Dr. Adriaan Bax on Wearing Masks While Speaking

Excerpts from our conversation with Dr. Adriaan Bax, a biophysicist at NIH in Bethesda, Maryland who began studying the effectiveness of masks during the COVID-19 pandemic.

NIHNiH: What do we know about why masks work?

Bax: Masks are most effective in protecting against virus exiting the mouth. It’s much harder to protect against incoming particles. And the reason for that is that when a respiratory droplet comes out of the mouth, it is 95 to 99% water. When that water evaporates in the atmosphere, the droplet shrinks and becomes what we call aerosol. And it will be about 20 to 100 times smaller in volume. It’s much harder to protect against that small particle once it floats around in the room, than to stop it from entering the room in the first place.

NIHNiH: Can you describe some of the experiments you’ve done to show how respiratory droplets spread?

Bax: Yes, this came about very early on during the pandemic. We realized that speech droplets were numerous, and that they had to play a role in disease transmission. That’s because without coughing or sneezing, it’s very hard to get the kind of numbers—in terms of droplets and volume—into the air needed for virus transmission without speaking.

But before seeing how these droplets spread, we first needed a way to observe them. This was done by shining a narrow bundle of very bright laser light through a pitch-dark large box that was filled with ultraclean air, to make sure there was no contamination from dust. Then, by speaking through a small opening in the side of the box we could see the speech droplets when they enter the light beam. These droplets quickly dry out and then become so light that they stay afloat in the air as “speech dust.” The speech dust becomes visible to the naked eye when it drifts into the laser beam.

Of course, because the particles are trapped inside the box, they can’t travel very far. But we can see how long they stay afloat, which is the important factor. How far they will travel in a real room depends on how long they remain afloat in the air. Air flow will carry them during that time, usually fairly rapidly. For example, when a smoker enters a room, very quickly you can smell smoke at the other end of the room. That’s because the air flow spreads the smoke particles, just like it spreads speech aerosol.

NIHNiH: So, these particles are just from speaking at a normal volume, not by shouting or singing?

Bax: Correct. It turns out that speech is a little bit more complicated than you normally might realize. There are vowel sounds that come from your vocal cords. And those make a ton of very small droplets. However, the total volume of those is rather small, even while the numbers are large and increase with loudness.

On the other hand, if you say a single “P” or a “B,” that produces a lot of droplets from your lips—anywhere between 100 and 1,000. But the number of droplets produced doesn’t depend as much on how loudly you speak, as with vowels. These droplets are relatively large before they dry out. And they consist mostly of saliva. So, the total volume of such “lip droplets” is actually much larger than for the vocal cord droplets, but they float in the air for less time. It’s typically less than ten minutes.

There was a very interesting NIH study that came out earlier this year which showed that, especially in asymptomatic people [with COVID-19], the viral content of saliva can be sky high. Suggesting that those droplets that are generated by speech—including at the front of the mouth—can be very, very infectious.

NIHNiH: So, knowing this, what would be the main message for our audience in areas with high numbers of COVID-19 cases?

Bax: Keep the mask on regardless of vaccination status.* But it’s especially important for people who are not vaccinated. And most importantly, don’t take your mask off when you’re speaking. I know it’s hard—especially for teachers and others who talk to people for a long time—to have a mask on because you have to speak louder. Masks muffle the voice. So, I use a voice amplifier. It was $30 online. Whenever I have longer discussions with people face to face, I use my voice amplifier. Even if I whisper, they can still hear me.

Be careful with restaurants and bars. People often go there to chat and catch up with loved ones—to talk, to socialize. They take their masks off because they’re eating or drinking, while also talking loudly to their friends in a noisy background. That speaking without masks on is how we’re going to keep on transmitting the virus.

*Editor's Note: Our question's phrasing was updated following publication for clarification. This response was based on CDC guidance at the time the article was written. To stay up-to-date with the latest guidelines for when to wear a mask, visit the CDC webpage.