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Advice for a yoga studio that wants to reopen?

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: (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-xxx-xxxx 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.



  1. anonymous says:

    Interesting, but before thinking about a comment of substance — remove your phone number from the post! :(

  2. I think room size HEPA filtration would be good. They are rated for the square footage they’re designed for, so you just buy 50% more than the rated quantity and hope.

    Another thing you didn’t mention is air ionization. It’s been shown to be effective specifically for viruses. It basically winds up electrostatically charging the small aerosols and then they’re attracted to surfaces like the floor and walls.

    I would encourage any yoga studio to do a mop down of the floor with light bleach solution between each class. I don’t know about you guys, but when I do yoga classes they’re pretty intense, and there’s sweat dripping all over the floor etc.

    Outdoors would really be ideal though. I personally wouldn’t consider an indoor yoga class at the moment. Of course, I’m in a hot zone virus wise with a lot of cases coming from the LA county area in general.

    • jhe says:

      Very interesting. IT guy thing: you may want to suppress the phone # or make it less readable by a bot.

    • Phil says:

      I didn’t know about ionization for virus removal, I’ll pass that along, thanks.

      I think they’re already doing large-scale surface disinfection but I’ll make sure that’s the case.

      • Here’s one example study:

        It seems like not only does it attract the virus out of the air towards surfaces, it also seems likely to damage the virus envelope and make it less infective.

        Unfortunately there are all sorts of questions about how effective these devices are, they probably vary widely by manufacturer. They’re also likely to work a lot better in dry climates than humid ones.

        Also note the difference between a negative ion generator (which generally produces trivial quantities of ozone), and an ozone generator. Some people use ozone generators to do things like de-mustify basements etc. You shouldn’t be in a room with an ozone generator going. Negative ion generators are a different thing.

  3. Joshua says:

    Phil –

    > 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.

    I’ve seen some stuff that suggests that wearing masks increases the humidity level behind the mask, which might reduce the distance that the aerosolized particles might travel. Of course, I certainly don’t know if that true, and and I think that even the experts are uncertain…

    • Navigator says:

      I believe any barrier, especially surgical or N95 masks will work well enough to minimize the risk at the population level, but only if everyone involved is using them. However, wearing anything that restricts breathing while exercising is not a very good thing.

      Exercise is ‘essential’ but exercise in a group is not. It’s a luxury.

      And also, let’s not confuse exercise and movement where only the latter has some benefits, if performed continuously throughout the day.


      • Phil says:

        For a while (not sure if it’s still true) some athletes wore masks to deliberately make it hard to breathe, in an effort to simulate altitude training. The ‘theory’, if we can call it that, is that they were training their bodies to work hard with less oxygen. It sort of makes sense but ignores several key factors that I won’t bother going into. But although it is silly, and I hope discredited, as a way of improving performance, I don’t think it had any major ill effects either. You just can’t work out as hard.

        It’s true that nobody has to go to yoga class. Also true that yoga studios are reopening and some people are going to them. I can either give advice or not give advice. I’m choosing to give advice.

        I’m not sure what you’re saying with your last point. Both of my parents have benefited a lot from yoga. As people get older we tend to lose range of motion, and to develop various posture issues. Some of these issues are due to cartilage or bone deterioration, but some are muscular, and even the ones that aren’t muscular can be helped by the right exercises. Both of my parents found that even an hour every couple of days is enough to make a noticeable difference: they regained mobility when they started doing yoga, and if they stop for a while they start to lose it. They do get other exercise too, but that was also true before they started with yoga.

  4. jim says:

    here are a few more relevant CDC case reports:

    fitness centers in South Korea;
    choir practice WA state;

    In the choir practice 53 of 61 people became ill from a single infection in a two hour choir practice. People were sitting and standing close to one another – not quite touching – but clearly they weren’t all in close proximity to the infected person the whole time.

    in part rephrasing a few of Phil’s points:

    1) People should be as widely spaced as possible, indoors or outdoors. 6ft probably isn’t enough for intense physical activity, even outdoors. Wildly guessing 12ft would be the min spacing.

