Research shows coronavirus spreads primarily through air. Here’s how to reduce risk

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• William Brangham:

  For more on what we are learning about just how this novel coronavirus gets into our bodies, I'm joined now by Dr. Linsey Marr. She's a professor of civil and environmental engineering at Virginia Tech, and her expertise is in the airborne transmission of viruses.

  Linsey Marr, very nice to have you on the "NewsHour."

  Could you help us initially with this a distinction over terminology? People have been hearing about aerosols vs. droplets. What do those things mean, and why do we really care about them?

• Linsey Marr:

  Droplets and aerosols are really kind of opposite ends of a spectrum. They really refer to the same thing, which is a droplet of liquid that comes out of your mouth. And when it's really large and we can see it, we call it a droplet. It flies through the air like a mini-cannonball and can land on someone who's close to you, in their eyes, nose or mouth.

  Aerosols are just microscopic droplets. And they come out of your mouth or nose. And they're small enough that they can remain floating in the air for quite a while.

  And those, rather than flying through the air like mini-cannonballs, kind of float around like cigarette smoke. And, just like that, you can breathe them in.

• William Brangham:

  So, people have been hearing a lot of different kinds of advice.

  Given what we're learning now, how should we be thinking about this virus, as it circulates in the air?

• Linsey Marr:

  We can think about it like cigarette smoke.

  So, the distancing really keeps you out of range of those big droplets that fly through the air, and then will fall to the ground quickly. They don't really fly more than six feet or so.

  But, at the same time, for every one of those that you see, there's hundreds or thousands of the microscopic ones, the ones we call aerosols, and those can stay in the air, again, like cigarette smoke.

  So, imagine you're interacting with a smoker. You want to stay as far away as possible from them, really, to avoid breathing in the smoke. The smoke doesn't stop at six feet. So, six feet is a guideline, and it keeps you kind of farther away from the most concentrated part of the smoke, the plume, but it doesn't guarantee that you're not going to be exposed.

• William Brangham:

  And I guess, too, that's a useful way to think about it if you're traveling in a car or inside a large room or sitting outside with someone.

  If I'm directly next to you and you're having a cigarette, I'm going to be sharing in a lot of that. But if you're sitting at a farther distance, and it's breezy, that might lessen my exposure.

• Linsey Marr:

  Exactly.

  The cigarette smoke is a great way to think about different specific scenarios, whether you're indoors. Do you want to be indoors with that smoker? Well, you think about what affects that is the ventilation in the room. If the room is really well-ventilated and there's lots of outdoor air coming in, then that smoke will be kind of pushed outside.

  But if the ventilation is poor, that smoke can build up, just like the virus can also build up in the air. And exactly as you suggested, if you're outdoors, you're really close to someone, and you're right downwind of them, and there's not much wind, you could end up breathing in a lot of that smoke.

  But if you kind of move a little bit, and it's really windy out, you would greatly reduce your exposure to smoke.

• William Brangham:

  As we know, the CDC has been waffling back and forth about exposure, aerosols, droplets.

  For the layperson, does this going back and forth about their guidance have a real, tangible world effect?

• Linsey Marr:

  The only thing it affects is the guidance on ventilation.

  So, whether it's droplets or aerosols, we still want to wear masks, we still want to maintain distance, because masks and distance help reduce our exposure to both the droplets and aerosols.

  Ventilation is the one thing, though, that really only matters if aerosols are important, because the large droplets are — just fall out close to you, whether you have good ventilation or not.

  But the smaller things, the aerosols, that's where ventilation really matters.

• William Brangham:

  Help me understand something else.

  We have seen President Trump has been recently holding some of these indoor rallies, very crowded, largely maskless. And he's been criticized for that. He and his supporters point to, well, what about all of these protests we saw, which were much bigger crowds, sometimes people wearing masks, but they're also yelling and crowded together.

  How do we know which of those things are risky? How do we measure the risk of those relative events?

• Linsey Marr:

  There are two important differences between an indoor rally and the outdoor protests.

  The indoor rally, first of all, was indoors, where we know that virus can accumulate or build up in the air. There's less opportunity for dilution, compared to being outdoors, for example, with the protests.

  The other important difference is that, at the rallies, not many people are wearing masks. And so, if someone there happens to be infected, then they are releasing a lot of virus into the air, which can float around, and other people can build in.

  With the protests, almost everyone was wearing a mask, and so the mask prevents that virus from getting into the air. It also helps protect the wearer and reduces the amount to a virus they might breathe in from the air around them.

  A third difference, actually, is that at the rally, people are standing close together, and they're in the same place for a long time. I think with the protests, in many cases, people are moving around. This creates more space between them. There's more airflow. So that's another factor that would reduce the risk in the outdoor masked event.

• William Brangham:

  All right, Dr. Linsey Marr of Virginia Tech, thank you so much for your help.

• Linsey Marr:

  Thank you very much.