How I'm rating the HEPA filters purchased by Canadian schools 🧵

I will tweet a thread soon comparing some HEPA filters purchased by Canadian schools. Before, I want to explain my rating system here.

1/8
Getting the info about the HEPA filters was very difficult - it isn't available. The proper info provided for each one should be the CADR (clean air delivery rate) and noise at each speed, but you can't find that, so I had to estimate based on the available information.

2/8
Many companies provide misleading info. For example, they state the CFM of the fan without the filter instead of the CADR. They state the noise at >6 ft away rather than the standard of 1 m. You have to sort through the nonsense to find the necessary info.

3/8
I then had to determine a rating system. You are looking for cost, noise, CADR and no ionizers. If something is too noisy in a classroom, it just gets turned down, so I decided to use my estimated CADR when on a speed with the noise under 55 db - usually medium speed.

4/8
This estimate isn't super accurate, but I think it is in the ballpark of the correct value. 2 categories CADR @ < 55db and $/CADR @<55db:

CADR @ <55db
CADR Points
250+ 5
200-250 4
150-200 3
100-150 2
50-100 1

5/8
$/CADR Points
<2 5
2-4 4
4-6 3
6-8 2
8-10 1

Electronic air cleaning = -2 points

In the end, they get a mark out of 10.

6/8
For reference, the one I recommended here is the Levoit LV-H133. 275 CADR @<55db and costs $1/CADR. Easy 10/10.

If you went to a hardware store and played ini mini miny moe, you would probably pick a better HEPA filter than in most classrooms.



7/8
The point is, I created a very generous scale because the HEPA filters in schools have low CADR and cost a fortune. If I hadn't done that, they would all fail. This scale allows you to compare between school boards. Stay tuned. The ratings will be out soon.

8/8

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More from @joeyfox85

Feb 22
If all the money that was spent on cloth masks, face shields, gloves and gowns would have been spent on N95s, everyone would have been protected.

If all the money that was spent on hand sanitizer would have been spent on CO2 monitors, people could keep themselves safe.

1/7
If all the money that was spent on deep cleanings would have been spent on HEPA filters, there would have been less transmission.

If all the money that was spent on plexiglass would have been spent on UVGI, those classrooms would have been safe.

2/7
If all the time spent on teaching people to wash their hands would have been spent on teaching people about airborne mitigation, people would know how to protect themselves.

If some of the media time given to PH and ID docs...

3/7
Read 7 tweets
Feb 15
We stopped COVID in our home! My wife felt sick early last week and then tested positive (RAT). Everyone in our family is now negative. My kids and I never got it. We didn't resort to extreme measures like isolation or kids wearing N95s all day. This is how we did it:
1/11
We followed public health advice:
-wore comfortable cloth masks
-washed our hands frequently
-cleaned and disinfected - especially the toys
-practiced safe physical distancing
-put up plexiglass barriers as an extra precaution.

Just kidding. That's not how it's done.

2/11
#COVIDisAirborne . Focus on keeping the air clean and you can stop the spread. These are the measures we took:

Vaccines - we're all fully vaccinated (including boosters) except my 3 yo. My 4 month old had 2 doses in utero. My 6 yo has 2 doses.

3/11
Read 11 tweets
Feb 15
What are good CO2 levels and what can you do if they aren't good?

When people breathe out they expel CO2. Depending on the level of ventilation, this can be exhausted and the CO2 levels will stay low (good ventilation) or if it isn't good, the CO2 will build up and rise.

1/7
There is no hard cutoff with the different levels, but here is a guide.

<800 ppm is quoted a lot, but in my experience, classrooms can be much higher. Up to 1500 ppm can occur with ventilation not working perfectly (very common). > 2000 happens with bad ventilation.

2/7
CO2 levels measure how much air is shared or rebreathed. @DavidElfstrom prepared a table comparing CO2 levels to the % of rebreathed air. The more rebreathed air, the higher the risk of airborne transmission.



3/7
Read 7 tweets
Feb 8
1 air change only removes 63% of the virus particles. Why? (Warning: Math)

Thought experiment 1:
Start with 100 virus particles (VPs).
Remove all the air in the room (100 VPs).
Refill the room with air.
You have 0 VPs left - 100% removal.

1/5
Thought experiment 2:
Start with 100 virus particles (VPs).
Remove half the air in the room (50 VPs).
Refill half the room with air and let it mix.
You have 50 VPs left.
Remove half of the air again (25 VPs).
Refill the room.
You have 25 VPs left - 75% removal

2/5
Thought exp. 3:
Start with 27 VPs.

Remove 1/3 of the air (9 VPs).
18 VPs remain. Refill the room.

Remove 1/3 of the air (6 VPs).
12 VPs remain. Refill the room.

Remove 1/3 of the air (4 VPs).
8 VPs remaining, 19 removed total.
Refill the room.

19/27=70% of VPs removed.

3/5
Read 5 tweets
Feb 6
Case study in bad school ventilation (not from work or my kids’ school):

There were outbreaks where most of the class got infected. There was a new ventilation system installed 2 years ago. It should have been great ventilation. I asked them to get a CO2 monitor to check. 1/4
CO2 was more than 2000 ppm throughout the day! I checked the airflow with a tissue and a broomstick. No airflow. 2/4
What happened?
It’s a VVT system that was designed to reduce airflow when heating wasn’t needed. Should be set to have a minimum amount of air. However: mechanical installed it with factory settings, air balancer only measured maximum airflow, consultant didn’t verify. 3/4
Read 4 tweets
Feb 4
Ventilation vs. Filtration vs. UVGI vs. Electronic Air Cleaning
What are they and what do they do?

Ventilation - Exchanging indoor air with outdoor air (slight oversimplification)

Filtration - passing air through a filter

1/7
Ultraviolet germicidal irradiation (UVGI) - kill or inactivate microorganisms with UV light.

Electronic Air Cleaning (EAC) - charge particles and discharge them into the space.

2/7
They all do different things with different types of indoor pollutants. There are many pollutants, but I'll deal with 3 types:

Gasses - carbon dioxide, carbon monoxide, radon
Particulate Matter (PM) - dust, smoke, pollen
Microbes (type of PM) - Viruses, Bacteria, Mold

3/7
Read 8 tweets

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