CADR: the number to buy on
Clean air delivery rate (CADR) is measured under the ANSI/AHAM AC-1 standard. It is the volume of filtered air a unit delivers, in cubic feet per minute, measured separately for smoke, dust and pollen. Smoke particles are the smallest and hardest to catch, so the smoke CADR is the most useful single figure. Units verified through the AHAM program carry a seal with the three numbers on the box.
AHAM's rule of thumb: the smoke CADR should be at least two-thirds of the room's floor area in square feet, for a room with 8 foot ceilings. Taller ceilings need proportionally more.
| Space | Floor area | Minimum smoke CADR |
|---|---|---|
| Private office | 150 sq ft | 100 |
| Small meeting room | 250 sq ft | 167 |
| Large meeting room | 400 sq ft | 267 |
| Open office zone | 600 sq ft | 400 |
CADR is measured at the highest fan speed. Most people run purifiers on a lower, quieter setting, so buy headroom: a unit rated well above the minimum can run slower and still meet it. For a large open space, two or three units spread around usually do better than one big one in a corner.
Air changes per hour
Some guidance is written in air changes per hour (ACH): how many times the room's volume passes through the filter each hour. ACH = CADR × 60 ÷ (floor area × ceiling height). A 400 sq ft room with 9 foot ceilings holds 3,600 cubic feet; a unit with a CADR of 300 gives about 5 ACH.
HEPA and MERV
- True HEPA, as defined by the US Department of Energy, captures at least 99.97% of particles 0.3 microns in diameter. Most portable purifiers of any quality use a HEPA or HEPA-type filter. "HEPA-like" and "HEPA-style" are marketing words with no set meaning.
- MERV (minimum efficiency reporting value) comes from ASHRAE Standard 52.2 and rates filters for HVAC systems from 1 to 16. It is how you upgrade the building's central filtration rather than a room. Higher MERV filters add airflow resistance, so ask your HVAC contractor what your system can take before fitting one.
- Activated carbon stages address odors and some gases. A thin carbon sheet does little; a heavy bed of carbon does more.
Portable purifiers and better HVAC filters work together. The purifier helps rooms the HVAC system serves poorly, such as a closed meeting room or a space near a copier.
What we do not claim
This guide is about filtering particles from air. It does not say an air purifier will prevent illness or meet any health requirement. If you have a specific health, safety or compliance concern, ask a qualified industrial hygienist or your local health authority.
The specifications that matter
- Smoke CADR, AHAM verified.
- Noise in decibels at each speed. Anything much above 50 dB is noticeable in a quiet office.
- Energy. ENERGY STAR certifies room air cleaners on CADR per watt. A purifier run all day, every day, can use more electricity than people expect.
- Filter cost and life. Replacement filters are the main running cost. Price a year of filters before choosing a model, and check they are easy to buy.
- Filter change indicator. A timer is better than nothing; a sensor-based indicator is better still.
- Particle sensor and auto mode raise the fan when the air gets dirtier. Useful, but sensors in consumer units are approximate.
The costs people miss
- Replacement filters, often every 6 to 12 months depending on use.
- Electricity for units running continuously.
- Someone to change filters on schedule across many units.
For humidity control, see the dehumidifier guide. For cleaning floors, see floor scrubbers.
Before you buy
- Measure each room's floor area and ceiling height.
- Buy a smoke CADR comfortably above two-thirds of the floor area.
- Check for true HEPA and an AHAM verified CADR.
- Avoid ozone generators; check the CARB listing.
- Price a year of replacement filters.
- Check the noise level at the speed you will use.
- Name who changes filters, and when.
Air purifiers for large rooms
Affiliate linksHEPA air purifiers with high smoke CADR for meeting rooms and open office zones. Aim for a smoke CADR of at least two-thirds of the floor area.
SIESEAN
Air Purifiers for Home Large Room Up to 4200 Ft², 289 CADR, 20dB Sleep Mode
SIESEAN
Hepa Air Purifiers for Home up to 3500 Ft², 235 CADR, 30dB Quiet Operation
PEIKETAO
Air Purifier for Large Rooms Up to 2550 sq ft, H13 True HEPA Filter with 568 CADR, PM2.5 Monitor, Auto Mode, 24h Quiet Operation for Home Smoke, Dust, Pollen, Pet Dander, Odors
Levoit
LEVOIT Smart Air Purifier for Home Large Room up to 1875 Ft² Vital 200S-P
Prices retrieved from Amazon on 2026-09-29 and may since have changed; the price on Amazon at the time of purchase is the one that applies. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. No manufacturer pays for a place in this list.