CALCULATING WHAT A HEPA FILTER DOES

Portable air purifiers (HEPA filters) are not magic machines that you turn on in the corner of the room and they make COVID go away.

They usually are advertised with a square footage, but that's not accurate. Here's what it means:

1/10
I'll use the Austin Healthmate HM400 as an example. It says it's designed for a 1500 sq ft room. Assume 9 ft ceilings. Total volume = 13500 ft^3.

The advertised CFM (cubic ft/min) is 400, but it's really 250 with a filter: cdn.shopify.com/s/files/1/1002…

2/10

austinair.ca/products/healt…
Using their advertised CFM, 13500ft^3/400 CFM = 33.8 minutes/air change = 1.8 Air Changes/Hour (ACH).

The true clean air delivery rate (CADR) is 250 CFM: 60*250/13500=1.1 ACH

The claim is it "effectively cleans an area of 1500 sq. ft". That's not what it does.

3/10
It provides 1.1 ACH for an area of 1500 sq ft when running on high speed. It also doesn't filter all the air in the room well when it's put in a corner. It should be placed near the center of the room.

4/10
What happens on low speed? Can't find the data on low speed with filter, but without the filter, it is 75 CFM. That gives you 0.3 ACH.

Typical classroom of 750 sq ft gives you 2.2 ACH on high and 0.6 ACH on low.

5/10
Another way to calculate - ignore the area and look at the people. 25 people in a room - 250 CFM = 10 CFM/person. 75 CFM = 3 CFM/person.

This adds to the existing system. If you have ventilation providing 3 ACH and your HEPA filter has 2 ACH, you now have 5 ACH.

6/10
If your existing system has 10 CFM/person and this adds 10 CFM/person, you now have 20 CFM/person.

What do these numbers mean? How to interpret?

7/10
For total CFM:
10 CFM/person=poor
15 CFM/person =typical
20 CFM/person=good
25 CFM/person = really good
30 CFM/person = good for smoking conditions

8/10
For ACH, see 👇


Note: Modern day ventilation calculation as per ASHRAE 62.1 combines the expected occupancy and square footage of the area. For classrooms, it ends up around 15 CFM/person.

9/10
Ultimately, ignore the "sq ft" they advertise. Find the clean air delivery rate (CADR). Use the volume of the room to get the ACH=60*CADR/volume or divide the CADR by the amount of people to see what the change is in CFM/person.

10/10

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More from @joeyfox85

30 Dec
WAYS TO KEEP YOU KID SAFE AT SCHOOL

Many are very worried about what's going to happen Monday. If you are sending your kid back to school, there are things that you can still do:

1. Ask the teacher to keep the windows open as much as possible.

1/5
2. If there is a HEPA filter in the classroom, ask the teacher to move it closer to the center of the room and run it as fast as possible (noise permitting).

3. Ask the teacher if they can check the ventilation themselves:

2/5
4. Get your kid a respirator. We get ours from Cangard because they have nice designs and my kids like them:
cangardcare.com/product-catego…

Here are other Canadian manufacturers:
cappem.ca/disposable-res…

3/5
Read 6 tweets
29 Dec
3 WAYS YOU CAN CHECK VENTILATION YOURSELF

Before checking, you need to find where the air is coming from. It's usually from a rectangular, circular or linear diffuser, or from a grill. Usually on the ceiling, but sometimes on a wall. Here are some examples.

1/7 ImageImageImageImage
The air returns through a grill as well. It can look like these.

2/7 ImageImage
1. LOOK AND LISTEN

I'm always looking up at the vents and listening when walking into a room. People think that's weird. Start doing that. You can usually hear the air moving. If you hear it turning on and off, you know it's not being run properly. It should always be on.

3/7
Read 7 tweets
27 Dec
If you want to make schools safe now, we can't do any half measures. Here's what to do:
1. Rapid test everyone, every day.
2. Windows open as much as possible
3. HEPA filters in every classroom. Double/triple up if possible. Use CR boxes.

1/4
4. HEPA filters near center of the room. Not in the corner.
5. Run the HEPA filters as high as possible. Not on low.
6. Everyone wears a respirator - N95 or KN95
7. Caretakers need to run ventilation checks every day.

2/4
8. Get creative about eating without masks - use larger spaces with good ventilation: libraries, gyms, auditoriums or outdoors. Quiet lunches.
9. No removing masks indoors - frequent outdoor mask breaks.
10. Change schedule to have more recess and let room air out.

3/4
Read 4 tweets
15 Dec
#covidco2 HOW CAN CO2 MONITORING HELP IN SCHOOLS? 🧵

When humans breathe out, 4-5% is CO2. If you are in a poorly ventilated area, it builds up. Measuring CO2 in a room tells you how much air has already been breathed in.

1/25
COVID spreads by breathing in air that an infected person has breathed out. It makes sense to closely monitor the CO2 levels. Higher CO2 levels indicate a higher risk environment where you are more likely to get infected.

2/25
If you are going to a public place with other people, you want to reduce your risk as much as possible. By monitoring the CO2 levels, you get a good assessment of your risk. If you are indoors with other people, it's best choose a place with lower CO2 levels.

3/25
Read 25 tweets
6 Dec
HUMIDIFICATION 🧵

Humidity control is one of the unused resources in air quality. However, it isn't something that can easily be used in the places where you would catch COVID, but it can be used in your home to prevent you from getting sick when you go out. Why?

1/7
Your body has natural defense mechanisms to prevent catching airborne pathogens. Exposing yourself to very low humidity over long periods of time in the winter weakens this. This can be easily remedied by keeping your home at 40% relative humidity.

2/7

In other words, having good humidity control in your home is actually a scientifically justified "immunity booster". It's very easy to run a humidifier and you will be healthier.

3/7
Read 7 tweets

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