How fast will it clear the room

Researched from the AHAM AC-1 test method, EPA and CARB guidance, and published manufacturer specifications

Quick answer

A 250 CFM machine in a 300 square foot room with an eight foot ceiling delivers about 6.3 air changes an hour, which removes 90 percent of airborne particles in roughly 22 minutes and 99 percent in about 44. Removal is exponential decay rather than a countdown, so the last tenth takes as long as the first ninety percent did.

How fast will it clear the room Worked answer below
Use the smoke figure where the machine publishes all three.
Air changes delivered 6.25 per hour
Room volume2,400 cu ft
Half of the particles gone in7 min
90 percent gone in22 min
99 percent gone in44 min
Verdictenough for smoke season at six air changes an hour
Largest room for 4.5 ACH417 sq ft

The last one percent takes as long as the first ninety did. That is what exponential decay means, and it is why a machine left running holds a low concentration while one switched on for twenty minutes does not.

A filter near the end of its life moves less air at the same fan speed, so the delivered air change rate falls quietly over months without the machine sounding any different.

How it decides: air changes per hour = CADR x 60 / room volume, and time to remove a fraction = ln(1 / (1 - fraction)) / ACH

Assumes a well mixed room, the machine as the only removal path, and nothing new arriving. While a source is active, cooking for example, removal and arrival compete and the concentration settles where they balance instead of falling to zero.

Equipment this calculator applies to

Clorox Medium Room Turbo Air Purifiers for Home With True HEPA Filter

A machine that publishes a figure you can actually compare against the answer above, rather than a coverage area with no air change rate behind it.

$93.21Check pricePrices change often

Prices and specifications change, so check the listing against the number this calculator gave you.

Machines with a published airflow figure

Common questions

How long does an air purifier take to clean a room?

It depends on the air changes an hour it delivers in that room, and the relationship is exponential rather than linear. At five air changes an hour, 90 percent of airborne particles are gone in about 28 minutes and 99 percent in about 55. At one change an hour the same two figures are roughly 138 and 276 minutes.

Should I run it constantly or in bursts?

Constantly, at whatever speed you can live with. Continuous running holds a low steady concentration, which is what you actually want. A burst on high clears a backlog quickly and then lets it rebuild, and in a room with an ongoing source the concentration settles wherever removal and arrival balance.

Which CADR number should I use here?

The smoke figure where the machine publishes all three. Smoke is the smallest particle size in the AHAM AC-1 test and usually produces the lowest number, so it is the conservative one. A single headline CADR with no particle named is normally the highest of the three.

Does an open door change this?

Substantially. An open door connects the room to a larger volume, so the same machine delivers fewer air changes an hour to the space it is actually serving. Either enter the connected volume rather than the room, or close the door during the hours that matter.

Why does the room still smell after the particles are gone?

Because odour is gas, not particles, and a HEPA filter does not remove gases. Odour needs activated carbon with real mass behind it, and a thin carbon coated mesh in a combination filter has very little capacity. Ventilation clears gases faster than most portable filtration does.

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