    2) Everyone should wear masks, indoors or outdoors;

    3) Indoors: select rooms with the with highest ceilings possible. This allows air to circulate upward and away from people;

    4) Indoors: Deploy fans to circulate air upwards, away from people, not around in circles. Ideally fans would pump air out of high windows (as Phil suggested) and just roughly guessing those windows should be higher than 20 feet; otherwise circulating and possibly contaminated air will flow by people at face-level.

    5) Bathrooms: IMO the biggest threat is the small air volume rather than the factors Phil enumerated. If two ppl are in a small bathroom and one is infected, chances are good the non-infected person will “consume” infected air. Crank the windows open, limit to one person, impose a empty time between uses – say three minutes – and if a fan is available pump air out the windows. Definitely if ceiling fans are available use them.

    I’m not an expert. But the basic geometry of the situation is simple enough: if people aren’t exposed to contaminated air, the probably won’t get sick, so doing everything possible to make air flow upward seems like the sensible proposition.

    • Phil says:

      Thank you. I knew about the choir practice report but not the fitness centers, which is very relevant and which I will pass along. Interesting that one instructor did not pass along the illness to her students and that they speculate that that may be due to the fact that her classes were low-intensity. I wish we knew more about the causality of these things.

    • Phil says:

      “Using fans to circulate air upwards” doesn’t help much when people are sharing a space for a long time compared to the mixing rate. What goes up must come down, and quickly in this case.

      Creating high airflow can help when people are close together and the concern is that someone will infect the person right next to them due to a short period of high-concentration, and there’s no harm in it, but it also makes concentrations more uniform so it could lead to more people being infected. Could go either way. But in any case it doesn’t matter how you angle the fans.

  5. Martha (Smith) says:

    There is another option to consider: Put the class online, via Zoom or similar method. My senior barre fitness class has been doing this for the past few weeks. I’d say the average attendance for the online version has been greater than the average was before it went online.

    Students need to supply their own equipment, but the class has been modified to use items that are available in most households. For example, instead of a mat, someone can use a large towel on top of carpet; instead of a ball to be squeezed between the legs, or as a prop in a floor position, use pillows. A large towel can also substitute for a stretch strap.

    One downside to online classes (in addition to getting whatever one needs to put the classes online) is that the the instructor can’t correct things that a student does wrong. In my class, this is partly mitigated by having one person on the studio acting as student, which gives the instructor a better way (than just illustrating the problem themself) to point out possible wrong positions and correct them (and also to show modifications for students who need them).

  6. John Mashey says:

    Online with Zoom: my wife and I do a Pilates class most weeks that way. Works well, and less time driving to/from gym.

  7. Winston says:

    +1 for online with Zoom. A friend of mine teaches Pilates here in NZ, and taught her classes online while we were in lockdown. She even preferred it in some ways, especially when teaching the youth – in person she has to spend a lot of time telling them to stop looking at their damned phones aand pay attention. She only teaches small groups though – not sure if it would work as well with a large class.

  8. JFA says:

    While it’s good to give advice on how certain businesses can operate safely, advice also needs to be given to participants about the risks in patronizing those businesses.

    There’s a scene in Mean Girls where the sex ed or PE instructor is talking about abstinence (if I recall correctly) and ends with the instructor saying something along the lines of “If you have sex, you will get chlamydia, and you will die!” I feel like this kind of advice (relating it to Covid rather than chlamydia) should be considered when discussing an elderly person’s going to a group activity with heavy breathing.

    One thing that might want to mentioned is the adverse selection issue (just in case your thinking of adjusting the advice based on level of community spread). The people likely to go to a group fitness class during a pandemic probably have a higher risk tolerance and are more likely to engage in risky behavior outside of just going to the group fitness class. The likelihood of a randomly selected person having Covid is, therefore, probably higher at the group fitness class than at the grocery store. The participant will also want to take account of the ethnic/racial makeup of the group.

  9. jim says:

    “when discussing an elderly person’s going to a group activity with heavy breathing.”

