The danger of COVID aerosol transmission and why masks are key to protection

Friday was a record for new COVID-19 infections in LA County, with just under 3200. Statewide, Saturday marked the fifteenth straight day of record hospitalization rates.

Now more than 200 scientists are demanding the World Health Organization revise its recommendations on COVID-19. In an open letter, they specifically address the issue of airborne transmission of the virus. They say the WHO is downplaying the danger that poses.

KCRW speaks with one of the scientists who signed the letter. Kimberly Prather is a professor of atmospheric chemistry at UC San Diego.

KCRW: What's wrong with the WHO’s current guidance?

Kimberly Prather“Right now it's focused on transmission through contact and droplets. It's geared towards other forms of transmission of viruses in the past. ... So people normally get sick,cough and sneeze, and they release large droplets. And those droplets can fall to the ground within six feet. That's what we're sort of hearing over and over. 

However, it's not really geared towards aerosols. When you when you just speak, and especially if you're someone that's infected and doesn't know it, you produce these … much tinier invisible droplets that basically can float in the air and travel much further than six feet, and can be inhaled. And that is not being really addressed by the WHO at this time.”

How far can aerosols travel?

“Basically what they can do if you're in an indoor space, a crowded space, it can build up and travel all the way around the room. … One of the really challenging things with this virus is there's [sic] a lot of people that have it that are infected and don't know it. And so they're speaking or yelling in a crowded bar, they can be releasing thousands of aerosols that are just accumulating in the room over time, and they get in the air. You don't even have to be right next to that person.”

If you're wearing a mask, then that protects you from aerosols?

“It protects you two ways. … The biggest reason to wear a mask is to block what's coming out of those that don't know they're sick. ... That's the main reason. But mask filters in both ways. And so it also will protect you. … If two people wear a mask that's not a perfect mask, it filters 50%, that’s 75% it's cutting back. You can think of it as sort of cutting the dose back by some 75%, which is significant, which can get it down to levels where people don't get infected.” 

Why has the WHO not said this, and not embraced mask wearing wholesale, and is not promoting this science?

“This is sort of conjecture on our part. We're all scratching our heads. … But it's evolved, right? Early on, if you recall, when the virus first was out there, there was a shortage of masks. And so we were told masks don't work. And basically that was an effort to pull back masks and make sure that the health care workers and first-line responders had masks. 

Now there's [sic] masks, and people can make masks, and there's an abundance of masks. And so the reason now, I'm being told, is there is concerned by the WHO in particular (because CDC has actually kind of come around) … they’re concerned, ‘What about countries that are more economically challenged? Those people can't afford or don't have access to masks. So if you impose a control that not everybody can access, that can be problematic.’ ... That's what I'm told is their thinking. 

My thinking and the scientists’ thinking is everyone has access to a t-shirt, everyone has access to things that they could put over their face that would protect them more. … It's really important that people are aware of this as another possible pathway to cut back the spread.”

The fact that a group of scientists have published this open letter to the WHO, what does that say?

“It says that we're hopeful that by speaking up, that we can actually get us out, get us unstuck, and acknowledge this route of transmission to protect the public. 

… As scientists, it's very frustrating to hear people saying, ‘Oh masks hold us back.’ Actually, masks are the key. And so we decided we needed to speak up, to be able to hopefully just get recognition that this is a very fixable problem. We can all go back and function in our jobs and in society if we just take the proper precautions. It will stop being spread.”

It’s confusing for the general public when health organization leaders are not adhering to or promoting the latest science, or there are conflicting recommendations from various groups. There's not one single message. What is the general public supposed to take away from this, and who are they supposed to believe?

“I think you should believe the scientists. … We have skin in the game, but we don't have any motivation. We just have facts. And everyone's entitled to their own opinions, but not everybody's entitled to their own facts. 

We totally get it. And I spent a huge amount of time speaking to the public and trying to get the right word out there. I wrote a paper about this to try and just sort of take away the mystery. It’s not a mystery. It's unfortunate that the public is confused right now. 

… I would just hope that by us speaking out, people will … start listening. But again, this will work. … We have evidence that it works in places like Taiwan. Taiwan never shut down. They have more people in New York. They never shut down. They just wore masks. They did simple measures, and they had seven total deaths. And so we really believe that this will flip things back around. It’s proven in other places. The U.S. just needs to listen.”

—Written by Amy Ta, produced by Nihar Patel



  • Kimberly Prather - professor of atmospheric chemistry at UC San Diego, and director of the National Science Foundation’s Center for Aerosol Impacts on Chemistry of the Environment