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
How it's really done: calculus

The rate of virus removal is proportional to the concentration of the virus:
-d(virus removed)/d(volume removed)= C * virus concentration
C is a constant.

This is the classic mixing problem - a differential equation.

4/5
Once you solve it, you get:
Old air remaining %=1/e^(air changes)

Fresh air % = 1-old air%=1-1/e^(air changes)

AC Fresh Air %
1 63
2 86
3 95
4 98
5 99

5/5

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More from @joeyfox85

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
Feb 2
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
Read 15 tweets
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
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
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

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