Source: US CDC. I added the red lines.
The A-words would be aerosol and airborne. The difference being that, up to this time, the CDC had only said COVID-19 was spread by droplets. Droplets are (conventionally) larger than 5 microns, rapidly fall out of the air (so-called “ballistic trajectory”), and are the basis for our 6′ social distancing rule. By contrast, aerosols are small (under 5 microns), can hang in the air for a long time, can travel far more than 6′, and can be inhaled.
The news here isn’t that aerosol transmission matters. The news is that, as of last Friday, the CDC is (finally) explicitly saying that. And that, in turn, has a lot of implications for Federal, state, and local policies for dealing with COVID-19.
A long and winding road.
The subject of aerosol transmission is something I’ve been returning to over the past half-year. By the end of March, it was clear that the CDC’s explanation of how COVID-19 spread (via “droplets”) could not possibly be right (Post #573). That’s when I decided to wear a mask whenever I was in public.
And I was hardly the only one to figure this out. In early March, a researcher at the University of Minnesota criticized the CDC’s failure to acknowledge airborne transmission if COVID-19. (That’s still good reading, and the illustrations immediately make the key point: COMMENTARY: COVID-19 transmission messages should hinge on science. ) By April, the National Academies of Sciences came to the same conclusion. (You can see the original National Academies of Sciences summary letter at this URL.) By July, several hundred experts in related scientific fields drafted a “commentary”, summarizing the evidence. It was their attempt to up the pressure on CDC and WHO to address aerosol transmission of COVID-19 (see Post #771). That document, listed in Post #771, is still the definitive summary of the evidence.
In fact, it got to the point where it was so obvious, the question wasn’t whether aerosol transmission mattered. The only real question was why the CDC would not change its mind about it (Post #771).
And, in truth, it was clear that the CDC had already figured it out. They just wouldn’t say it. When the CDC changed its guidance on using masks (on April 3, 2020), they quietly implied that aerosol transmission was possible (Post #602). In other words, you could only make sense out of the changes they made if you understood that, beneath it all, they were talking about aerosol transmission of COVID-19.
And now, as of last Friday, the CDC has made a grudging and partial admission of the obvious. I say grudging and partial because, while their advice now contains the A-words, that really hasn’t trickled through to all of the guidance that the CDC offers. And I don’t think people quite realize the full implications of this.
In particular, I would not be surprised to see this language disappear, once those in power do realize the implications. Hence the snapshot, above.
So what’s the big deal?
I mean, if everybody already knew this, what’s the big deal with the CDC finally saying it. The fact of airborne transmission of COVID-19 appears to be common knowledge within the expert scientific community. Some CDC guidance (starting with “wear a mask”) implicitly acknowledged it. And the guidance from states for re-opening the economy was quietly based on the potential for aerosol transmission. E.g., the fact that outdoor venues (parks, outdoor dining) opened before indoor venues only makes sense if you realize that aerosol spread of COVID-19 is real. (Droplets, by contrast, fall to the ground within six feet and so are equally dangerous in outdoor and indoor settings.)
And so, what’s the big deal if CDC finally says it? The big deal is that this implies that we should be routinely taking a lot more actions to prevent spread of disease.
As it stands, various bits of guidance are implicitly or quietly based on the understanding that COVID-19 can be spread by aerosols. There’s a little bit of piecemeal acknowledgment. But that’s a far cry from designing the guidance from the ground up, with the understanding that aerosol spread of COVID-19 in the community is a significant hazard.
First, any guidance that has the phrase ” … if adequate social distancing cannot be maintained” is now obsolete. That includes some of the current guidance in Virginia. It’s obsolete because once you admit that aerosol transmission matters, then you admit that 6′ social distancing does not provide adequate protection. And so, all guidance that says you only need to wear a mask in situations where social distancing cannot be maintained needs some sort of rewrite.
Second, citizens in high-risk occupations now have a reason to demand high-quality masks capable of filtering out aerosols. E.g., N95 respirators. Particularly those who work indoors, with a lot of other people in the room, such as teachers. I’ve made this point before, and I’ll make it again. If school districts are going to re-start face-to-face learning, they ought to be burdened to provide masks capable of stopping aerosols (N95s). Staying 6′ away from a classroom of students does not provide adequate protection. And it’s just a convenient fantasy for a school system to pretend that it does. And with this change from the CDC, that’s now explicit.
Third, building ventilation systems and standards need to be re-examined for any location where crowds gather. And some energy-efficiency measures may need to be overridden for the duration of the pandemic.
The idea being that in modern energy-efficient buildings, you bring in as little fresh air as possible. Modern, tightly-sealed, energy-efficient buildings are exactly what you don’t want when a disease is spread by aerosol (airborne) transmission. (In fact, under the LEED standards, many systems for ventilating meeting spaces don’t introduce any outdoor air until until it gets “stuffy”, defined by LEED standards as 1200 PPM C02). In the COVID-19 era, by contrast, you’d want to have as much fresh air exchange as possible. You probably also want to upgrade to filtration systems capable of capturing aerosols, such as Filtrete ™ instead of standard air filters.
Fourth, you have to admit that there’s an inherent, unavoidable risk in activities such as indoor dining (including bars) and indoor aerobic exercise. Which, in fact, the new CDC guidance explicitly says. Any place where a bunch of people take off their masks, talk, eat, drink, or breathe hard — that’s a place with high risk of COVID-19 transmission. As with classrooms, you can no longer pretend that 6′ spacing alone provides mystical protection against spread of COVID-19.
(I have already published my proposed “air curtain” solution to this issue, in Post #810. I do hope people will take that seriously now).
Finally, you want to rethink the advisability of any mask-off indoor gathering, social distancing or no social distancing. Not that having a large indoor gathering in these times was smart in any case. But now with explicit acknowledgement of aerosol transmission, I would not be surprised to see states further restrict any type of large indoor gathering. You could pretend that social distancing alone was adequate, up to now. You can no longer pretend that.
The bottom line is that when the CDC says it, yeah, that’s a big deal. It’s a big deal because, in effect, all official guidance on how to deal with COVID-19 is, at root, based on what the CDC officially says. By putting the A-words down in black and white, they’ve changed how every government official needs to think about COVID-19, and ways to prevent spread of COVID-19.