Step 3: Top tips on what to do once you have your purifier including best placement

Where should you place your air purifier?


The best placement for an air purifier is in the middle of the room, but this is rarely feasible. Most classrooms will have more than one unit so look to place them (a) where the most contaminants will enter and (b) wherever has the most airflow. Air movement is important as it lifts pollutant particles making it easier for the HEPA filter to trap it and clean the air.


Good placement

  • Close to the classroom door as the air moves each time the door is open, plus you may find a high level of contaminants from the corridor

  • Windows let in draughts and possibly outdoor pollution making this the next perfect place for your purifier

  • Air moves vertically and horizontally. The tall Blast is ideal to capture both but if you are using the smaller Sqair, it is best to position this on a desk or unit to enable it to also access horizontal air movement

  • Make sure you know the recommended distance from obstructions / walls. Smart Air tested their products found that their purifiers should be 10cm away from the wall. If you have different purifier, contact the supplier to find out the correct placement. Smart Air explains why placement is important:

“When the purifier was flat against the wall, the airflow was only about 5%! Moving the air purifier 4 cm from the wall increased the airflow from 5% to 94%. At 10 cm from the wall, airflow was back up to 100% of its normal value.”

Bad placement

  • Avoid corners as they have the least airflow

  • Avoid obstructions so don’t squeeze it into small spaces to keep it out of the way

  • The Smart Air purifiers are all mechanical rather than electronic. If you have an electronic purifier, it uses wavelengths to function so should not be kept near other electronic items. Ideally, a minimum 1.5m distance between electronic items is recommended


Running your air purifier


If your purifier gives you 6 air changes an hour, you will have clean air in 10 minutes. This means you only need to run the purifier during class sessions as there is little point in shortening the filter life by cleaning an empty classroom.


When you first switch on the air purifier(s), it will be louder than usual as it can take around 15-25 minutes for the noise levels to regulate. We recommend that you set a timer for the air purifier to come on 30 mins before class. When class starts, not only will the noise be lower, the air will also have been cleaned 3 times. If your purifier doesn’t have a timer (like Smart Air products), a low-cost mechanical timer switch means you are not dependent on teachers to remember to switch the purifiers on or off.


Changing your filters


The Blast and Blast Mini have pre-filters which should be washed regularly and put back when fully dry. It prolongs the life of your HEPA filter, catching the larger particles and leaving the HEPA filter to trap the smaller particles. How regularly you wash the pre-filter depends on the particle levels in your classroom. In a very polluted classroom, you may need to wash as frequently as every four weeks.


We recommend placing a sticker at the back of your purifier to log the date the HEPA filter was opened and for the dates the pre-filter was cleaned.


The simplicity of the Blast design means more space for a larger filter, resulting in a 2-year filter lifespan for the Blast and 13 months for the Blast Mini (based on running the purifier for 8 hours a day). Each air purifier company will provide recommendations on HEPA replacements, but these recommendations are rarely data-backed and you can’t go by looks. The image below is a Smart Air test on their DIY HEPA filter. On day 90, the HEPA looks quite dirty, but data shows it only lost 4% effectiveness at that point. By day 200, the effectiveness dropped significantly by 50%.

How long a HEPA lasts depends on many factors such as outdoor air quality and how often the purifier is used which is why we added this to your purchase ‘checklist’.


Smart Air filters are tested in Beijing air where the average PM2.5 in 2020 was 38 µg /m3 compared to 13 µg/m3 in London. This mean that the filter will last longer in the UK than we can declare but without official tests, we will stick with the Beijing test results. As lab tests are different to real-world tests, Smart Air runs both. If you are interested in the data, you can find it on Smart Air’s website:



That’s the end of our 3-part guide, we hope you find it useful. If you have any questions, please get in touch with us.


If your school can afford it, we recommend purchasing air quality monitors with both PM2.5 and CO2 for each class. Understanding the quality of the air you breathe is an important step in maintaining healthy buildings and ensuring the short and long-term health of the children in your school.


Breathe safe!


<< Step 1 - Learn the lingo

<< Step 2 - Choosing your classroom purifier