Published on December 4, 2024 4:10 PM GMT
I help organize a contradance that requires high filtration masks (N95 etc) at half of ourdances. When we restartedafter shutting down for covid we started requiring masks and then stoppedallowing cloth and surgical masks before switching to our currentapproach inmid 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]
This week I'm attending 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 they 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, lets assume thatregardless of how we change policy the people who are trying toprotect themselves continue to wear N95s for their own respiratoryprotection. Is there something we could do to maintain the sourcecontrol benefits of N95s but that's more pleasant for dancing?
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 are going to 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, and options for dropletreduction aren't great, I think we'd need to stick with at leastsurgical masks for source control. These do a good job with droplets,but not much with aerosols. What would our options be for aerosols?
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 room with a highceiling is safer than one with a lower one, but it's pretty good.ANSI/ASHRAE Standard 62.1-2022 recommendsa minimum of 20 CFM/person for dance halls [2] and gyms [3], but weprobably want something higher if we're trying to match theperformance of N95s for source control. Let's target 80 CFM/person.The dance has a peak attendance of ~290 people, sothat would be 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. [4] This is above the ASHRAErecommendation (20 * 290 = 5.6k), but still leaves us 15kCFM shy ofour target.
What are our options for covering the rest?
More ventilation. This is practical in a lot of cases,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 Coway AirmegaAP-1512HH I got on the Wirecutter's recommendation a few yearsago) removes particles from the air with filters. They're evaluatedby their clean-air delivery rate (CADR), which tells you the CFM ofoutdoor air you'd need to have a comparable effect. [5] A commercialpurifier will list a CADR in its specs, and the AP-1512 says 233 CFMon high. To get 15k CFM we'd need 64 (!!) of these, at $13k. That's justbarely practical: it's one every three feet around the permiter of theroom. But storing them when not in use, or building permanentmounting, would be a ton of work. And they put out enough noise that64 of them would be pretty bad.
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.
Upper-room UV. Upper-room ultraviolet light is atraditional (ex: recommendedby CDC) but not widely used method for reducing infection riskfrom airborne particles. A bulb is cheap and simple: a fluorescentlight is a mecury vapor lamp (producing 254nm UV) with a phosphor thatconverts the UV to visible light; leave off the coating and you have254nm gemicidal light. On the other hand, this light is bad forhumans, so you need to ensure through fixture design, placement, andmanagement of reflections that people aren't covered by the light.Installations need to be professionally designed, and there don't seemto be goodoptions here.
Far-UV. Instead of using 254nm light, which isdangerous for humans, you can use 222nm("far UVC") light, which is much safer. And because it'slight, it (a) does really well in large rooms with high ceilingsbecause the beams pass through a lot more air and (b) it doesn't havea noisy fan. Talking to UV researcher Vivian Belenky at Columbia, she estimatedthat for a room this size (40x60ft, with a 25ft ceiling) you'd do wellwith four emitters on portable 10ft stands. With [product TBD] thiswould cost [cost TBD] and be equivalent to [CFM TBD] CFM.
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 lower level than what's used inentertainment, just below where it condenses out of the air and makesvisible fog. Talking to Aman Patel at the non-profit researchorganization BlueprintBiosecurity, it reduces pathogen concentrations by over threeorders of magnitude [6]. He recommended a standard fog machine(~$100) with triethylene glycol (instead of the more common propyeleneglycol) 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]
.I think most of the options achihere look like If we lowered our maskrequirement to surgical masks, for droplet control,
[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] "Occupant activity is high. There is considerable aerobicactivity. There are often considerable quantities of open drink,creating high levels of space-related contaminants related to thepeople using the space."
[3] "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."
[4] 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?
[5] 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.
[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 ways 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.
That's the amount we need to emit each minute, so over 3hr it would be53cm3. This is just 2oz of liquid TEG, or $0.78 at $50/gal.
(I'm not confident I did my math right here!)
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