The day it became about the air

So, in my morning scan of the news, etc., I was excited to see an Op-Ed by none other than aerosologist supreme, Linsey Marr in the NYT.  Great stuff, of course:

This confirms the results of a study from late May (not peer-reviewed) in which Covid-19 patients were found to release SARS-CoV-2 simply by exhaling — without coughing or even talking. The authors of that study said the finding implied that airborne transmission “plays a major role” in spreading the virus.

Accepting these conclusions wouldn’t much change what is currently being recommended as best behavior. The strongest protection against SARS-CoV-2, whether the virus is mostly contained in droplets or in aerosols, essentially remains the same: Keep your distance and wear masks.

Rather, the recent findings are an important reminder to also be vigilant about opening windows and improving airflow indoors. And they are further evidence that the quality of masks and their fit matter, too…

Here is another common misconception: To the (limited) extent that the role of aerosols had been recognized so far, they were usually mentioned as lingering in the air, suspended, and wafting away — a long-distance threat.

So what does this all mean exactly, practically?

Can you walk into an empty room and contract the virus if an infected person, now gone, was there before you? Perhaps, but probably only if the room is small and stuffy.

Can the virus waft up and down buildings via air ducts or pipes? Maybe, though that hasn’t been established.

More likely, the research suggests, aerosols matter in extremely mundane scenarios…

I agree that long-range transmission by aerosols probably is not significant, but I believe that, taken together, much of the evidence gathered to date suggests that close-range transmission by aerosols is significant — possibly very significant, and certainly more significant than direct droplet spray.

The practical implications are plain:

  • Social distancing really is important. It keeps us out of the most concentrated parts of other people’s respiratory plumes. So stay away from one another by one or two meters at least — though farther is safer.

  • Avoid crowds. The more people around you, the more likely someone among them will be infected. Especially avoid crowds indoors, where aerosols can accumulate.

  • Ventilation counts. Open windows and doors. Adjust dampers in air-conditioning and heating systems. Upgrade the filters in those systems. Add portable air cleaners, or install germicidal ultraviolet technologies to remove or kill virus particles in the air.

It’s not clear just how much this coronavirus is transmitted by aerosols as opposed to droplets or via contact with contaminated surfaces. Then again, we still don’t know the answer to that question even for the flu, which has been studied for decades.

But by now we do know this much: Aerosols matter in the transmission of Covid-19 — and probably even more so than we have yet been able to prove.

So, the good news is this doesn’t really change the basic 3C’s guidance that many have been preaching for a long time now.  Avoid other people indoors without a mask as much as you can.  But, we can add to this, definitely pay attention to the quality of ventilation in any indoor space that contains more than a handful of people for more than a handful of minutes.

As soon as I finishted reading that this morning, lo and behold, Zeynep Tufekci had posted an even more thorough piece covering much of the same ground in the Atlantic (unlike Marr, not limited by the NYT’s Op-Ed word counts, she was able to really cover a lot of stuff, and rely on Marr extensively, as well as other experts).  But, the end result is the same…pay attention to ventilation, damnit

Under an aerosol regime, we would have different rules for the indoors and the outdoors (especially since, in addition to the diluting power of air, sunlight quickly deactivates viruses.) We would mandate masks indoors regardless of distancing, but not necessarily outdoors. Marr told me that she wears her mask outdoors only if she’s interacting with people, if she’s in a crowd, or if she cannot maintain distance. Yet, in the United States, many locales are mandating masks indoors and outdoors under the same rules, forcing even the solitary person walking her dog to mask up. And there are places, such as Chicago, where beaches are closed because officials fear crowds, but indoor restaurants and gyms remain open with mild restrictions…

As another example, you may have seen the many televised indoor events where the audience members are sitting politely distanced and masked, listening to the speaker, who is the only unmasked person in the room. Jimenez, the aerosol expert, pointed out to me that this is completely backwards, because the person who needs to be masked the most is the speaker, not the listeners. If a single mask were available in the room, we’d put it on the speaker. This is especially important because cloth masks, while excellent at blocking droplets (especially before they evaporate and become smaller, thus more likely to be able to float), aren’t as effective at keeping tinier aerosol particles out of the wearer’s mouth and nose once they are floating around the room (though they do seem to help). We want to see the speaker’s mouth, one might say, but that is a problem we can approach creatively—face shields that wrap around the head and seal around the neck, masks with transparent portions that can still filter, etc.—once we stop ignoring the problem. In fact, designing a high-filtration but transparent mask or face shield might be an important solution in classrooms as well, to help keep teachers safe.

Once we pay attention to airflow, many other risks look different…

There are two key mitigation strategies for countering poor ventilation and virus-laden aerosols indoors: We can dilute viral particles’ presence by exchanging air in the room with air from outside (and thus lowering the dose, which matters for the possibility and the severity of infection) or we can remove viral particles from the air with filters.

Consider schools, perhaps the most fraught topic for millions. Classrooms are places of a lot of talking; children are not going to be perfect at social distancing; and the more people in a room, the more opportunities for aerosols to accumulate if the ventilation is poor. Most of these ventilation issues are addressable, sometimes by free or inexpensive methods, and sometimes by costly investments in infrastructure that should be a national priority.

Last week, I walked around the public elementary school in my neighborhood while thinking about what we could do if we took aerosol transmission more seriously. It’s a single-story building, all the classrooms have windows, some have doors that open directly to the outside, and many have a cement patio right outside. Teaching could move outdoors, at least some of the time, the way it did during the 1918 pandemic. Moreover, even when indoors or during rainy days, opening the doors and windows would greatly improve air circulation inside, especially if classrooms had fans at the windows that pushed air out.

When windows cannot be opened, classrooms could run portable HEPA filters, which are capable of trapping viruses this small, and which sell for as little as a few hundred dollars. Marr advises schools to measure airflow rates in each classroom, upgrade filters in the HVAC system to MERV 13 or higher (these are air filter grades), and aspire to meet or exceed ASHRAE (the professional society that provides HVAC guidance and standards) standards. Jimenez told me that many building-wide air-conditioning systems have a setting for how much air they take in from outside, and that it is usually minimized to be energy-efficient. During a pandemic, saving lives is more important than saving energy, so schools could, when the setting exists, crank it up to dilute the air (Jimenez persuaded his university to do that).

In her tweet sharing this story, Tufecki, who was super-prescient on masks early in this mess, had a great line:

Ventilation is like masks in March. We’re ignoring even simple but effective steps.

Yep.  Of course, since I’ve been closely following experts like Marr for a few months, now, all the guidelines here are already pretty much what I had internalized, have been going by in my own life, and pretty much telling everybody I know.  But, what’s frustrating, as Tufekci extensively describes, is how much this important advice is being ignored and how much truly foolish policies exist that ignore the importance if indoor vs outdoor air and ventilation.

As I think I mentioned, I’ve been pleased to see how seriously NC State is taking the role of ventilation.  My big concern now is how seriously Wake County public schools will take it for the Special Ed kids (like my son) who will be in the building.  I’m it’s remotely allowed (I’m good with gray areas) I think I’ll be buying one of those air purifiers for my son’s classroom.  My wife says to wait, but if this really is like masks in March, there may well be a run on them.  Though, it would be good news to see us taking air quality seriously.

About Steve Greene
Professor of Political Science at NC State

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