    Is that true? What percentage of elderly deaths were in nursing homes? It matters because nursing homes are the almost ideal high-transmission environment: lots of people with health issues, trapped for extended periods in close quarters – possibly with an air circulation system that shares infected air around the building.

    My guess is that the susceptibility of elderly people in the general population is much lower than for those in nursing homes and thus the age effect isn’t nearly as strong as it appears.

    I agree that group exercise activity has a higher risk and I personally probably wouldn’t go. But that’s as much out of ignorance as it is out knowledge. I suspect that, with good upward ventilation and increased spacing and limited duration the risk is of a large outbreak is reduced substantially.

    • confused says:

      The age effect definitely exists and is very strong even if no nursing homes are in the sample population: see the Diamond Princess (2% IFR – strongly skewed to the elderly) vs. the USS Theodore Roosevelt (less than 0.1% IFR … so far at least, but that outbreak was over 2 months ago, so the number is probably valid). Singapore also has an extremely anomalously low IFR (less than 0.1%) since the vast majority of its cases are in relatively young and relatively healthy foreign workers.

      Nursing homes are an extreme case (something like 40-45% of *all* US COVID deaths are in nursing homes – possibly more, as deaths in hospitals are likely better reported) but not the only driver for the age effect.

    • JFA says:

      Long term care facilities have certainly been hard hit, but the age effect exist independent of that. For example, here’s Indiana’s dashboard: (total deaths at 2240). 51% of their deaths are from those in the 80+ category, and about half of their deaths were in long term care facilities (1011 deaths in LTC facilities). But even the 60-69 and 70-79 age categories make up a lot of their deaths (16% and 24%). So 91 percent of Indiana deaths (that’s 2016 deaths) are those over 59. Those over 59 make up only 30% of the cases.

      If you assume that all those LTC deaths are over age 59 (not unreasonable) and remove those deaths from the total, you still end up with those over 59 making up 82 percent of total deaths in Indiana. Removing LTC cases (again, assuming all those cases are in the 60+ category), you are left with only 21 percent of the cases being from the 60+ age group.

      Now a 79 year old woman who does yoga is probably in pretty good health relative to her peers, but I certainly wouldn’t be taking the chance of getting Covid just to do some stretching if I were her.

  10. Dzhaughn says:

    Why is her yoga studio going to take your advice if your mom won’t? I mean maybe, but what is the model?

    I read that advice as “Just fold; consider HVAC installation as new career.”

    • Phil says:

      My mom is taking my advice too.

      I think a yoga studio should probably be able to make it work: limit group sizes, enforce mask-wearing, provide adequate ventilation and filtration. Maybe even ask old people and other at-risk people not to come, if it’s legal to do that; if it isn’t legal then at least make the dangers clear to these people.

      If they can’t do those things and stay in business, then yes, of course they should fold. That’s true of any business that can’t operate with reasonable safety.

      • Martha (Smith) says:

        Phil said,
        “If they can’t do those things and stay in business, then yes, of course they should fold. That’s true of any business that can’t operate with reasonable safety.”

        Yes. In addition to the barre fitness classes, I had been going to a yoga class once every other week at an establishment offering a variety of classes. They have now folded.

        I think a big part of the reason that the barre fitness studio seems to be surviving is the competence of the owner — she just has a lot of savvy in lots of ways: getting along with people, giving individual attention, good business sense, ability to adapt to new circumstances, rising to new challenges, etc. She was a Radio City Rockette for thirteen years, but then developed hip problems requiring surgery — and took the initiative to develop an exercise routine to help her recover and maintain fitness — and then decided to open an exercise studio focusing on those techniques. I have been impressed at how she has been able to modify her basic methods to adapt them to the needs of old people, pregnant women, women with pregnancy-induced injuries, etc. (even football players!)

  11. Linch Zhang says:

    Re: bathrooms, I don’t *think* bathroom transmission is a large effect in the West but the last few months I’ve been recommending that my roommates and I flush toilets with the lid closed. Intuitively this should be a large effect size at a minimal time cost.

  12. Gregroy C. Mayer says:

    There’s no virus in sweat: ; it’s saliva and mucus you have to worry about. The latter, can, of course, get on anything contacted by hand.

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