE Director of Teacher Support Lin Andrews
Short Bio

Lin Andrews is NCSE Director of Eduation.