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Joey Fox, P. Eng, M.A.Sc Profile picture
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Nov 20 • 9 tweets • 3 min read
HEPA vs. MERV - What's the Difference?

First, there are two things you need to know about filters - efficiency and pressure drop.

Efficiency is what % of particles of different sizes can be filtered out.

Pressure drop is how much the filter restricts the air flow.
HEPA filter stands for high efficiency particulate air filter. It has extremely high efficiency: >99.95%

They are rated H-13 or H-14. H14 goes up to 99.995%. It provides no real benefit over H13 in any normal application.
Other organizations have different ratings for high efficiency filters.

HEPA has a very high pressure drop and significantly restricts flow. It can't be used in most HVAC applications as the units are not designed for this. Portable air cleaners are.

en.wikipedia.org/wiki/HEPA
MERV stands for Minimum Efficiency Reporting Value. It goes from really poor filters (MERV 1) up to very efficient filters (MERV 16).

The efficiency required for MERV is defined in AHSRAE standard 52.2. It can be found here: ashrae.org/technical-reso…
MERV 17 = HEPA isn't a real thing. Here's confirmation from the former chair of 52.2.

HVAC applications are usually designed for MERV 7-10. These aren't that effective with small particles. MERV-13 is kind of a sweet spot between the ability to filter small particles and low pressure drop. Many HVAC units can be upgraded to MERV-13 filters.
Portable air cleaners should always be HEPA, unless they are a bad knock-off. Don't get any portable air cleaners without HEPA.

cleanaircrew.org/air-cleaners/
An exception to all of this is the Corsi-Rosenthal box. To make it less expensive, it uses MERV-13 filters which are much less efficient than HEPA filters. However, it has a much larger surface area for the filters and has a much more powerful fan than most HEPA filters.
Ultimately, because of the increased airflow and decreased airspeed through more filter area, it out-performs the vast majority of HEPA filters and is much cheaper.

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More from @joeyfox85

Joey Fox, P. Eng, M.A.Sc Profile picture
Nov 21
Can you create a bubble of clean air around you?

Personal HEPA filters and personal far-uv are tempting options as an added layer of defense or possibly as a complete defense. With difficulty, you can make them both work. Here's how.
The tools are ventilation, filtration and UV. There are no portable ventilation devices but sitting beside an open window helps. Portable HEPA filters do exist. How can they be used effectively?

I give a few options here:
If you hold it close enough to your face it could work. These tests show it would be helpful within 20-40 cm (8"-16"). That's really close and not practical for long periods. Might help on an airplane while at the gate.

Read 13 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Nov 20
Purchasing a Far-UV Lamp

Here's a review of what I know about the Far-UV market and what you can consider when purchasing a lamp.🧵
First, I'm hesitant to post this as I have not done extensive research into every possible option that exists and this will open me up to a lot of criticism (I've avoided a lot of tweets for this reason), but no one else is offering guidance. This is what I know.
There are 5 main sources I know of: Ushio, Ergo Health, UV Can Sanitize/ First UVC, Sterilray, Eden Park.

Ushio produces filtered lamps, I think they are the most common. Usually around 15 W. Here's a long list of companies that sell them:
care222.com/care222-partne…
Read 17 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Nov 17
Does Far-UV Generate Ozone or have Volatile Organic Compound byproducts?

I tested it out in my basement! Here's the experiment.

1/10 Far uv lamp and some indoor air quality monitors
I used the Carnation lamp. It's 15 W and what I intend to put in my dining room. The room in which I ran the experiment is 15'x15'x7' (4.5m x 4.5m x 2.1m). I didn't want the ozone/VOCs escaping, so I blocked the door.

2/10
I stayed there throughout the experiment and you can see the CO2 levels rising in the room indicating poor ventilation. I first let the equipment run for 10 minutes to get a baseline and at 10 minutes, I plugged in the lamp.

3/10 Indoor air quality monitors. Results described below.
Read 10 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Nov 14
I've seen it written that HEPA filters are more efficient with the windows closed.

That's speaking about pollen and not COVID. Keep the windows open.
More detailed explanation:

If your concern is pollen, keeping the windows open will let pollen keep on coming into the room.
If your concern is COVID, windows let the virus out of the room and don't bring it in.
Even more detailed explanation:

The steady state concentration of any pollutant is related to the rate it is supplied and the rate it is removed.

For pollen, it is supplied from the outdoors through the windows and removed through the HEPA filter.
Read 4 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Nov 14
Making School Buses Safe from COVID

We have the tools to make school buses both safe and comfortable. They are dealt with in detail in this video by industrial hygienist Mike Van Dyke:

Also found here: pubmed.ncbi.nlm.nih.gov/35311626/

Here's a summary 🧵

1/6
I'll first discuss the options and then results. There are 3 main tools:
- windows
- defroster
- hatches

Windows

The driver has a sliding window and there are windows at every seat. They open from 2" to half way down.

2/6
Defroster

It's the same as the defrost in your car. It blows hot air on the window. It can recirculate or intake air from the grille shown in the picture. Just like in your car, there are different speeds.

3/6
Read 6 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Nov 9
@juhashi_ Here's what I know about 15 W devices:
1. It was used here at an IAQ conference.
@juhashi_ 2. This publication indicates 1 15 W device can give 33-66 ACH for a pathogen that is harder to kill than COVID, although it doesn't use respiratory particles, so there is some variability.
nature.com/articles/s4159…
@juhashi_ 3. This data sheet shows that at 1 m away from a 15 W lamp with 100 deg beam, you will not exceed the limits after 8 hours of exposure.
uvmedico.com/lamps/uv222-sp…
Read 6 tweets

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