This post is by Phil Price, not Andrew.
My 79-year-old mom likes to go to yoga classes, although of course she has not done so in months. Her favorite yoga place is cautiously reopening — they’ve had a few sessions with just eight or ten people in a rather large space (I’m going to guess 25 feet by 50 feet, based on memory from several years ago, but that could be pretty wrong and for our purposes the details don’t matter). When my mom described one of these classes I wasn’t thrilled that she went but it didn’t sound too terribly risky…but she says the place is now going to greatly increase the number of people, that otherwise they will fail financially. She asked if I have any advice that would let the place operate safely.
The following is what I wrote to the owner of the yoga place. I’m inviting comments: what did I get wrong, and what else can I say?
I have a modest amount of expertise in this area: for about ten years I was in the “Airflow and Pollutant Transport” group at Lawrence Berkeley National Laboratory, where I worked on understanding the spread of contamination in buildings, including things like chemical weapons and anthrax spores. (Most of that time was after both 9/11 and a domestic terrorism release of anthrax spores via the mail, so there was a lot of interest in this general subject). A lot of stuff that I worked on is not directly relevant to protecting against the pandemic — specifically we were not focused on infected people, and that’s pretty different — but there are some ways of thinking about this stuff that are useful, so here I am.
I’ll start with some general background and then get to some specifics about your business. By the way, my parents brought me to a couple of yoga classes at your place. I really enjoyed them, and I want you to be able to reopen and for my mom to be able to keep going to your classes! But I want her and all of your customers to be safe, and I know you do too.
So here goes:
I think contact transmission is pretty easy to understand: if an infected person leaves a drop of sweat or spittle or whatever on an object, and someone else later touches that object and wipes their mouth or eyes or nose, there is a possibility of infection. As recently as two months ago this was thought to be the most common way transmission was occurring, but it seems that now the experts think most transmission is from inhaling particles that contain the virus. I don’t know anything more about that than anyone else who reads the papers; not my area of expertise. And I don’t think anyone is really sure of the details, for instance there may be some people who shed a lot of virus particles in their breath and others who generate a lot by coughing or sneezing. You should certainly disinfect yoga mats and doorknobs and anything else that is touched by a lot of people. That’s a pain but it’s pretty straightforward, and at least if you do disinfect something then it really is safe. Preventing spread through airborne transmission is a lot harder, but it’s also thought to be really really important with this virus, so that’s what I’ll focus on.
For any single person, the most important parameters that determine whether they get infected via inhalation, are: concentration, time, breathing rate. Specifically:
1. What is the concentration of virus-containing particles (‘virions’) in the air they are breathing (how many particles per cubic foot)?
2. How long is the person breathing this air (How many minutes)?
3. How much is the person breathing (how many cubic feet per minute)?
If you multiply those three numbers together, you get the number of virions they have inhaled during the time they were exposed.
These are not (at all) the only things that determine whether that person will get sick. For instance, when I say “virus-containing particles”, that could be an individual airborne virus (one particle) or a little droplet that has a hundred virus in it (also counts one particle), but those don’t carry exactly the same risk. And whether the person breathes through the nose or through the mouth makes a difference. Other things too. But those three are the way to start thinking about it.
And of course, probably most important of all: Even if several people inhale the same number of virus-containing particles in the same amount of time, they won’t all have the same health outcome: the same ‘viral load’ that is easily handled by the immune system of a young, healthy person might cause a severe infection and even death in an old or sick person.
If you are going to have a half-hour or hour-long yoga session in your place, then you know #2 already and there’s nothing you can do about it.
Although you can affect #3 a little bit by avoiding strenuous exercise, the difference in breathing rate between a quiet yoga session and a strenuous aerobics session people is only a factor of three or so. Still, it’s probably a good idea to not have strenuous sessions: I assume (though I’m not sure) that if an infected person is breathing really hard they will be putting out more virus particles per minute so the concentration of particles in the air will be higher (so #1 will be higher), _and_ the non-infected person will be breathing more air per minute (#3), so there is probably a multiplicative effect or something like it. Putting those together, this means that, if you have one infected person among a bunch of non-infected people, the probability that one of the non-infected people will get sick is several times higher if everyone is doing strenuous exercise compared to something calmer. That’s enough of a difference that you might want to consider it when deciding what kinds of activities to do, and how many people to allow in the room at a time, but it’s not such a dramatic difference that one would ever say “it’s dangerous to do this aerobics class but safe to do quiet yoga.” So take this into account but don’t rely on it.
So much for issues 2 and 3. The one that really counts, the place where you can look for factors of 10, is in trying to reduce the number of virus-containing particles per cubic foot in the air people are breathing.
Let’s start with the obvious: masks. A really well-fitted mask that can filter out really fine particles can greatly reduce the number of virus particles that people actually breathe, even if the concentration right in front of their mouth is high. Unfortunately you really can’t count on this and nobody should: there is almost always leakage around the masks, most people don’t have the right masks anyway, etc. etc. You have to assume that the air that enters the person’s mouth will be approximately as contaminated as the air directly in front of them. So what good are these cloth masks or regular ol’ surgical masks that don’t exactly conform to our faces, is this just theater? No, these are actually useful, but they do not help protect the person wearing them. They help protect _other_ people. They do this in two ways. One is that they catch the larger droplets from a sneeze or cough. That’s not nothing! Unfortunately, the individual virus particles are so tiny that they can easily go through the weave of a typical cloth mask, and a lot of tiny droplets do that too.
But there’s another way the mask helps: it greatly reduces the velocity of the air that you exhale. If you say “Peter Piper” loudly while you hold your hand in front of your mouth, you can feel your exhalation a foot or even 18 inches in front of your face. Normally, if two people are carrying on a conversation, standing three or four feet apart, they are going to be inhaling a substantial amount of each other’s exhaled air. But if they are both wearing masks, much less of the air will be intermingled in a short conversation: I’m breathing mostly ‘my’ air and you’re breathing mostly ‘your’ air.
Unfortunately, the paragraph above only applies for relatively short periods of time, or if there is a steady supply of uncontaminated air that is constantly replacing the contaminated air being exhaled. If an infected person wearing a mask walks into a room, initially only the air around them will be contaminated with a substantial amount of virus. But this contaminated air will gradually spread out to fill the rest of the room. The concentration will always be higher near the person and lower farther away, but it will climb everywhere until an equilibrium is reached: eventually the number of viral particles per minute that are exhaled by the victim will be equaled by the number of particles per minute that either deposit on surfaces or are removed to the outdoors by the ventilation system, and at that point the average concentration in the room will remain steady.
Maybe you’ve seen two of the more compelling examples of airborne transmission, both very relevant to your yoga business:
https://wwwnc.cdc.gov/eid/article/26/8/20-1274_article. (Note Figure 2 especially)
What does all of this mean for your business? It means you have to find a way to either get the virus out of the air, or to get the virus-containing air out of the building. Simply moving air around with fans will not do anything to reduce the number of particles in the air. You will reduce the airborne concentration of particles near the infected person, yes, but you will increase it everywhere else, and the number will continue to increase until it reaches equilibrium as described a couple of paragraphs above. It’s possible that by moving the air around you can decrease the exposure of the people close to the infected person to an extent that nobody gets sick, but it’s also possible that by spreading the air around you’ll infect everyone in the class rather than just the people closest to the infected one (more or less like in the NY Times article above). Basically you cannot simply move the air around, you have to get it out of the place and replace it with uncontaminated air.
Putting it all together, what are your options? I’ll list a few but these won’t be exhaustive:
1. Hold classes outdoors. This would be by far the best solution if you can find a way to make it work. Set up in a park, or on campus, or in the parking lot. Keep people far enough away from each other — make it ten feet, say — and this should be fine: there is pretty much always enough of a breeze that the concentration of virus-containing particles will not be high anywhere except very close to an infected person. It’s possible that someone directly downwind from an infected person could inhale enough to get sick, but that could probably only happen if the breeze is very, very steady in direction and too weak to cause turbulent mixing. You could help make this kind of transmission even less likely by spacing people farther apart in the upwind-downwind direction, even if that means a little less distance in the cross-wind direction.
