ACH Explained: Air Changes Per Hour

CADR tells you how much clean air a purifier can deliver. ACH tells you how much clean air it delivers relative to the room you're actually putting it in. ACH is the number that determines whether your purifier is doing its job.

What ACH Measures

ACH stands for Air Changes per Hour. It measures how many times per hour the purifier cycles the entire volume of air in a room through its filter. Each cycle, particles are removed from the air sampled. The more cycles per hour, the lower the steady-state concentration of airborne pollutants in the room.

ACH is a rate of dilution of pollutants by repeatedly replacing contaminated air with filtered air. It is not a measure of single-pass filtration efficiency (that's what HEPA certification measures). A unit with lower single-pass efficiency but very high ACH can achieve excellent air quality — and vice versa.

Unlike CADR, ACH is specific to a room size and ceiling height. The same purifier has very different ACH in a 150 sq ft bedroom versus a 500 sq ft open-plan space.

How to Calculate ACH

Formula (imperial):

ACH = (CADR in CFM × 60) ÷ Room volume in cubic feet

Room volume = Room length (ft) × Width (ft) × Ceiling height (ft)

Formula (metric):

ACH = CADR in m³/h ÷ Room volume in m³

Examples at standard 8 ft (2.4 m) ceilings:

CADR (CFM)	Room size	Room volume (cu ft)	ACH
145 CFM (Levoit Core 300)	150 sq ft	1,200	7.25
145 CFM (Levoit Core 300)	300 sq ft	2,400	3.6
246 CFM (Coway AP-1512HH)	200 sq ft	1,600	9.2
246 CFM (Coway AP-1512HH)	400 sq ft	3,200	4.6
410 CFM (Levoit Core 600S)	400 sq ft	3,200	7.7
410 CFM (Levoit Core 600S)	600 sq ft	4,800	5.1

ACH Targets for Different Use Cases

Use case	Minimum ACH	Recommended ACH	Context
General air quality improvement	2	3	Basic pollution reduction
Dust and pollen allergy	3	4–5	Bedroom and main living space
Pet dander	4	5	Continuous room occupation
Asthma management	4	5–6	Bedroom especially important
Smoke (cigarette or wildfire)	4	5–6	Higher concentration requires more cycles
Virus / airborne pathogen reduction	4	5–6	CDC and SAGE guidance converge here
Hospital isolation room standard	6	12	Clinical setting — not residential

Practical benchmark: 5 ACH is the target that consistently appears in allergy research, asthma studies, and infectious disease guidance as delivering measurable benefit. Design your room sizing around achieving 5 ACH at your intended operating speed — not at the unit's maximum speed.

ACH vs CADR — Which Matters More?

CADR is a property of the air purifier — it's fixed by the unit's fan and filter. ACH is a property of the air purifier in a specific room — it changes depending on the room you put it in.

CADR is useful for comparing purifiers to each other. ACH is useful for evaluating whether a specific purifier is right for a specific room.

The critical mistake people make: choosing a purifier based on CADR alone without calculating ACH for their actual room. A 400 CFM purifier achieves 3 ACH in a 700 sq ft open-plan space — adequate for general use but insufficient for allergy control. The same purifier in a 300 sq ft bedroom achieves 5 ACH — well suited for allergy management. Same unit, very different outcomes.

See our CADR guide for the full explanation of how CADR is measured and how to use it.

How Fan Speed Affects ACH

Most CADR figures are published at maximum fan speed. Effective ACH drops significantly at lower speeds:

Fan speed	Approximate CADR as % of max	Effect on ACH
Maximum / Turbo	100%	Full rated ACH
High	75–85%	~80% of rated ACH
Medium	50–65%	~57% of rated ACH
Low / Sleep	30–45%	~37% of rated ACH

This table explains why a unit with 400 CFM CADR running on low speed (delivering ~140–180 CFM equivalent) achieves only 2–2.5 ACH in a 400 sq ft room — well below the 5 ACH target. This is the most common cause of disappointing results from apparently well-sized units.

Key implication: If you plan to use the purifier in a bedroom overnight on low/sleep mode, you need a unit with CADR roughly 2–3× the theoretical minimum for your room — so that at low speed you still achieve 4–5 ACH. Under-specifying for bedroom use and then running quietly is the most common sizing mistake.

