What air changes per hour should you target for your room?
ACH targets differ by room type: homes, offices, clinics and procedure rooms each need a different figure. See the recommended ranges and how to apply them.
Key takeaways
- Recommended ACH targets are 3 to 4 for homes, 4 to 5 for offices, 5 or more for clinics, and 6 or more for dental and procedure rooms.
- Higher-risk spaces need a higher ACH target because more people, closer contact, or aerosol-generating procedures raise the airborne load faster.
- An ACH target only becomes useful once it's converted into a required clean-air delivery rate for the specific room's dimensions.
- The free size calculator applies the formula automatically once you enter floor area, ceiling height and your target ACH.
Why ACH targets differ by room type
Air changes per hour measures how many times the entire volume of air in a room gets cycled through a purifier in an hour. A living room used by one family and a dental operatory used by a stream of patients through the day have very different exposure patterns, so it makes sense that they need different targets rather than one universal number.
The target isn't about the room's size, a large bedroom and a small one both use the same ACH target for homes, it's about how the room is used and who's exposed to what's in the air.
Thinking in terms of use rather than square metreage also helps when a single space serves more than one purpose, such as a home office that occasionally hosts client visits.
Recommended ACH for homes, offices, clinics and procedure rooms
AIRE's general recommendations are 3 to 4 ACH for homes, 4 to 5 for offices, 5 or more for clinics, and 6 or more for dental and procedure rooms specifically. These aren't arbitrary bands, they roughly track the level of shared exposure typical of each setting: a home has a stable, known group of occupants, while a clinic waiting room sees a constant turnover of visitors.
Within these categories there's room for judgment. A home with a resident who has asthma or allergic rhinitis might reasonably target the higher end of the home range, just as a busy GP clinic during flu season might lean toward the higher end of the clinic range.
Why higher-risk spaces need a higher target
The logic behind a higher ACH target in clinical or procedure settings comes down to what's generating airborne particles and how often. Dental aerosols, laser plume in aesthetic procedures, and general bioaerosols from patient turnover all add to the airborne load faster than typical home or office activity does, so the air needs to be cycled more frequently to keep pace.
This is also why a dental operatory sits above a general clinic waiting room in the recommended range: the aerosol-generating activity happening directly in that room is more intensive than simply having people pass through a waiting area.
The same logic applies within a home too: a nursery or a bedroom used by someone with allergic rhinitis reasonably sits toward the higher end of the home range, even though it's still a home rather than a clinic.
Turning your ACH target into a CADR requirement
Once you know your target ACH, the formula for the clean-air delivery rate you need is straightforward: floor area in square metres, multiplied by ceiling height in metres, multiplied by your target ACH. This gives you a required CADR figure in cubic metres per hour, which then points to a specific model rather than leaving the decision as a vague size category.
Two rooms of the same floor area but different ceiling heights will need different CADR figures for the same ACH target, since ceiling height changes the actual air volume being cycled, even though floor area looks identical on a floor plan.
This is a common blind spot in older buildings with unusually high ceilings, where a room can look modestly sized on paper but actually holds significantly more air volume than expected.
Using the calculator to confirm your figure
Rather than doing the multiplication by hand, AIRE's free size calculator takes your floor area, ceiling height and target ACH and returns the required CADR along with a suggested model that meets or exceeds it. This removes the risk of under-sizing a room because a target ACH wasn't properly converted into an actual purchasing decision.
For a room that doesn't fit neatly into one category, a home office used daily for client meetings, for example, it's reasonable to size toward the higher of the two relevant ranges rather than defaulting to the lower one.
If a room still feels like an edge case after using the calculator, a room-by-room Air Report can settle it with an actual site visit rather than a self-reported estimate.
Next step
Confirm your own room's figure with the free Air Purifier Size Calculator, which applies this formula automatically.
Frequently asked
Why do recommended ACH targets differ by room type?
ACH targets reflect how a room is used and the level of shared exposure it typically involves, not the room's size. A home with a stable group of occupants needs a lower target than a clinic waiting room with constant visitor turnover, even if the rooms are similar in floor area.
Why do clinics and procedure rooms need a higher ACH than homes?
Clinical settings, especially dental and procedure rooms, generate airborne particles faster through aerosols, laser plume or higher patient turnover, so the air needs cycling more often to keep pace. AIRE recommends 5 or more ACH for general clinics and 6 or more for dental and procedure rooms specifically.
How does an ACH target turn into a CADR requirement?
Multiply the room's floor area in square metres by its ceiling height in metres, then by your target ACH, to get the required clean-air delivery rate in cubic metres per hour. That figure points to a specific model rather than leaving the sizing decision as a rough size category.
Related reading
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