Source: COMMENTARY: COVID-19 transmission messages should hinge on science. March 16, 2020, Lisa Brosseau, ScD, author, on line at the University of Minnesota Center for Infectious Disease Research and Policy. Used without permission here, under a claim of fair use.
So, today’s headlines include:
‘Speaking causes airborne virus transmission’:
Source: Marketwatch
A minute of loud talking can generate more than 1,000 coronavirus-laden droplets that linger in the air
Source: Businessinsider
Loud talking can leave coronavirus in air for up to 14 minutes
Source: Yes, even Fox “news” gets it.
Source: COMMENTARY: COVID-19 transmission messages should hinge on science. March 16, 2020, Lisa Brosseau, ScD, author, on line at the University of Minnesota Center for Infectious Disease Research and Policy. Used without permission here, under a claim of fair use.
God bless the National Academy of Sciences. Created by President Lincoln, they have offered advice on scientific, engineering and technical matters to the US government since that time. They are a little untainted voice of sanity in this pandemic.
Today, research in the Proceedings of the National Academy of Sciences made a strong case that COVID-19 is being spread by aerosol transmission from normal speech. That is, spread by tiny droplets that remain suspended in air, emitted simply by talking (as opposed to coughing or sneezing). They conclude that:
These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.
What’s interesting to me is that there is very little new information in this research. To a large degree, this research merely confirms research from more than a year ago showing that:
- Speaking generates aerosol droplets (small enough to float in the air rather than fall to the ground);
- Talking loudly generates far more droplets than talking softly.
- Some people (super-emitters) generate vastly more droplets than others.
I would say that there were only two new bits of information here. One is that ventilation matters — the aerosol concentration was measured as if it were in stagnant indoor air. That’s pretty obvious, I think, and argues for the safety of outdoor as opposed to indoor venues. Which people have already figured out, as many state re-opening plans call for the resumption of outdoor dining first, along with opening parks, golf courses, and other outdoor venues.
The other is the likely viral load of a single “aerosol-sized” droplet. Their calculation is that each tiny (4 micron) droplet has about a one-third chance of carrying a virus particle, from a typical asymptomatic-but-infectious carrier. Normal speech emits such droplets at the rate of about 1000 per minute, on average. And they note that all that matters for probability of infection is the number of virus copies that you inhale. Doesn’t matter a bit if they come from separate droplets.
This phenomenon of aerosol transmission is why smart countries like German have banned singing in church. Singing generates as many aerosol droplets as coughing. This is an issue which mainstream US churches appear to be fully aware of (Post #682), though what they will do about it remains uncertain.
One bit if information that I should pass along, not contained in that analysis, is that you don’t have to worry (much) about those tiny floating “aerosol” droplets landing on your skin or clothing. That’s because they are so tiny, you push them out of the way as you move through the air. Really, the only risk is inhaling them.
People who have been paying attention have been aware of this for months. I was hardly the first, but you can read Post #573 (March 26, 2020) to see when this became obvious enough that I shaved my beard and started wearing a mask in any indoor public space. Followed by Post #585 (March 30, 2020) to fill in some of the details. And post #671 (4/27/2020) when I first saw some mainstream acceptance of aerosol transmission of disease.
But the US CDC continues to repeat what it began saying months ago. The CDC still says that the main mode of transmission is “droplets” from coughs or sneezes of people who have symptoms. Those larger “droplet” particles rapidly fall out of the air, and those are the basis for the 6′ social distancing norm. But it was obvious that that “droplet transmission by symptomatic individuals” was eliminated as a mode of transmission months ago. Nobody was dumb enough to be out in public when sick, and social norms strongly enforced that (Post #601, 4/4/2020).
(Honestly, I think that’s an artifact of the CDC’s contact tracing methodology. They can easily find spread within a household, and that’s plausibly attributable to droplet spread. But cases outside the household, spread by asymptomatic individuals — it’s not clear that the CDC can even find those, via contact tracing. These then get chalked up to “community spread” of disease (i.e., no known origin), and the CDC doesn’t even bother to track the count of such cases separately.)
The interesting thing to watch here will be the CDC. Will they change their advice, once again, and fully admit that aerosol transmission from speech is real? Because it makes a huge difference if the CDC will say that. At that point, every state has the right (or maybe obligation) to take steps to halt transmission from that mechanism.
Given the asymmetric risk here, I’m astonished that the CDC hasn’t done this already. Consider what the penalties are for being wrong, in either direction. If there is no aerosol transmission of disease, but CDC says there is, people will unnecessarily wear masks and increase ventilation in indoor spaces. A minor inconvenience. By contrast, if there is aerosol transmission from speech, and CDC fails to say that, we will continue to see indoor spread of disease from individuals who think that they don’t need to wear masks, because they (e.g.) are maintaining proper social distancing at work.
And that’s just plain stupid, given the large amount of evidence now suggesting that aerosol transmission is likely.
At the least, if they’ll accept the likelihood that this is occurring, then as I said six weeks ago, there’s something they could do right now:
There is a simple piece of public-health advice that the CDC could offer. Refer back to Post #573: Please shut up in public.
Your takeaways.
Wear a mask any time you are in an indoor space, other than your home. You might also want to wear one in any crowded outdoor situation.
Above all: Wear a mask at work. My strong suspicion is that most of the transmission we are seeing, outside the home, is settings where people talk to one another in close proximity in the work space. That’s based on indirect evidence, among other things, that the peak incidence of disease is NOT among the vulnerable retiree population, but among the middle-aged working population. (Plus, just look at what happened in the White House.)
Source: Virginia Department of Health.
But, mostly, you should start looking for a mask that will fit tightly and has some ability to filter aerosol-sized (five micron or smaller) droplets. Most masks that I see being worn are the type that will protect other people from you (by stopping larger droplets, and by slowing the stream of air you emit and reduce spread of aerosol droplets). But they don’t to much to protect you from other people’s aerosol emissions. Mainly, the masks I see around town tend to be “surgical masks” (unrated single-use masks), or cloth masks, both of which tend to leak air at the edges, and neither of which has much ability to filter aerosol-sized (five micron or smaller) droplets.
The need for masks that would filter aerosol-sized drops is why, some weeks ago, I offered “mask liners” capable of filtering aerosol-sized droplets. I made these from a combination of Filtrete 2500 filter material, and other non-woven (plastic fiber) cloth. (See Post #593 for an extended discussion of filtration and filtration standards.) But I got no takers, so I withdrew the offer, and have only made a few improved versions of these masks, for friends, since.
People either didn’t realize they needed them, didn’t think mine would work, or something. What I’m pretty sure of is that most people don’t already have them, based on the masks I see being worn around town. And if you pay attention to what just got published by the National Academies of Science, if you think you need to wear a mask, then, logically, you think you need to wear a mask that can filter out small (5-micron) droplets.
Wear the best mask you own. I guess that’s the bottom-line advice.
