How to size an air purifier for your room (CADR explained)

An air purifier is only as good as the amount of air it can clean in a given time. That's what the CADR number tells you, and it's the most useful spec on the box.

What CADR means

Clean Air Delivery Rate (CADR) is a rating from a test program run by AHAM, the appliance manufacturers' association. It measures how many cubic feet per minute of clean air a purifier delivers, separately for three particle types:

• Smoke (the smallest particles)
• Dust
• Pollen (the largest)

Higher numbers mean faster cleaning. The smoke number is usually the most demanding and the most useful for comparison.

The two-thirds rule

AHAM's rule of thumb: the smoke CADR should be at least two-thirds of the room's floor area in square feet, assuming a standard 8-foot ceiling.

Room size	Minimum smoke CADR
150 sq ft	about 100
300 sq ft	about 200
450 sq ft	about 300

That works out to roughly five air changes per hour. If you have allergies, asthma or deal with wildfire smoke, size up beyond the minimum. Higher ceilings also call for more.

Be careful with "covers up to 1,000 sq ft" claims. Some brands calculate coverage with only one or two air changes per hour. CADR is a more comparable number.

Filters: true HEPA vs. "HEPA-type"

• True HEPA filters meet a standard for capturing very small particles with high efficiency. Look for the word "true" or a stated efficiency.
• "HEPA-type" or "HEPA-like" filters aren't held to that standard.
• Activated carbon layers help with odors and some gases. Thin carbon pre-filters do less than heavy carbon beds.

Purifiers to avoid

• Ozone generators sold as "air purifiers." Ozone is a lung irritant, and the U.S. EPA advises against using ozone generators in occupied spaces.
• Ionizers without information on ozone output. In California, air cleaners must be certified by the California Air Resources Board for low ozone emissions; CARB certification is a useful sign even outside California.

Running costs and noise

• Replacement filters are the ongoing cost. Check price and recommended replacement interval before you buy.
• Noise: CADR is measured at the highest speed, which is often too loud for a bedroom at night. Look at noise levels in decibels at each speed, and consider sizing up so you can run it at a quieter setting.
• Energy use: ENERGY STAR certified models use less power, which matters when a purifier runs all day.

Placement

• Put it where you spend the most time, often the bedroom.
• Keep a little space around the intake and outlet, away from walls and furniture.
• Close doors and windows while it runs, or it's trying to clean the whole outdoors.

Questions people ask

Does an air purifier help with allergies?

It can reduce airborne particles like pollen, dust and pet dander. It doesn't remove allergens settled in bedding and carpets, so regular cleaning still matters.

Should I run an air purifier all the time?

Many people do, at a lower speed, because particles keep coming in. Running it constantly at a quiet setting is often more effective than short bursts on high.

Do air purifiers remove mold?

They can capture airborne mold spores, but they don't fix the moisture problem causing mold. Address leaks and humidity first.