How Long Does It Take for an Air Purifier to Clean a Room?

How long an air purifier takes depends on the room's volume and the purifier's clean air delivery rate (CADR), which is measured in cubic feet per minute (CFM). One full air change takes room volume divided by CADR. A purifier sized to the two-thirds rule turns a room over about 4.8 times an hour, which is one air change every 12.5 minutes.

At The Indoor Report, we work from published specifications. Our rule on this site is to publish a figure only when a named source gives it or the arithmetic is shown. No number here is a timed reading from a room. The figures come from the U.S. Environmental Protection Agency (EPA) air cleaner guide, from the two-thirds rule as this site states it, and from arithmetic on those values.

EPA says the most effective ways to improve indoor air are to reduce or remove pollutant sources and to ventilate with clean outdoor air. EPA adds that filtration can be an effective supplement.

The Timing Math Is Room Volume Divided by CADR

Take a room 12 feet by 15 feet with an 8-foot ceiling. Its volume is 1,440 cubic feet (12 x 15 x 8).

At a CADR of 180 CFM, one air change takes 8 minutes (1,440 divided by 180).

The two-thirds rule corresponds to 4.8 air changes per hour. Sixty minutes divided by 4.8 air changes per hour gives 12.5 minutes per air change.

EPA says CADR is intended to help consumers select an air cleaner based on the size of the area. It also says a higher CADR removes more particles and serves a larger area.

The 8 minutes covers one pass of the room's air. It does not describe a finished room.

One Air Change Is Not a Clean Room

Each air change is one pass of the room's air through the purifier.

Each pass removes only part of the particles.

The EPA guide gives no timing for the first hours, so none is given here.

To see the change in your own room, take a reading before the purifier starts and another after. Start with the air quality monitor guide or the how to test air quality at home guide.

Alen publishes an article on how long an air purifier takes to work, at alen.com/blogs/usage. The text and claims of that article are not repeated here.

The EPA Sizing Chart Works Out to About 12.3 Minutes

EPA publishes a sizing chart for particle removal. It is an estimate for an 8-foot ceiling, and higher ceilings may need a higher CADR.

Every row gives about the same result. Multiply floor area by 8 feet to get cubic feet, then divide by the chart's CADR. The 300 square foot row is 2,400 cubic feet divided by 195 CFM, or 12.3 minutes.

The chart's CADR rises in step with floor area, so the minutes per air change stay flat down the column.

The two-thirds result of 12.5 minutes and the chart's 12.3 minutes sit within a fraction of a minute of each other.

Fan Speed, Ceiling Height, and Filter Condition Change the Time

EPA says CADR is typically measured at the highest fan speed. It also says an air cleaner filters less air at a lower speed.

At 180 CFM, the example room takes 8 minutes per air change. A lower speed moves less air, so the same room takes longer.

No lower-speed CADR figure is listed here, so no lower-speed time is given.

The EPA chart assumes an 8-foot ceiling, and EPA says higher ceilings may need a higher CADR. More height means more cubic feet at the same floor area, so each air change takes longer at the same CADR.

EPA says all filters need regular replacement, and a dirty, overloaded filter will not work well.

The EPA guide does not address open doors or windows, so no figure for them is given here.

Sizing for a room is covered in the air purifier size for room guide.

The minute figures here would change if the rated CADR, the ceiling height, or the fan speed changed.

How Long to Run an Air Purifier Each Day

EPA says higher fan speeds and longer run times increase the amount of air filtered.

The EPA guide does not publish a daily run time, so none is given here.

Levoit lists the Vital 200S at 50 watts of rated power and 250 CFM of CADR. That pair of numbers is one example of how a CADR is published.

Running the Vital 200S at its 50-watt rating for 24 hours uses 1.2 kilowatt-hours (50 x 24 = 1,200 watt-hours).

The air purifier electricity use guide covers running cost.

Odors and Gases Fall Outside the CADR Rating

EPA says the CADR rating system is for particles only.

It also says there are no widely used rating systems for gas-removal air cleaners.

A CADR figure therefore says nothing about how fast a room's odor clears.

EPA says activated carbon can be effective when a large amount of material is used.

The carbon filter air purifier guide is the place to read about carbon filters.

Smoke Events Are Covered in Separate Guides

EPA says most air cleaners are rated for tobacco smoke, dust and pollen, which represent small, medium and large particles.

No timing for a smoke event is given here, because the EPA guide gives none.

Wildfire smoke is covered in the wildfire smoke indoor air guide. Smoke-specific purifier guidance sits in the air purifiers for smoke guide.

When a Purifier Is Not the Fix

EPA says air cleaners will not resolve a mold and moisture problem.

For a mold or moisture problem, start with the how to get rid of mold guide.

EPA says to avoid air cleaners that intentionally produce ozone, because ozone is a lung irritant.

Start With the Room Math Before Setting a Run Time

Multiply the room's length, width, and ceiling height to get cubic feet.

Divide that volume by the CADR your purifier lists, in CFM, to get minutes per air change.

A 1,440 cubic foot room at 180 CFM gives 8 minutes. The EPA chart's 300 square foot row gives 12.3 minutes at its listed CADR of 195.

Then read how to test air quality at home and take the before reading before the purifier starts.