THEY BLOW THE VIRUS AROUND
This is one of the most prevalent lies against HEPA filters. It’s been used by school boards and politicians as an excuse to not put HEPA filters into schools. This myth needs a thorough debunking.
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This is one of the most prevalent lies against HEPA filters. It’s been used by school boards and politicians as an excuse to not put HEPA filters into schools. This myth needs a thorough debunking.
1/9
AEROSOL SPREAD IN A CLASSROOM
The air in a classroom is well mixed. Anyone who is concerned about blowing the virus around needs to know that it's already been blown around. Here's a diagram of what classroom airflow looks like:
2/9
The air in a classroom is well mixed. Anyone who is concerned about blowing the virus around needs to know that it's already been blown around. Here's a diagram of what classroom airflow looks like:
https://twitter.com/healthyheating/status/1481041411776520192?s=20
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Here's a document from ASHRAE discussing the well-mixed assumption for classrooms.
"...the well mixed assumption in the Wells-Riley model may hold better for these [smaller] particle sizes."
ashrae.org/file%20library…
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"...the well mixed assumption in the Wells-Riley model may hold better for these [smaller] particle sizes."
ashrae.org/file%20library…
3/9
The full paper is here.
H/T @DavidElfstrom
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H/T @DavidElfstrom
https://twitter.com/DavidElfstrom/status/1447269292811202575?s=20
4/9
Aerosol concentration is higher at the source, but distancing in a classroom all day long will not protect you. If someone is smoking in a classroom all day long, being 4 m or 6 m away won't help reduce your smoke inhalation. You need to remove the smoke from the room.
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5/9
WHAT THE HEPA FILTER DOES
The HEPA filter takes the air in the room, removes aerosols and outputs clean air. Once this clean air mixes throughout the room, it dilutes any aerosols and reduces how many you breathe in.
6/9
The HEPA filter takes the air in the room, removes aerosols and outputs clean air. Once this clean air mixes throughout the room, it dilutes any aerosols and reduces how many you breathe in.
6/9
Is there a concern that if the HEPA filter is blowing on someone infected, that someone downstream gets a higher concentration of their aerosols? Maybe.
If it blows air sideways towards someone, it should be at least 2m away.
7/9
If it blows air sideways towards someone, it should be at least 2m away.
https://twitter.com/MichaelYartys/status/1481690170902650884?s=20
7/9
It should be placed properly, but if it's not, there might be a possibility of a slight increased risk for a few. However, there is a certainty of reduced risk for many people. When it comes to minimizing risk for everyone, there's no doubt that a HEPA filter is worthwhile.
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8/9
Because of the possibility of increased risk to a few people, it's best to have a HEPA filter that discharges the air upward. It avoids any issues and makes this entire line of thinking moot. Most of the top HEPA filters are designed like this.
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9/9
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