Masks4Canada Room Ventilation/Filtration Guide

and Tip Sheet

Protect yourself and your family from more contagious strains of the virus.  We do not endorse any particular company and receive no funding or royalties from, and have no ties to any of the listed products.  You can contact us here if you want to suggest changes, or reach out to @kashprime, @lisa_iannattone or @davidelfstrom on Twitter.

Improving ventilation has been one of the biggest missed opportunities in this pandemic, especially with greater and greater recognition that Covid is airborne.  We have long advocated this position, with our open letters and vocal advocacy in the media, together with an international group of scientists.   Below we will explain basic terms and and give you the tools you’ll need to secure your office or classroom and protect everyone better.

CO2 Monitoring

Generally room CO2 can be used as a rough marker of fresh air in a given room.  The higher the count, the more chances you are rebreathing air from another person.  If that person has Covid-19, you will inhale infectious particles that will give you the disease.  This neat spreadsheet from @davidelfstrom does the math, also described by Dr. Richard Corsi.   Many classrooms in older buildings can have CO2 levels as high as 3000 ppm, in which 7% of the air in every breath you take comes directly from other people in that room.

You can use commonly available devices to measure your room CO2 levels.  These are best done when the rooms are occupied.  Generally you want levels to be as low as possible, as low as 700 ppm or lower.

CO2 Monitors that we have used:

HEPA Filters

You can use HEPA (High Efficiency Particulate Air) filters to remove infectious particles from the air in rooms where you can’t improve with outdoor ventilation.  These are measured with the CADR (Clean Air Delivery Rate).  This is given in units of CFM or Cubic Feet per Minute.  

Let’s take a typical medical clinic office.  It is usually about 75 square feet, with 8 foot high ceilings, which is typical for a home or small office.  With a small HEPA filter with a CADR rating of 50, you can get 5 air changes per hour on the highest speed setting.  Add to the 1 ACH you get in a badly ventilated room and you get close to the standard of 6 ACH used in most hospital settings.   Here’s the math:

Reducing Noise

One issue is noise; the CADR rating is usually measured at the filter’s highest setting, which can be quite loud.  This unit in the picture makes 60 dB at its highest setting, about the same as a conversation.  This would be distracting in a classroom or office setting.  You can mitigate this by placing 2-3X as many filters and running them on a low or medium setting, and running them on the highest setting during meal times.

This is an excellent graph created by @marwa_zaateri that compares various commercially available HEPA filters, charts them by cost and the noise they generate at their highest settings.

How to Choose a Good Filter

1. High CADR (Clean Air Delivery Rate)

2. Low noise level on the highest setting

3. Relatively low velocity fan, preferably directed upwards

4. High energy efficiency - try to get one Energy Star rated if possible

5. Carbon filters don’t matter (meant to stop odours), and can actually make the fan blow harder

6. UV filtration probably not useful, better used by specialists and in ‘upper rooms’

7. Ionization is possibly not a great technology.  It adds cost with questionable health impacts, maybe some issues from the Ozone that’s generated as a byproduct. If present, it should be turned off.

For Classrooms

Harvard Healthy Buildings has a number of resources that can be used to calculate the ventilation needs in a large classroom.  This spreadsheet can help you calculate this (make a copy so that you can enter your own data)

Improvised HEPA Filters

Understandably,  demand for filters has exploded in the last year, and supply might become an issue.  You can improvise an air filter using MERV 11 or 13 grade furnace filters sealed to a portable fan. The simplest version attaches one filter to the face of the fan. You can improve the efficiency by increasing the surface area of the filters: 1 thicker filter (2 inch instead of 1), 2 filters in a wedge shape or 4 filters in a cube, dubbed the ‘Corsi Box.’  This article goes in more detail.  More details here, and this article discusses a modification that you can use to improve the efficiency of the filter by directing the air through the middle of the fan.  Box fans are in short supply these days, but you can use cardboard to fit most fan shapes, as @ladyscorcher does here:

Here’s a single 2 inch filter sealed to a box fan and another example of the 2 filter wedge design:

These improvised fans can be quite effective, one tested by CBC Marketplace produced a Clean Air Delivery Rate (CADR) of 100 CFM, in line with commercial filters that cost hundreds of dollars.

Examples

1. Classroom with very poor ventilation -

1. Your CO2 detector shows >700 ppm with people inside
2. 800 sq.ft. 25’ x 32’ with 10 foot ceilings = 8000 cubic feet.
3. You will need air filters that total a CADR (Clean Air Delivery Rate) of 800 CFM (cubic feet per minute to exchange the air 6 times an hour ( (8000 cu.ft. x 6) / 60 minutes )
4. Assume the room itself provides only 1 air change an hour (which is terrible)
5. You can buy 2 of these models, which each provide a CADR of 323 CFM each to get close to that number (323 x 2 = 646 CFM).  You can also use 3 improvised home-made Corsi Boxes (each has a CADR of around 200 CFM)
6. Plug these numbers into this spreadsheet, the filters bring it up to 5.8 air changes an hour (ACH).  This is very close to hospital grade (which is 6 ACH).  
7. For $600 (~$25 per student) you have protected them incredibly well from an airborne virus.  Filters are good generally for one year.
8. You can buy 4 of them (twice as many as required) and run them at lower settings if you want to limit noise (these ratings above are when the filter is running at full speed)

2. Medical Office with poor ventilation

1. Your CO2 detector shows >700 ppm with people inside
2. 75 sq.ft  7.5’ x 10’ room with 8 foot ceilings = 600 cubic feet.
3. You will need air filters that total a CADR of 60 CFM ((600 cu.ft. x 6) / 60 minutes)
4. Assume the room itself provides only 1 air change an hour
5. You can buy one of these models which provides a CADR of 50 CFM
6. Plug these numbers into the spreadsheet to find that you can bring your room circulation up to 6 air changes an hour (6 ACH), which is hospital grade.

7. For ~$90 you can protect yourself and your patients much more effectively
8. You can buy two or three of them and run them at a lower setting if you want to minimize noise

General Advice

This pandemic will be over for most of us soon.  This is some general advice for you to follow to make things safer for unvaccinated family members (i.e. kids <12) to keep safe until then.