This is a complex topic - but so important. Thank you to all educators out there. I hope you can stay safe for yourselves and for those around you .
One of the problems with COVID guidelines is that they are written by medical people who did not learn about ventilation in med school. Many have learned along the way, but I have found some of the best guidance from mechanical engineers - who are the experts on air quality. From a new study from U of Wisconsin (the parenthetical remarks are mine from the article):
**Leakage rates around masks are typically in the range of 20-80% when fit reasonably well to the user’s face, resulting in effective filtration efficiencies that are much lower than the material filtration efficiency. (Because of leakage even surgical procedure masks can be grossly inefficient.)
**Use of masks and ventilation simultaneously are synergistic (providing multiplicative reductions) and together can provide greatly reduced aerosol transmission infection probability. (Ventilation at the rate of 5 air exchanges seems to be a good compromise between efficacy and efficiency. This can be achieved using appropriate portable air filtration systems.) From studies by Dr. Richard Corsi, dean of engineering at PSU, most school ventilation systems are inadequate. A really good one might achieve 3 air exchanges an hour, most are much less. And they usually can be upgraded only a little. The study above determined that 5 air exchanges an hour is a good compromise between safety and efficiency. So, use of portable air filtration systems in the classroom is important. Dr. Corsi - my go-to expert on this, Dean of PSU Engineering School and an international expert on school ventilation has this recommendation:
You may want to go straight to the manufacturer if you can and tell them that these are for K-12 schools and you'd like to inquire about a bulk discount. If there are 30 classrooms and some office spaces in a school you could get by at around $7,000 to $8,000/school. I spent $5K on the engineering building at PSU. He made the point that this wasn't an endorsement, rather information about one system that worked for his situation: Honeywell HPA300
A couple of tips. The filters will need to be changed perhaps twice per school year (every four months or so). Before changing the filter switch the system off on a Friday afternoon and let it sit all weekend. This makes sure that no more viruses are collected on it and many viruses will inactivate over the weekend. replace it on Monday morning with gloves, mask and goggles. Place it in a plastic garbage bag with tie down.
Portable HEPA filters should be placed at least three feet away from walls to avoid recirculation in a stagnant zone. They can be a bit noisy, but that's the price one pays for a system that actually works. It is best to leave it on high setting and also to leave it on when students exit the classroom for some period of time (to reduce particle levels in the air before everyone comes back in).
Dr Corsi's presentation to East Washington Dems on December 7, 2020 can be seen here: