Air Purifiers: Applying Science to Rehearsal Spaces

Our district started this fall with in-person learning, and families have the option to choose remote learning. While the weather holds, we are rehearsing our bands and choirs in small groups outside. This has given us time to make preparations for the move indoors. Air quality for the indoor rehearsal spaces is one aspect our department has been considering.

The Performing Arts Aerosol Study recommends the use of HEPA air purifiers for indoor band and choral rehearsals. Beyond this, a team of active researchers investigating aerosol transmission of COVID-19 have compiled an extensive FAQ, which includes specific information on filtering and “air cleaning”.

The evidence is clear: adding HEPA air purifiers to indoor rehearsals can have a measurable impact on lowering risk for the spread of COVID-19.

In this post, I’ll walk you through how we applied this information to our indoor rehearsal spaces to augment our COVID-19 mitigation strategies.

Our virus exposure risk is a function of aerosol concentration and time.  For environments where we can’t reduce time, we can reduce the concentration. Portable air cleaners can effectively reduce indoor concentrations of virus relevant aerosol sizes.

— FAQs on Protecting Yourself from COVID-19 Aerosol Transmission

The Tool for the Job

The Harvard-CU Boulder Portable Air Cleaner Calculator for Schools is relatively easy-to-use spreadsheet for getting a sense of the needs for rehearsals spaces.

We started our calculations with these following information and assumptions:

  • the square footage for the room

  • the ceiling height for thee room

  • an assumed an ACH (air changes per hour) rate of 1.5

Using the 1.5 ACH rate as a starting point was a bit of a guess on our part. The ASHRAE engineering standard is 3 ACH for classrooms, and a study of California schools found an average rate of about 0.4 ACH. The Harvard-CU Boulder Tool lists an average school classroom at 1.5 ACH. Our thinking is that 1.5 ACH was a good assumption of our current facility, erring on the side of caution by using a conservative “guesstimate”. 

Applying the Parameters

The Harvard-CU Boulder Tool has several tabs, and the “Expanded tool” on the third tab gives a good overview and flexibility in applying the different parameters. After experimenting with the spreadsheet a bit, and researching various manufacturers and  models, we settled on Blueair air purifiers for several reasons.

(Now that we have used them a bit, I've found the Blueair 211+ very easy to set up and use.)

To get started, download the Harvard-CU Boulder Tool as an Excel file. In the “Expanded tool” tab, you can use any of the rows, changing the values to match your rehearsal space and air purifier. The models listed are there as examples.

One of our rehearsal spaces looks like this:

  • 1415 square feet

  • 10 foot ceiling height

  • CADR of 350 cfm (cubic feet per minute) for Blueair 211+

With one Blueair 211+, the resulting ACH is 3.0.

Ex. 1: one Blueair 211+

Though this ACH would meet the ASHRAE standard, we had a goal of exceeding the standard, before taking other mitigations like opening windows and increasing outdoor air mixture with the mechanical air handlers.

Here’s the good news: adding more air purifiers has a cumulative effect.

Ex. 2: two Blueair 211+

When we add a second 211+ to the equation, raising the CADR to 700 cfm, we now have an ACH rate of 4.5. Our hope is that this underestimates the actual ACH when factoring in the other mitigations.

Clearing the Air

Admittedly, all of this is well beyond our traditional role as music educators, and it can feel a bit uncomfortable making safety recommendations based on a “best guess”.

If you feel this way, you are not alone. At the most basic level, we are charged with making decisions for the safety of our students. Taking steps like adding these air purifiers has made me more confident in my ability to teach my students, knowing that I’m taking extra precautions to keep everyone healthy and safe.

In “normal” times, we integrate music advocacy to support our programs. With the tools and information available, you can be an informed safety advocate for your program.