Air Changes Per Hour (ACH) Explained
If CADR is the "engine power" of your air purifier, then Air Changes per Hour (ACH) is the "speed" at which you're driving. It is the most important setting for determining how clean your air actually stays.
What is ACH?
Air Changes per Hour is a simple ratio: it is the number of times an air purifier cycles the entire volume of air in a room through its filters in 60 minutes. If you have a room with 1,000 cubic feet of air and a purifier that cycles 4,000 cubic feet per hour, you have 4 ACH.
Why Does ACH Matter?
Indoor air isn't static. New pollutants are constantly entering the space—pollen from an open door, dander from a moving pet, or VOCs from a new piece of furniture. If your ACH is too low, the purifier cannot keep up with the rate of new pollution. The "effective" clean air in the room will never reach a healthy level.
The Recommended ACH Tiers
Not every situation requires the same intensity. We categorize ACH needs into four main tiers:
- 2 ACH (The Bare Minimum): Often used by manufacturers to claim large "coverage" areas. We do not recommend this for anything other than very low-use guest rooms.
- 4 ACH (The Healthy Home Standard): Recommended for general air cleaning in bedrooms and living rooms for healthy individuals.
- 5 ACH (The Allergy Target): Recommended for individuals with seasonal allergies or pets. This higher rate helps capture allergens before they settle on surfaces.
- 6 to 8 ACH (The High Intensity Zone): Necessary for wildfire smoke events, asthma-sensitive sleepers, or homes with multiple large pets.
The Formula: How to Calculate ACH
To find the ACH of a specific purifier in your room, you need to know the unit's CADR (in CFM) and your room volume.
ACH = (CADR × 60) ÷ Room Volume
For example, if you have a purifier with a 200 CADR and a 2,000 ft³ room:
(200 × 60) ÷ 2,000 = 12,000 ÷ 2,000 = 6 ACH.
Let Us Do the Math
Our sizer automatically calculates the CADR needed to hit these specific ACH targets for your room volume.
Calculate My Target →CADR vs. ACH: The Relationship
CADR and ACH are inextricably linked. If you move a 300 CADR purifier from a small bedroom to a large living room, its CADR stays the same (the engine power), but the ACH will drop because it now has to clean a much larger volume of air. This is why "one size fits all" air purifier recommendations are fundamentally flawed.
ACH and Fan Noise
Remember that a purifier's rated CADR is based on its highest fan speed. If you buy a unit that gives you exactly 4 ACH on high, but you run it on medium (where the CADR might be 50% lower), you are actually only getting 2 ACH. This is why we always recommend sizing up—buy a unit that gives you 6 ACH on high, so you can run it on medium and still get a solid 4 ACH.
FAQ
Can I have too many air changes per hour?
In terms of air quality, no. More ACH is always cleaner. However, extremely high ACH (10+) can create noticeable drafts and be unnecessarily loud without providing significant additional health benefits for most people.
Is ACH more important than the filter type?
They are equally important. A high ACH with a bad filter just moves dirty air around faster. A great filter with low ACH won't clean enough of the room's air to make a difference. You need both a True HEPA filter and the correct ACH for your concern.
Related Guides:
CADR vs. Square Feet | Full Sizing Guide