Published on December 8, 2024 2:00 PM GMT
This is a personal post: I'm not speaking for SecureBio orBIDA.
I help organize a contradance that requires high filtration masks (N95etc) at half of our dances. When we restartedin 2022 we required masks at all our dances, before switchingto half in 2023. We just rana survey of our dancers, and while there are people who would liketo not have to wear masks there are also a lot of people who are onlywilling to come if they know all the dancers will be masked. [1]
Last week I attended a conference for work with a lot of peoplethinking about biosecurity, which has me wondering about ways we couldhave a hall as safe as one where the dancers are all wearing N95s butwithout the ways N95s make it harder to dance.
We're using N95 for two purposes:
Source control: reducing the amount of droplets and aerosols thatare breathed out which go on to enter the air of the room.
Respiratory protection: reducing the amount of droplets andaerosols in the room air that end up inside people.
Since the overall goal is to have a space where people can come anddance at a lower risk of respiratory infection, let's assume thatregardless of how we change policy the people who are trying toprotect themselves will continue to wear N95s for their ownrespiratory protection. Is there something we could do to equalthe source control benefits of N95s but that's more pleasant fordancing?
If we moved our dancing outside (we can't), or managed to getequivalent ventilation to being outside (really not practical) wecould potentially stop with source control entirely. With somuch ventilation the aerosols would disperse extremely quickly,giving air that's much cleaner than the N95-filtered air we currentlyhave. While the benefits for droplets aren't going to be as high, Ithink that's outweighed by the effects of the former.
Since that's not on the table, though, I think we'd need to stick withat least surgical masks for source control. These do a good job withdroplets, but not much with aerosols. What would our options be foraerosols?
The first question is, how much clean air do we need? Ventilation isusually given in CFM/person: cubic feet of clean air, per minute, perperson. This isn't perfect, since all else equal a larger room issafer, but it's pretty good. ASHRAE Standard 241-2023 ("Control ofInfectious Aerosols") recommends80 CFM in gyms, their closest scenario to dance halls. [2] Since thedance has a peak attendance of ~290 peoplewe'd like 23k CFM.
We currently have two 42" barrelfans, each rated for 15k CFM. But that's their performance ifunobstructed, and (a) their location isn't ideal in our space and (b)one is blowing in and one is blowing out. I'm going to guess we getabout 8k CFM from the two fans. [3] This is above the 20 CFM/personASHRAE recommendationfor regular ventilation in a dance hall (20 CFM/person * 290 people =5.6k CFM), but still leaves us 15k CFM shy of our 80/CFM infectiousaerosol control target.
What are our options for covering the rest?
More ventilation. This is practical in a lot of venues,either by running the HVAC at a higher flow or with more windows. Inour case, however, we're already at the limit of what is possible inthis space with its physical layout and agreements with the city andneighbors.
Commercial air purifiers. An air purifier (ex: the AirFanta3Pro, review)removes particles from the air with filters. They're evaluated bytheir clean-air delivery rate (CADR), which tells you the CFM ofoutdoor air you'd need to have a comparable effect. [4] A commercialpurifier will list a CADR in its specs, and the AirFanta 3Pro says 413CFM on high. To get 15k CFM we'd need 36 (!) of these, at $13k.That's just barely practical: it's one every 5.5 feet around thepermiter of the room. But storing them when not in use, or buildingpermanent mounting, would be a lot of work. Also, at 55dB each, 36would be a bit noisy, though not impractically so. [5]
DIY air purifiers. During covid a lot of people built Corsi-Rosenthalboxes. These are cheap to make and have higher CADRs: Wikipedia citesthis2022 study as something like 700 CFM for their best design. Somedances (ex: Montpelier)have mounted them permanently from the ceiling, which gets them out ofthe way:
Unfortunately you still need a lot: even at 700CFM you'd need 22 toget the 15k we're looking for. And this hall would be unlikely to letus hang them.
Upper-room UV. Upper-room ultraviolet light is atraditional (ex: CDC)but not widely used method for reducing infection risk from airborneparticles. A bulb is cheap and simple: a fluorescent light combines amercury vapor that puts out 254nm UV with a phosphor coating thatconverts the UV into visible light; leave off the coating and you havejust the UV. On the other hand, UV in this range is bad for humans,so you need to ensure—through fixture design, placement, andmanagement of reflections—that human exposure is very low.Installations need to be professionally designed, and, while I'vecalled several companies on the UVSolutionsBuyers Guide for ambient air treatment, I haven't foundsomeone. I'd still be excited to see someone startsomething in this area.
Far-UV. Instead of using 254nm UV, which isdangerous for humans, you can use 222nm UV("far UVC") light, which is much safer because it is mostlyblocked by our top layers of dead skin. But because it is still akind of light, it(a) does really well in large rooms with high ceilings because thebeams pass through a lot more air and (b) it doesn't have a noisy fan.Talking to UV researcher Vivian Belenky at the Columbia Center for RadiologicalResearch, they used their modeling software to estimate that for aroom this size (40x60ft, with a 25ft ceiling) four lamps on portable 10ft stands would be in the right ballpark:
They recommended I write to OSLUVto ask about event kits, but I haven't received a response. (I didonly write on Thursday, so they may still reply!)
