Thread by @joeyfox85

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

https://twitter.com/fishtron/status/1479980573430980612?s=20

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

https://twitter.com/jljcolorado/status/1479250147330637825?s=20

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

AIR HANDLING UNITS (AHUs)

Depending on the design of the school, the AHUs can supply a single zone as shown here. Some can supply 3 or 4 classrooms. There can also be a single AHU for an entire floor of 20 classrooms or even one for a whole school.

4/8

https://twitter.com/DavidElfstrom/status/1484560585589391365?s=20

If the AHU supplies one zone, then using a MERV-13 filter will mitigate shared room transmission as it will filter out the recirculated air in the room.

5/8

If one AHU supplies an entire school, the air is returned from the entire school and mixed with fresh air before being supplied back to the school. Using a MERV-13 filter here would prevent long range transmission which is a lower risk.

6/8

The risk increases as there are more infected people in the school which means more infectious aerosols circulating, so MERV-13 filters still help. However, it doesn't provide additional shared room mitigation.

Space HEPA filters prevent shared room transmission.

7/8

Ultimately, it's possible that mitigation measures for long range transmission are being put in place and are being used as an excuse not to use mitigation measures against shared room, which is much more likely to cause infection.

8/8