Claire WhitakerBy Claire Whitaker

Portable air purifiers are labeled with room sizes, but those labels are calculated in different ways. Understanding two figures, CADR and air changes per hour, lets you compare models on a consistent basis and avoid buying a unit that is too small for the space.

Note: this guide covers how to compare equipment. We do not make health claims. If you are buying an air purifier for asthma, allergies or another health condition, talk to a medical professional.

What CADR means

CADR stands for Clean Air Delivery Rate. It describes how much filtered air a purifier delivers, in cubic feet per minute, for three types of particles: tobacco smoke (small), dust (medium) and pollen (larger). A higher CADR means the unit cleans more air in a given time.

CADR is measured under a standard developed by the Association of Home Appliance Manufacturers (AHAM), whose verification program is described at AHAM Verifide. Look for CADR values on the box or in the specifications, ideally with indication that they are verified.

The quick sizing rule

AHAM’s rule of thumb, which it calls the 2/3 rule, is that the smoke CADR should be at least two-thirds of the room’s floor area in square feet (AHAM: air filtration standards). The rule assumes ceilings of up to about 8 feet.

Room area Minimum smoke CADR by the 2/3 rule
150 sq ft 100 cfm
225 sq ft 150 cfm
300 sq ft 200 cfm
450 sq ft 300 cfm

If your ceilings are higher than 8 feet, your room holds more air, so you will need a higher CADR to achieve the same result.

Air changes per hour (ACH)

Some manufacturers describe coverage in terms of air changes per hour: how many times the purifier can filter the full volume of air in a room each hour. You can estimate it yourself:

ACH = (CADR in cfm × 60) ÷ (room area in sq ft × ceiling height in ft)

For example, a purifier with a CADR of 200 cfm in a 300 sq ft room with 8-foot ceilings: (200 × 60) ÷ (300 × 8) = 12,000 ÷ 2,400 = 5 air changes per hour.

This is why two purifiers can both claim to suit “up to 500 sq ft.” One may assume a lower number of air changes per hour than the other. When room coverage is listed, check the ACH it assumes or calculate it from the CADR.

The U.S. Environmental Protection Agency’s Guide to Air Cleaners in the Home explains CADR and other considerations in more depth.

Other factors that affect your choice

Filter type

  • HEPA filters capture fine particles. Our filter types guide explains the HEPA standard.
  • Activated carbon filters are included in some units to help reduce certain odors and gases. Their effectiveness depends on how much carbon is used.
  • Pre-filters catch larger particles such as pet hair and extend the life of the main filter.
  • Ozone: the EPA advises consumers about ozone generators sold as air cleaners. If a unit uses ionizing or other technologies, check whether it produces ozone and whether it is certified to applicable emission limits, such as those required in California.

Noise

Purifiers deliver their rated CADR at the highest fan speed, which is also the loudest. If you plan to run a purifier in a bedroom at a low setting, choose a unit with a higher CADR than the minimum so it still cleans effectively when turned down. Manufacturers often list noise levels in decibels for each speed.

Running costs

  • Replacement filters: check the recommended replacement interval and the availability of replacements before you buy.
  • Electricity: ENERGY STAR certified air cleaners meet efficiency criteria. Wattage is usually listed for each fan speed.

Placement

Follow the manual’s clearance guidance. Purifiers need open space around the intake and outlet to work as rated. Keep doors and windows closed while the unit is running if you want it to clean the room’s air effectively.

Who should not rely on a portable air purifier

  • Anyone trying to address the source of a problem, such as mold, gas appliances or smoke indoors. Source control and ventilation come first.
  • People wanting to clean a whole house with one small unit. Portable purifiers are designed for individual rooms.

Frequently asked questions

Can I use one air purifier for multiple rooms?

A purifier works best in a single enclosed room. Moving it between rooms is possible, but it will not clean rooms it is not in.

Is a bigger air purifier always better?

A higher CADR cleans faster and can run more quietly at lower speeds. The trade-offs are size, price and filter cost.

How often should I replace the filter?

Follow the manufacturer’s schedule. Replace sooner if the filter is visibly clogged or the unit’s indicator calls for it.

Claire Whitaker

About Claire Whitaker

Claire Whitaker is part of our editorial team and reviews and edits our non-vacuum cleaning gear guides, checking product details against manufacturer specifications and owner feedback.