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Step 2: Choosing your classroom purifier

We have put together a nifty Excel calculator that lets you calculate how many purifiers you need per class; the number of air changes you need; and the total cost per class. This calculator does all the work for you in 3 simple steps.

Calculator for Schools
Download XLSX • 49KB

This might not work well in a browser so save the calculator to your desktop then follow the instructions below:

  • On the tab, enter the area (length x width) of the classroom and the ceiling height and select if you are entering in feet or metres

  • The blue box will show you the total CADR you need per class under ASHRAE and NEU guidelines

  • Toggle the 3 boxes in Step 3 to see the results of Smart Air purifiers on different settings

That’s it. There are notes at the bottom of the calculator to help you understand how we have derived at these figures. Essentially you need to factor in the following with all purchases:

  • Noise levels – ideally you want below 50dB in total for one or multiple units, maximum of 55dB. A purifier is an expensive choice if the teacher switches it off because the class can’t hear them.

  • Filter replacement costs can be substantial so work out the cost over a 4-year period

  • Go for a higher CADR than you need to give you faster clean air

  • Will you be opening the windows as part of your COVID/Ventilation policy? If so, the air purifier will work but at a reduced capacity. The Smart Air tested their purifier with the window open and found that it still cleared particle pollution, but at 60% capacity: Do Air Purifiers Work with the Window Open?

Compare other purifiers with Smart Air purifiers

(you can skip this step if you are only interested in Smart Air purifiers)

If you want your teachers to be heard in class, you will need to gather more details than most manufacturers share online. Don’t be afraid to approach suppliers with a checklist of questions, after all, you are going to be spending a substantial amount of money on filtration, so you need to know if a product will meet your specific needs. Here is a handy checklist of the information you need to do a comparison:

  1. What is the noise range (dB) on each setting? If you are running 3 purifiers each at 60dB, you will have a total noise level of 66dBA which is too loud for teachers to be heard comfortably

  2. What is the CADR on all settings of the purifier? Without this, you can only work out the clean air on the highest setting and it will be pure guesswork as to whether the purifier will give you the air changes you need on a lower setting

  3. If CADR hasn’t been linked to Smoke, Dust or Pollen, ask if CADR is based on Airflow or what it has been certified with If they can’t answer this, calculate 60% of published CADR to account for air mixing – see learn the lingo

  4. What is the HEPA lifespan based on? For example, Beijing air, Swiss air, UK air – also, is it based on running 24 hours a day or 8 hours a day This information is not required for the calculator, but it can help you to know when the filters really need changing

  5. What is the recommend distance away from the walls the purifier should be placed? This will help with the third part of this blog 'Step 3 - Best placement for your purifier'

  6. Does it emit any ozone? Please avoid any air purifiers that omit ozone

Great, now you hopefully have enough information to compare. Go back to the Excel calculator, enter the information you have gathered on the 'Air Purifier' tab, then return back to the 'Schools.tool' sheet to see the results.


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