School Ventilation

Jesse Smith - August 18, 2020

Advice

I don’t really know if this is totally appropriate for this forum, but I’ve been getting a ton of questions about school ventilation and filtration systems. So I made a video that outlines these systems Hopefully this helps parents and school staff work to verify the function of these systems in their school districts. Feel free to post any questions specific to this issue. If there’s even slight interest I’d like to do some stuff shortly on measured flow and filtration with simple devices you can readily install in your kid’s classroom. 

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I watched the video and appreciate your effort to make it more understandable. I have a question.

Using an average school’s ventilation system, does a smaller classroom have an advantage over a larger classroom for Covid infection? I would guess the smaller size increases the number of air changes per hour which is good. Or does the smaller size make it easier for high concentrations of the virus to happen? Because there’s less space for it to kinda spread around?

 

Really good question! This is very difficult to answer because of unknowns. Basically, the vast majority of schools are either unventilated altogether or under-ventilated. So assuming the number of occupants in the classroom is the same, it’s likely that the larger classroom would have a slight advantage over the smaller one. 

However, it’s worth contextualizing this a bit better. The bottom end of ventilation that I mention in the video is 3 ach. For most classrooms this will mean at least 250 cubic feet of air per minute (cfm), and many classrooms will need a lot more. It’s just an insanely high ventilation rate relative to what most classrooms are currently experiencing. The protection afforded by room size is really low, and larger rooms will tend to have more occupants which will offset this protection. The focus here needs to be on getting the ventilation airflow right.

I’m working on another video outlining how to cheaply verify ventilation for laypeople. The cliff’s notes are that people should stock up on CO2 monitors. Basically the best way to tell if a space is underventilated is to use CO2 thresholds. Normal outdoor levels of CO2 are in the range of 400 ppm. Indoor environments in the range of 800-1000 ppm are considered underventilated. For classrooms in the time of covid I would use the lower threshold and make 800 ppm the cut-off where either people leave the space or increase the ventilation rate. As with other supplies during covid, I’m really shocked that as of last week one could still readily purchase reliable co2 monitors. 

 

This is incredible, Jesse, thanks so much. As a teacher being forced to return to work in person in a 110+ year old NYC building with minimal window opening and only in-room ac/heat (ie, no HVAC), I am immediately looking to purchase a CO2 monitor for just the purpose you described. Thanks for this guidance. 

Do you have a model you recommend or, better, a lower-cost model that might seem good but you know is junk?

Loving the expertise here, and just another reason I’m so grateful for the sharing in this community. 

 

So glad you found it helpful! I have 2 of these, and it looks like amazon has 1 left as of this post: https://www.amazon.com/gp/product/B07YY7BH2W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

Assuming moderate occupancy and little to no ventilation, there’s basically no chance that you’ll be in a room that would be considered safe. I suppose the great thing about having a CO2 monitor is that you can demonstrate this to people in charge of the building, but it’s not really an experiment that’s worth running.

Can you get room unit specifications? It’s really worthwhile to pursue these.  <boring anecdote alert> I was in a school last week that had room heaters that essentially blow airflow across a heating coil. However, the units had openings to the outside so some fraction of outdoor air was being drawn into the room. I pulled specs on the unit, which had max airflow of 1000 cfm. However, I didn’t have time to measure airflow from the outdoors, and the unit specs don’t appear to have them. My guess is that under current operation the unit might pull 20% outdoor air, which would be 200 cfm – better than nothing but insufficient for the room demand (~400 cfm ventilation). My point is that many people are in the dark about the mechanical devices they’re surrounded by every day.

In contemporary schools with ducted ventilation systems it’s likely that most of these systems will be central, so it’s also important to get specifications on these. Pulling specs will allow teachers to quickly determine if the system could conceivably pull enough ventilation air to be safe. However, it doesn’t absolve us of the responsibility to verify proper ventilation rates to every room. 

Specific to school communications – I’m seeing a lot of statements designed to reassure teachers that have very little real substance. For instance, districts might say something like, “we’ve increased our ventilation system to 100%.” The claim is probably rooted  in the fact that some ventilation systems will fix airflow as a constant but allow users to program some fraction of air to recycle through filters to avoid overwhelming improperly designed heating and cooling equipment. But what quantity of airflow = 100% in this context? What if the 100% of the system capacity is 1/4 of the flow rate required to keep occupants safe? 

TL:DR takeaway:

1. Get equipment specs
2. Verify safety on a room-by-room basis

 

Another teacher here – are there currently available models of CO2 monitors you consider reliable? The one you linked is completely out of stock on Amazon.

 

Unfortunately I haven’t tested many other units, aside from in combustion sensors, which aren’t really applicable. My best guess is that you would probably have reliable monitoring down to the $70 price point, but loss of functionality. Most teachers probably don’t need data logging or an app that can link and track multiple sensors, so here’s a mid-range that would probably be fine: 

And here’s a super cheap one that’s basic, but probably also OK: 

I’d love to hear experiences from teachers tracking CO2 levels in their classrooms, so feel free to report back with the results!

 

Thanks for this! I was in my doctor’s office and noticed they had air purifiers everywhere. Do you think adding a .1 micron purifier to a room helps?

 

Good question! It’s sort of complicated. Yes, the scientists who study this widely believed that air purifiers are helpful. In addition, HEPA level filtration probably isn’t totally necessary for reducing Covid risk – mounting a 20×20 MERV 13 filter to the side of a box fan is probably effective and can be done for ~$40. 

How a MERV 13 Air Filter and a Box Fan Can Help Fight Covid-19

OTOH, ventilation (i.e. replacing indoor air with fresh air from outdoors) is widely viewed as superior to filtration. I suspect that most MD offices have very poor ventilation. HVAC is what I refer to as a landscape of endemic failure. So even when people genuinely believe that they have functional systems, they usually don’t. Consider carrying a CO2 monitor into densely occuppied commercial spaces. 1200 ppm of CO2 is a nearly universal standard by which rooms need increased ventilation, and nearly everyone working on Covid thinks a lower threshold of 800 ppm is reasonable. 

The best filter info I’ve seen seems to be coming from CU. Tons of good stuff, but here are 2 names to follow:  

About Me

http://cires1.colorado.edu/jimenez/

 

Great info – thanks again! Very much appreciated.