An introduction to buying air cleaners for schools
This is a simple guide to a complex topic to help you avoid bear traps when buying an air cleaning device using filters.
We have a much more advanced guide available for those wanting to understand more about the reasoning behind the principles, considerations and checklist we have created.
Four key principles:
For air filtration to work most effectively it needs to process enough air per unit time so that it is continuously removing particles. (There are some simple calculations to ensure it is sized correctly to provide the air changes needed per hour which are explained in our advanced guide.)
It must run at the speed you have calculated constantly and not in auto mode which will override all your calculations and make it run too slowly and thus ineffectively at times. (Auto mode is a very generalised setting which almost never will provide the particular solutions required in a school classroom. Devices are better if they don’t have this setting as then it can’t be used!)
It needs be quiet enough to be tolerated. In UK classrooms, 45dBA is considered the highest acceptable noise level.
So that we can monitor performance and measure improvement, use separate PM2.5 sensors (ie don’t rely on those included with the filtration device), to record data continually.
Additional considerations
It is best to avoid all added tech included in the device, such as sensors, ionisers, plasma, UVC etc. These have no proven benefit but add to cost, power usage and often, unwanted air chemistry. Carbon filters have limited utility beyond initial use.
Filters need to be replaced and it’s important to choose a machine where the replacement filters are readily available and also not overly expensive.
The ambition is to have sufficient devices to provide the equivalent of 6 air changes per hour (ACH) in a room. There is no one size fits all solution, and using several devices in a classroom sometimes provides the optimal solution. However, available space and amount/location of plug sockets often become limiting factors. Adding more machines also adds additional noise.
In the first instance, we would suggest seeking a solution using one device per classroom if possible. (Of course, more devices would probably be required in larger shared indoor spaces, such as assembly halls and canteens, but there is likely to be additional available space in those settings.)
The price of units varies tremendously. High-ticket price does not equal better by any means. It often means additional unwanted tech, clever marketing, sellers taking huge margins or reliance on brand reputation.
There are many different grades of air filter. Although many would see H13 grade as the minimum rating, lower grades can also be extremely effective. This is a complex topic but don’t allow a specific grade of filter grade to exclude potentially better performing, more cost-effective solutions. Our more advanced explainer provides more information on this topic.
Machine performance
There are 3 key performance parameters:
Clean Air Delivery Rate (CADR): The amount of air a machine can process per hour, generally in m3/hr. (These devices don’t change the air in a room, they clean it, so CADR is required to allow you to calculate its equivalent air changes every hour.)
Noise: Usually provided as dBA measured 1m from the device.
Power usage: Wattage used.
Ideally, we want high CADR, low noise and low power usage.
Checklist
It can be difficult to obtain performance data for all of the speeds at which the machines operate. All too often, the marketing information will provide a skewed picture, for example showing the CADR at highest speed whilst detailing the noise and power usage at the lowest speed.
So, before you buy, check you can comprehensively answer all of the questions in this list:
Do you know what CADR is required to produce the equivalent of 6ACH running at the speed you are prepared to use for the volume of the room in which it will be used?
Do you know the sound level and power usage of the machine(s) running at the speed you have calculated is needed?
Have you checked that no unwanted tech is added and that the machine does not have an auto-mode function?
Do you know the price and availability of replacements filters?
Have you identified sensors that you will use to monitor and record performance constantly?
Have you checked that the cost being paid per classroom isn’t over-the-odds versus all competitors that can meet the required specifications?
