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Joey Fox, P. Eng, M.A.Sc Profile picture
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Feb 2 • 15 tweets • 4 min read
1/ HEPA Filters and the #corsirosenthalbox
12 Threads

It's just a fan and a filter, but there's a lot to discuss.

Table of Contents

4/ HEPA Filter Myth vs. Fact
5/ Comparing HEPA Filter and CR Box
6/ HEPA Filter Selection for Classrooms

Continued...
2/ TOC Continued

7/ HEPA Filter for your Home
8/ Calculating What HEPA Filter Does
9/ Windows and HEPA Filters
10/ Convincing Schools to Put in a CR Box
11/ Mini CR Box
12/ They [Don't] Blow the Virus Around
13/ HEPA Filters Don't Interfere with Ventilation
3/ TOC Continued

14/ Myth: HEPA Filters not Required with Good Ventilation
15/ MERV-13 Filter in Air Handling Unit vs. Space HEPA Filter
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More from @joeyfox85

Joey Fox, P. Eng, M.A.Sc Profile picture
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
Joey Fox, P. Eng, M.A.Sc Profile picture
Feb 1
WHEN PLEXIGLASS IS USEFUL AND WHEN IT ISN'T 🧵

Plexiglass can be useful in specific situations, but harmful in others. To know why, you need to know the different methods of airborne transmission.

1/7
#COVIDisAirborne and transmits in three general situations:

1. Short Range
2. Shared Room
3. Long Range

2/7
Plexiglass stops short range transmission as it prevents you from breathing in the concentrated aerosols directly leaving the infected person's mouth/nose.

3/7
Read 7 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Jan 31
When we go back to the office, small personal HEPA filters are an additional mitigation measure you can use.

You won't have control over the entire environment and there might be COVID containing aerosols circulating in the room, but...

1/5
You aren't breathing in the air from the whole room. You are breathing in the air next to your face. If you can ensure that the air in your immediate vicinity is clean, you won't be breathing in virus particles.

2/5
You can purchase a small HEPA filter, but it's important to keep it close to you. Just like short range transmission can occur in a room with good ventilation, short range mitigation can work in a room with poor ventilation.

3/5
Read 6 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Jan 30
MERV 13 in Air Handling Unit vs. Space HEPA Filter 🧵

Many schools have chosen to upgrade the filters in the air handling units from MERV 8 to MERV 13 instead of putting in stand-alone HEPA filters. These 2 methods are not equal in risk reduction.

1/8
TRANSMISSION

There are 3 situations for airborne transmission:

1. Short Range
2. Shared Room
3. Long Range

Distancing stops short range transmission. Distancing doesn't prevent shared room transmission.

2/8
For shared room and long range transmission, ventilation and filtration reduce the concentration of infectious aerosols.

The important point is shared room is much higher risk than long range transmission. This is how super-spreader events occur.

3/8
Read 8 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Jan 26
COMPARING HEPA FILTERS AND THE #corsirosenthalbox 🧵

Both HEPA filters and CR Boxes do the same thing - they filter the air. Which one is better? I'll compare the CR Box to 2 HEPA filters: Levoit LV-H133 & Austin HM400.

Criteria are: CADR, noise, cost, structure.

1/7
CLEAN AIR DELIVERY RATE (CADR Dust CFM)
CR Box: 290-600
LV-H134: 274
HM400: 250

NOISE (55 dB is about the threshold for tolerable in a classroom.)

CR Box: 49-54 dB
LV-H133; 54 dB
HM400: 62 dB

COST (CAD before tax)
CR Box: $90-120
Levoit LV-H133: $329
Austin HM400: $850

2/7
STRUCTURE

CR Box: 10 lbs
Levoit LV-H133: 21 lbs
Austin: 47 lbs

CR Box is bulky and easy to break. It can be easily damaged with a pencil. But the large shape is what allows it to filter so much air. The filters have a lot of surface area and the fan is 20".

3/7
Read 8 tweets
Joey Fox, P. Eng, M.A.Sc Profile picture
Jan 26
PUTTING MERV-13 FILTERS IN AIR HANDLING UNITS 🧵

Many schools have replaced the standard MERV-7 or 8 with MERV-13 filters in the ventilation systems. It's a simple method to filter viruses from recirculated air, but doesn't always work. Here are some issues to consider:

1/6
MERV-13 filters are efficient at filtering out aerosols that contain viruses unlike MERV-8, but they also create a greater pressure drop and restrict flow. This is why ASHRAE recommends units to be rebalanced when filters are upgraded:

ashrae.org/technical-reso…

2/6
What happens if this isn't done? Air handling units could have 2 purposes: ventilation only or ventilation + temperature control.

If it's ventilation only, you will be providing less fresh air to the space. There will still probably be a net benefit, but this isn't ideal.

3/6
Read 6 tweets

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