ACH by Model in Common Rooms

The following table shows ACH for each model at maximum CADR in typical room sizes. For bedroom night use, assume approximately 40% of these figures at low/sleep speed:

Model	CADR	ACH in 150 sq ft room	ACH in 250 sq ft room	ACH in 400 sq ft room
Levoit Core 300	145 CFM	7.3	4.4	2.7
Winix 5500-2	232 CFM	11.6	7.0	4.4
Coway AP-1512HH	246 CFM	12.3	7.4	4.6
Dyson TP07	192 CFM	9.6	5.8	3.6
Blueair Blue Pure 211i Max	350 CFM	17.5	10.5	6.6
Levoit Core 600S	410 CFM	20.5	12.3	7.7

All figures assume 8 ft ceilings. See the full model comparison for noise levels and filter costs.

How to Improve ACH Without Buying a Bigger Unit

If your current purifier is underperforming its rated ACH, these measures improve real-world effectiveness:

Use the unit in a smaller room — moving from a 400 sq ft living room to a 200 sq ft bedroom doubles ACH with the same unit.
Close doors to separate the room from a larger space — an open doorway effectively increases the volume the purifier must service. Closing doors dramatically increases ACH in the room the purifier is in.
Improve placement — central placement with unobstructed intake and exhaust improves effective air circulation. A purifier wedged in a corner with the intake against a wall can deliver 30–40% less effective airflow than the same unit centrally placed. See our placement guide.
Increase fan speed during high-pollution events — use maximum or high speed when cooking, during pollen peaks, or when smoke is present. The higher ACH clears the event faster; you can return to low speed once air quality recovers.
Maintain filters regularly — a clogged HEPA filter reduces airflow and effective CADR, which directly reduces ACH. Clean the pre-filter every 2–4 weeks and replace HEPA on schedule.

Summary: How to Use ACH When Buying

The buying process using ACH:

Measure your room (length × width × ceiling height = volume in cubic feet)
Choose your target ACH (5 is the standard for allergy/asthma; 3 for general use)
Calculate required CADR at max speed: Volume × Target ACH ÷ 60 = required CFM
Account for your intended operating speed: multiply by 1.8× if you'll run at medium; 2.5× for low/sleep
Select a purifier with at least that CADR — see our ranked models and comparison table

FAQ

Is higher ACH always better?

Above about 6 ACH, the incremental benefit diminishes significantly. Studies show that moving from 2 ACH to 5 ACH produces dramatic improvements in air quality. Moving from 5 ACH to 10 ACH produces modest further improvement. The practical implications: 5 ACH is the target; beyond that you are spending extra money (or running the fan louder) for small marginal gains. Focus on consistently achieving 4–5 ACH rather than maximising ACH.

Does ACH decrease when I open a window?

No — ACH measures how many times the purifier recirculates the room's air volume through its filter. Opening a window doesn't change the purifier's output. However, opening a window introduces unfiltered outdoor air (containing pollen, particulates, or smoke) that the purifier then has to clean. This increases the total pollutant load in the room, which means the purifier must work harder to maintain low indoor concentrations — in effect making your ACH less "useful" per cycle because each cycle starts with more pollutants.

How do I know if my purifier is achieving adequate ACH?

Calculate it using the formula above with your room dimensions and the purifier's CADR at your actual operating speed. Units with PM2.5 sensors and auto mode provide indirect feedback — if the air quality indicator consistently shows poor air quality even on high speed, either the room is too large for the unit or filters need replacing. If it consistently shows clean air on medium speed, you are likely achieving adequate ACH.

Does ACH apply to HVAC systems?

Yes — HVAC systems also have ACH ratings, measured differently (total system airflow relative to total conditioned space volume). Residential HVAC typically delivers 0.5–1 ACH through a whole house. For comparison, a portable HEPA purifier sized for 5 ACH in a bedroom delivers 10–20× more air cleaning in that specific room than the HVAC system does. This is why bedroom HEPA purifiers are recommended even in homes with good HVAC systems.

Related guides and rankings

Our ranked models: Levoit Core 600S · Coway AP-1512HH · Winix 5500-2 · Blueair Blue Pure 211i Max · Levoit Core 300 · Dyson TP07