Blown Away

See the source image

I want to offer some advice to all of my friends who are school teachers, as well as anyone who wants to make sure that a classroom is safe for both kids and teachers as we open school again in 2020. It is unlikely that government or districts will have the money to make all of the building ventilation modifications that are needed. There is a work around, but individual teachers are unlikely to be able to afford it, so some crowd sourcing will do the trick.

The MAIN Issue is Ventilation

VENTILATION is the key to beating airborne transmission by aerosols. Being outside is best, but many places have weather that is prohibitive. Opening doors and windows is nearly as good, but many places don’t allow for that for all sorts of reasons. Most buildings are poorly ventilated, especially schools, because we build those systems for comfort and energy efficiency more than we do for air quality.

Other than everyone wearing masks, changing the air out once every 10 minutes or faster will substantially lower the risk of airborne transmission. BTW … hospital isolation rooms change the air out every 5 minutes. It’s called Air Changes per Hour, or ACH. The cubic feet per minute (CFM) times 60 divided by the room volume gives you ACH.

How do you do win the ventilation game? Get an in room air purifier with a CADR (clean air deliver rate) of at least 300. DO NOT get UV lights, or ion machines, just the HEPA. The other stuff is gimmick at this time in consumer grade products. Those technologies do work, but I don’t trust them yet in the consumer market.

However, most rooms need more than 300 CADR, so I’ve created a formula for picking the right CADR spec per unit(s). If you just want a quick way to know the CADR required to get an ideal ACH = 6, then use the formula given below before picking a simple HEPA filter in room air purifier. The formula above can be used by inputting the number of ACH you need given other sources of ventilation that the in-room purifier is supplementing.

This CADR value (cubic feet per minute BTW) will deliver an hourly air exchange rate (ACH) of 6. In other words, every 10 minutes, all of the air in the room goes through the filter. Put it in the middle of the room away from people at least 2 feet off of the floor for best circulation. Make it the centerpiece of the room.

You could also use this spreadsheet put out by some folks at Harvard School of Health. It’s all good.

Here I’ve developed a spreadsheet with two tabs … one for ACH to CADR to CFM conversions and another to compute ACH based on the wind speed of a fan unit. It is a Google Sheet, so in order to use it you’ll need to be signed in to you Google account and choose “Make a copy” or “Download > as Excel” from the File menu.

I estimate that this will cost about $500 per room, maybe less for small rooms, but it depends on the brand.

If your kids teacher can’t afford it or the school won’t buy what I described, then chip in and get it for them and for your children/grandchildren/nieces/nephews/etc.

DIY Approach

In the event that purchasing an air purifier is NOT possible, go for the DIY version of a simple fan and a MERV12 (or higher) filter that can be purchased at most any big box store. Be sure to place the filter on the intake side of the fan for better seal. THIS VIDEO shows how well they work.

Here is the evolution of a basic design supplanted by a better design of THE most basic DIY version possible. The original design works well but suffers from excessive load, whereas the improved design increases efficiency in a novel way by increasing the surface area for filtration and thus reducing the load on the fan. Better for the fan, better for the filters, better for the user.

Placement is the next question so here are some things to consider.

  1. You want to draw the air that people are exhaling away from everyone in the room rather than blowing exhaled air into other peoples breathing space.
  2. Drafting the room is ideal because it gets all of the air moving out of the room at once.
  3. Most of what you exhale will rise, so drafting at floor level is likely to do one of two things … (a) pull it back down to where people are breathing or (b) let it rise and concentrate. So put it on a table or chair.
  4. Central placement in the room, regardless of the number of air units is KEY. Cluster people around the air purifier such that no one is behind the person(s) that the air is blowing directly on. If the unit exhausts through the top, then never mind.
  5. Placing unit(s) in the corner(s) is another option since the air in the room is already moving even if you do not feel it. Thermal body plumes are driving much of the aerosol content upward and away from people.
  6. Here’s a great design by folks with advanced fabrication skills, and a follow up improvement. Then this guy takes it to a whole new level.

NOTE: there is a difference between MERV filters and HEPA filters. See these articles by Jim Rosenthal from Tex-Air Filters.

How Do you Know It’s Working?

Finally, purchase a CO2 (carbon dioxide sensor) for about $150. Indoor CO2 concentrations are an indirect measure of good ventilation and good ventilation means that COVID aerosols are not concentrating in the room. Indoor Air Quality (IAQ) experts leading the field in the fight against COVID recommend keeping indoor CO2 concentrations less than 800 ppm. Outdoor CO2 is about 400 ppm.

What do you do if the in room CO2 sensor goes above 800ppm? Open doors or windows, take a break, etc. Do something that ventilates the room. Dr. Jose Jimenez at the University of Colorado at Boulder is an atmospheric chemist who discusses how this is done here.

How to Compute the Risks?

It’s not easy … unless you borrow this spreadsheet created by Dr. Jose Jiminez, an Atmospheric Chemist who works with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder. Basically, given the dimensions of your room, the number of people, and a bunch of other data points, you can compute the absolute risks for one event or recurring ones. A simpler version put out by Duke uses the same tool, but has less input option; however, if your concern is just for a classroom, this is perfect.


Let’s do this. Share this post. Email it to people. Etc.

NOTE: I am glad to help anyone who needs clarification or more details. We can setup Zoom calls etc.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: