HEPA Filters - 3 Key Things You Need to Know for Classrooms

This Article Updated September 2024

I bought and tested HEPA (high efficiency particulate air) portable air filter for my main living space to help reduce viral and allergen loads. It's doing a good job. The specifications for the 170 s.f. space are shown to the right. Each model shows three ‘CADR' (Clean Air Delivery Rate) measurements, one for tobacco smoke, one for dust, and one for pollen. The tobacco smoke CADR addresses the sizes of particles that are hardest to filter. 

On the consumer site that I bought this from, they just show 110 cfm AHAM CADR, with no distinction.

If you want some more information on how air cleaners work and to find the test certificates, check out AHAM Verifide.  

Healthy Indoor Air Quality Starts With Clean Air

As a consumer product it does the job I want it to do (I have an air quality sensor, so I can back that statement up with documentation). But this is based on my knowledge of building science in the residential world. I have done a significant dive into healthy indoor environments over the past 30 years of my career as an R2000 inspector, Energy Advisor, and independent researcher/consultant.

The fundamentals of building science apply to all buildings when it comes to air quality: source control, dilution (ventilation), and filtration.

A School's HVAC System Needs Engineering

I wrote an article about opening schools last year. However, I am not an expert on controlling, maintaining and improving the indoor environment in institutional (school) buildings. And there are some great resources out there for you to dive into. But it's a lot to wade through if you're not conversant in the terms used in the field.

So I asked my colleagues David Elfstrom (an Energy Engineer & ASHRAE member in Ontario on Twitter @DavidElfstrom) and Aaron Smith (an energy Engineer & ASHRAE member in Nova Scotia, on Twitter @aaronrobsmith), to tell me simply and clearly: what should we be looking at when it comes to putting HEPA portable air cleaners for school classrooms. 

I also pulled a couple of pieces of guidance from Marwa Zaatari (Twitter @marwa_zaatari Mechanical Engineer, IAQ Consultant, USGBC Board Member, ASHRAE Epidemic Task Force, PHD with focus on IAQ/Energy/filtration. Mom & Lego Master - I am looking forward to getting to know this person!).

The List

First: Just Clean Air

Just air cleaning.

• No ozone generation option

• No ionization features

• No essential oil option

• No plasma

• No UV

HEPA is tried and true technology, and the gold standard for filtration.

Rely on it.

We're looking for high quality air filtration that cycles the volume of air in the classroom through the filters to add to existing outdoor air ventilation to achieve a total of six equivalent outdoor air changes per hour. This is the number of air passes that you need to minimize the viral load, as well as airborne particles, smoke, dust, dust mites, mold spores and other airborne pathogens.

Second: Placement

HEPA filters are no good if they're tucked into a corner. Restricting air passes restricts the filter's ability to clean air.

Here's what Marwa Zaatari has to say about placement:

1. 2-3 ft away from walls or open windows and doors.

2. Do not block the unit's air inlet or outlet, and place it as close as possible to the teacher and between the teacher and the students.

3. Use a cable cover to prevent tripping over the cord!

Third: Sizing

What are you starting with? Is there a ‘passive' ventilation strategy (ie, opening windows) or does the building have a mechanical ventilation system? There are different sizing requirements for these two situations, and both are predicated by how many people are in the room.

Marwa notes that sizing of a HEPA filter should be based on the ‘Tobacco CADR' equating to ⅔ the square footage of your classroom.

Here's what David said about sizing:

1. For classrooms WITHOUT mechanical ventilation: at least 34 m3/h (20 CFM) per person. So 875 m3/h for 24 students + 1 educator. 68 m3/h (30 CFM) per person would be better. This should be delivered at no more than 45 dB(A) as measured at 1 m from the device. So that means multiple units on lower speed for quiet operation, unless a very low noise/high flow unit is chosen.

2. For classrooms WITH mechanical ventilation: 25 m3/h (15 CFM) per person, subject to the same noise limit. This will double ASHRAE (American Society of Heating Refrigeration and Air Conditioning Engineers) and lead to 6 ACH (Air Changes Per Hour - how many times the total volume of air is changed in a 60 minute time frame).

Fourth: Noise Ratings

They need to be quiet to be used (Oh! Just like a bathroom fan or range hood in a house - imagine that!). According to David, noise has been greatly overlooked. Specifications are usually provided on highest speed only, which is higher noise level than acceptable for classrooms. An acceptable rating for a classroom would be under 50 dBA, preferably 45 dBA or lower.

By restricting the noise level and encouraging units with higher capacity, the units can be operated at lower fan speed. This allows for noisier air cleaners on high fan speed for critically important situations (unmasked eating in class) where the higher noise level will not interfere with instructional time.

Hire a Professional 

According to Aaron (and David) (and me), hiring an experienced professional familiar with industry guidance is important to ensure your installation is successful. 

ASHRAE, an industry organization of heating and ventilation professionals who write the ventilation standards for commercial and institutional buildings across North America has prepared COVID-19 response guidance.  They have information specific to various building types including schools.   As engineer-speak can be daunting, ASHRAE has prepared easy to digest one page guidance documents including one on portable air cleaners. 

This is very good, concise information.

Air Cleaner Comparison Chart

Here's another good, concise resource for those making decisions about classroom portable HEPA air cleaners: a comparison chart by Marwa Zaatari for a range of air cleaners (download it here or click on the image below).

• The horizontal axis indicates the money required to purchase and replace air cleaner filters in the first year.

• The vertical axis shows the CADR - the volume of clean air in CFM

• The colour of the dots associated with each model indicates the noise level in decibels (db).

You can find links to more resources in this October 2020 BHE Blog article: Top 8 Resources on Ventilation in Schools