With the BioabundanceUVPro, built around a UshioCare 222 B1 filtered KrCl emitter (OSLUVevalluation), each lamp would cost $1.7k for a total of $6.8k.This would give UV disinfection equivalent to a CADR of 5k to 25kdepending on the study and pathogen.
Glycol vapor. Running a fog machine, which puts outglycol vapor, is a surprisingly effective way to reduce risk fromairborne pathogens. You run it at a much lower level than what's usedin theaters, low enough that it's not making visible fog. Talking toAman Patel at the non-profit research organization Blueprint Biosecurity, itreduces pathogen concentrations by something like three orders of magnitude [6].He recommended a standard fog machine (~$100) with triethylene glycol(instead of the more common propyelene glycol) which costs about $50/gal. Ifyou're bringing in 8k CFM via the fan, then you need to be putting outenough TEG to keep that flow saturated. I calculate [7] this is 0.3 mlper minute, so over a three hour event it would be 53ml. This is just2oz of liquid TEG, about $0.78. This seems very promising; I wonderwhy more people don't do it?
Overall, it looks to me like glycol vapor would be ideal, combininglow costs and practical logistics. The main downsides are:
- People may be concerned about safety, but we're talking about levelswell below what's common in entertainment.Efficacy is not as established as I'd like.
Since the efficacy is still somewhat unclear, I'd like to combineglycol vapor with another system to supplement the ventilation. Ithink the second-best option is UVC: if the lamps run for 1k hoursbefore burning out then the system costs $7/hr to run, or $20/dance.
I'm considering suggesting trialing using far-UVC and TEG [8], and ifthat goes well would propose allowing surgical masks as an alternativeto N95s.
[1] Sometimes I hear people complain about long-running covid caution,but I actually think its great. It keeps mask-wearing and otherinfection-reduction precautions normal, is insurance against the riskthat the general non-cautious public is massively underestimating theharm of infection, and means that in the next pandemic some peoplewill be protected even before we know there's something to protectagainst.
[2] We can also check ASHRAE 62.1-2022 ("Ventilation and AcceptableIndoor Air Quality") which is general ventilation, not specific toinfectious aerosols. It givesthe same values for dance halls ("Occupant activity is high. Thereis considerable aerobic activity. There are often considerablequantities of open drink, creating high levels of space-relatedcontaminants related to the people using the space.") and gyms("Occupant activity is high. There is considerable aerobicactivity. There are no significant space-relatedcontaminants. Occupancy is variable, and the high area outdoor airrate compensates for the varying occupancy and local source.").
[3] Harris took CO2 readings atseveralof our dances. Combining his readings with minefrom 2022-05-19, I see the following CO2 levels:
| Recorded Attendance | Peak CO2 |
|---|---|
| 171 | 1,400 |
| 214 | 2,200 |
| 174 | 2,100 |
| 180 | 1,100 |
| 177 | 2,000 |
| 74 | 900 |
Note that "recorded attendance" is on the low side, since it doesn'tinclude the band and caller. This is especially relevant for thedance with 74 people, since that was an open band, adding another ~15people.
The correlation between CO2 levels and attendance is pretty weak, butI think it ought to be possible to estimate our current CFM/personfrom this data?
[4] Which tells you that HEPA filters are overkill for thisapplication. Instead of removing 99.97% of airborne particles on eachpass you can use, say, filters that remove 95%, and your CADR willonly drop by 5%. You can make up for this by filtering slightly moreair, which is not hard because it's much easier to move air through a95%-effective filter than a 99.97% one.
[5] Each doubling increases perceived loudness by 6dB, so if they wereall in the same place going going from 1 to 36 would take you from55dB to a quite loud 86db: 55db + 6log_2(36). But they're spreadevenly around the perimeter of the room, so the increase should bemuch smaller than that.
[6] He cited Gomezet al. 2022, Sultanet al. 2024, Ratliff etal. 2023, Mellody and Bigg1946, and Styleset al. 2022. I haven't read these, but if you do please let meknow what you think!
[7] TEG will have fully saturated the air when its partial pressureequals TEG's vapor pressure. The vapor pressue is very low, just 0.02Pa. How many liters of TEG would give a partial pressure of 0.02Pa? Dalton'slaw of partial pressures says that the partial pressure of a gasis the total pressure times the mole fraction of the gas. We'retalking about 8k cubic feet, which is 225 cubic meters. A cubic meterof air weighs about 1.2 kg, so this is 270kg of air. The average molarmass of air is about 30g/mol, so ~9k mol of air.
P_teg = P_total Mol_teg / Mol_total 0.02 Pa = 100k Pa Mol_teg / 9k mol Mol_teg = 0.02 Pa / 100k Pa 9k mol = 0.0018 mol
The molar mass of TEG is 150g/mol, so 0.0018 mol is 0.27g. Thedensity of liquid TEG is 1.1g/cm3, so 0.27g is 0.3cm3.
[8] I would probably buy the devices personally and lend them to BIDA,since BIDA isn't the only situation where I'm interested in tryingthis sort of thing out.
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