top of page

Everything you need to know about buying an air purifier for your class

We often get enquiries from schools or parents wanting to find out how to best protect their children in school. There is a lot of information out there, so we have simplified this blog to help you choose the right purifier for your classroom.

To avoid information overload, we have split this into three blogs:

  1. Learn the lingo – What the heck is CADR and how can you compare purifiers if there are different ways of calculating CADR?

  2. Choosing your classroom purifier – Step by step guide with a simple Excel calculator designed to do all the work for you

  3. So you have your air purifier, now what? – Top tips on the best placement (how 4cm changed filtration from 5% to 94%), timings and filter lifespan

Step 1 - Learn the lingo

Air purification is an unregulated industry with plenty of techno-babble, false promises and misselling – not always but if you don’t know your CADR from your ACH, how will you know what if what you are purchasing will truly keep children safe from airborne transmission and particle pollution?

Reading time is 6mins, but I promise you will have a greater understanding of what you should be looking for when sifting through sales brochures 😊.

Air Change per Hour (ACH)

ACH is the number of times air is cleaned through your filter an hour. The guideline for US schools from ASHRAE is 5-6 but in the UK, the NEU recommends 6 ACH.


Airflow is the flow rate of air measured in CFM (cubic feet per minute – US) or cubic metres per hour (m3/hr – UK and EU). Online calculators allow you to convert from one to the other easily.

Airflow assumes that all the air coming out is always 100% clean rather than telling us how much actual clean air is coming out, or how efficiently the purifier is mixing the air in the room.

Clean Air Delivery Rate (CADR)

CADR is the most important metric you need for comparing purifiers on a like for like basis – assuming it is certified (see caveat below). There are two ways to certify CADR:

  • Smoke, pollen and dust (US)

  • Smoke/formaldehyde (China) - All Smart Air purifiers are certified in China using smoke

A CADR test is run in a sealed room with elevated pollution levels. PM2.5 concentration readings are taken every 2 minutes for 20 minutes and using complex equipment and data analysis (particle degradation and air mixing) to produce the ‘rate of purification’ (the CADR).

As CADR tests are carried out in a small room, there is a ceiling limit of 890m3/hr. Any purifier above this will not have been certified using US or Chinese guidelines and are likely to be based on airflow.

Important caveat – Airflow derived CADR

This appears to have become an accepted method of calculating CADR outside a lab but Airflow and CADR are not interchangeable and referring to both as CADR makes like-for-like comparisons impossible. We’ll use the Blast as an example to show you how different the figures can be:

  • Blast Mk II certified CADR using smoke: ceiling limit CADR of 890m3/hr

  • Blast Mk II ‘airflow derived CADR’ is 1999m3/hr (Airflow of 2000m3/hr x HEPA 99.95%)

Labelling Airflow derived CADR as ‘CADR’ makes it impossible to know if you are comparing a certified figure with an inflated figure based on airflow.

If CADR is not assigned to smoke, pollen or dust, there is a strong likelihood it is uncertified and measured using airflow derived CADR

A final word on CADR is that a very well-known brand of purifiers adds up all 3 CADR figures for smoke, dust and pollen to promote an amazingly high CADR figure in their marketing campaigns. However, if you have the time to go through the FAQs, somewhere on page 3-4, you will see the true figure but I doubt everyone who purchases is aware of this.


Air purifier companies either knowingly or unknowingly mis-sell their purifiers on the basis of PM0.3. What's so important about this figure? This is the smallest particle size that travels in a straight line so is the hardest size to capture. Particles smaller than this, including viruses, travel by zigzagging (called the Brownian Motion) making it close to impossible to pass through the HEPA filter.

According to NASA and numerous peer-reviewed research, HEPA filters will trap virtually 100% of nanoparticles, also referred to as ultrafine particles. Yet here is what a very well-known Swiss brand is touting on their UK website:

“The air we breathe has a direct impact on our health and well-being. It is especially the fine and ultra-fine pollution that causes the most severe damage to our immune system. 90% of all ultra-fine particles are smaller than 0.3 microns, and most air purifiers only filter particles larger than this. This means that most air purifiers are only filtering 10% of the particles in your air.”

Bottom line: HEPAs are graded on their ability to trap PM0.3, This means that if a HEPA filter is graded as 95%, it will trap 95% of PM0.3 but virtually 100% of other particle sizes including viruses.

You shouldn't need to know about PM0.3, it is only here because this is how companies get you to part with spending more money than you need. to.

Medical grade HEPA

Filters either meet or fail the HEPA standards so the terms ‘medical-grade’ or ‘true HEPA’ are simply marketing terms. The Blast units have HEPA13 filters but we don’t call these ‘medical-grade’ as this is not an official standard.

The unspoken sacrifice is the higher the HEPA grade, the lower the airflow which means you will get less air changes per hour unless you have a crazy powerful fan.

Here is an example of how the Sqair performs in a 15m2 room with both filters:

  • HEPA11 - CADR of 315m3/hr. Cleans the air in 7 mins and gives you almost 8ACH

  • HEPA13 - CADR of 220m3/hr. Cleans the air in 10 mins and gives you almost 6ACH

For standard Indoor Air Quality, this isn't a lot of difference but during COVID times, that's 3 extra minutes of lingering aerosols.


Like HEPAs, UVGI (Ultraviolet Germicidal Irradiation) has been around for the same amount of time and is very efficient at killing pathogens. It can usually be found in washrooms and hospitals to create sterile environments but in the pandemic, it is beneficial for crowded spaces where airborne transmissions are likely to occur before the HEPA filtration can capture it.

It is important to note that UVGI isn’t the same as indoor air quality as it only kills pathogens. Filtration is essential for particle pollution which has short and long-term health impact on kids. HEPA filters will trap viruses (which can’t survive on surfaces for long) and other particles so can serve both functions. We won’t be in a pandemic forever and UVGI is expensive, but schools can benefit from this in areas like canteens and corridors with large crowds and raised voices in addition to filtration – the two work well together. In classrooms, HEPA filtration will do an amazing job of keeping the virus and other particles at bay.

So will a purifier with both HEPA filters and UV allow you to kill two birds with one stone? Sadly not, UV is not the same as UVGI and the vast majority will not have strong enough UV. This Smart Air article explains it in detail:

The UV in an air purifier will only kill the virus which as already been trapped by the HEPA filter so why risk UV exposure?

Last word on air purifiers

All you need for clean air is a fan and a filter, that’s it. The ‘7 layers of filtration’, plasma, patented technology etc are all designed to help you part with your money. HEPA filters have been around for 80 years, and they work…

  • Used by the British military in WWII for gas masks

  • Used by the US military for the Manhattan Project to prevent the spread of radioactive contaminants

Pair this with a powerful fan and you have all you need for clean air.


bottom of page