Besides CADR, there is another concept that often comes up when choosing an air purifier: how often the air in a room should be refreshed per hour. This is known as ACH, air changes per hour. In this article, we explain what a good number is and why this may differ for your situation.
What exactly is ACH?
ACH indicates how many times the total air volume of a room has passed through the air purifier in one hour. With 4 air changes per hour (4x ACH), the equivalent of the total amount of air in the room has flowed through the filter within an hour. This does not mean that every air molecule has literally been filtered, but it does provide a good indication of how quickly pollution in the room is reduced.
What number is recommended?
There is no universally "correct" number; it depends on the situation:
- 2x per hour: minimum level, sufficient for general air quality without specific complaints
- 4x per hour: recommended by many experts and manufacturers as a comfortable level for most living spaces
- 5 to 6x per hour: for people with asthma or severe allergies, households with pets, or rooms where smoking occurs
Many people notice a significant difference in the freshness of the air and a decrease in dust accumulation at 4 to 5 air changes per hour, compared to a lower level.
Why not always as high as possible?
A higher number of air changes per hour sounds attractive but also has drawbacks. To achieve a high ACH level, an air purifier must work harder, which entails more noise and higher energy consumption. For most living rooms and bedrooms, 4 to 5 times per hour is a good balance between effectiveness and comfort.
How do you calculate the ACH level of an air purifier?
You can calculate the number of air changes per hour using the following formula:
ACH = CADR (m3/h) divided by the volume of the room (m3)
Take, for example, a room of 40 m2 with a ceiling height of 2.5 meters, resulting in a volume of 100 m3. With an air purifier that has a CADR of 260 m3/h, such as the Dynter AirPure Plus PA244, you end up with 260 / 100 = 2.6 air changes per hour on the highest setting. For a smaller room of 25 m2 (62.5 m3), this increases to more than 4x per hour.
Bedroom versus living room
You often spend eight consecutive hours in a bedroom, so it is worth calculating more generously here than in a living room where you only spend part of the day. Many devices offer a night mode or sleep timer for this purpose, so the device can run quieter at night while still achieving sufficient air changes. The AirPure Plus PA244, for example, has a sleep timer and night mode, and a noise level starting at 28 dB on the lowest setting, making a good ACH level achievable without disturbing your night's rest.
Practical overview
- First calculate the volume of your room (length x width x height)
- Determine the desired ACH level (2x for basic, 4 to 6x for allergies or pets)
- Multiply the volume by the desired ACH level to find the required CADR
- Choose a device whose CADR is well above this result, so it does not have to run at full power continuously
Our advice
We recommend aiming for at least 4 air changes per hour for living rooms and bedrooms, and increasing this to 5 to 6x for allergies, pets, or smoking at home. Choose an air purifier with sufficient CADR reserve, such as the AirPure Plus PA244, so that you can still achieve your goal on a lower, quieter setting.