2. If you can’t hold classes outdoors, the next best thing would be greatly increasing the rate at which ‘old’ air is removed from the building and replaced by outdoor air. As discussed above, air movement is not the goal — moving contaminated air around in the room probably won’t help at all and could make things a lot worse. The goal is to get contaminated air out of the building, and since every cubic foot of air removed from the building has to be replaced, this is the same as moving uncontaminated air into the building. I think I recall that there are big windows along one wall of the building; if you could replace those with windows that slide all the way open, and use fans or some other means to increase the amount of air that flows through the resulting openings, that would greatly decrease the airborne concentration of virus. I know it gets hot there in the summer, but hey, people do “hot yoga” all the time, maybe you can make it work.
3. If you can’t move outdoors, and you can’t greatly increase the amount of outdoor air that you move indoors, then your only remaining option is to decrease the virus concentration in the indoor air in some other way. Really that means filtration (although see below for ultraviolet radiation). Conceivably you could have enough HEPA filters to greatly reduce the particle concentration. If you knew who was sick you could surround them with filters, but of course if you knew who was sick you wouldn’t let them in in the first place! But conceivably you could put a filter next to every participant, or something. My wife and I own a [specific type of air filter, redacted here because I don’t want to put an implied endorsement on Andrew’s blog] which we use in our bedroom to try to reduce problems with my mild allergy to some kinds of pollen. (We bought it ourselves, have no relationship with the company whatsoever, and I don’t know if this is the best brand. We like it because it’s quiet even at fairly high airflow). I know it would be expensive to buy 20 of these or whatever, but I think something like this is the only possible answer if 1 and 2 above aren’t in the running…with the possible except of uv radiation, discussed below.
4. You could also look into ultraviolet radiation. People think of it as a bit exotic at the moment, but if you have strong ultraviolet lights that fill the upper part of your space with UV radiation, you can inactivate the virus. In this paradigm you aren’t removing the virions, you’re rendering them harmless. You’d use baffles so that the people aren’t exposed to the UV. I have not looked into commercial systems for doing this but I know they exist.
Oh, none more thing that doesn’t really fit the enumerated list but seems relevant: bathrooms are apparently an issue. For one thing, flushing a toilet can supposedly create a sort of jet of tiny droplets that can contain virus. Also, a bathroom is a ‘high touch’ environment: people touch the toilet seat, the handle, the sink, the handles. The safest thing would be to simply not allow people to use the bathroom, but I don’t know if that’s workable (especially for employees who have to be there all day). Setting the bathroom fan to maximum, requiring a break between users, and frequent disinfection, can probably help with this. I don’t know anything about this issue but I encourage you to learn about it and figure out what you can do.
Finally, in addition to all of the above you can of course affect the degree of transmission risk by choosing how many people are in your space and who they are. Maybe you could have larger classes for young people who are willing to take a higher chance of getting sick from being close to an infected person, and smaller classes for people like my mom who I, at least, hope will not participate in a large class under pretty much any circumstances because of the chance she would be near someone who is unknowingly infected. Charge more for the smaller classes, even a lot more if you have to. But please, if you do this, also buy a bunch of filters.
Unfortunately I can’t give you a defensible quantitative estimate for how much you can reduce the transmission risk by moving outside vs using 20 HEPA filters vs using 40 HEPA filters. Maybe there’s enough information by now to come up with a useful estimate for that, but if anyone has done that I don’t know about it. What I can say is that I am going to ask my mom to stay away from any activity that would have her indoors with several people within ten feet of her for an hour, because if one of those people is sick there is just too high a transmission risk (see those two articles I cited earlier). The fewer the number of people in the same room, the bigger the distance between them, the lower the duration, and the better the ventilation, the more comfortable I feel.
I would be happy to discuss any of this stuff with you on the phone — really would be very happy to, just call me at 510-909-8863 if you want — but I reiterate that although I know a fair amount about the general topic of airflow and pollutant transport in buildings, this email puts me pretty much at the limit of things I can say with any confidence, so please don’t take what I say as definitive when it comes to what is safe.
I really hope you can make something work for your business while keeping my mom safe (and others like her. Maybe even my dad will start coming again, that’s an even higher standard!)
Best of luck.