How Long Should You Run an Air Purifier

Run your purifier as many hours as you can, because the longer an air cleaner runs, the more air it filters. If your unit makes ions or plasma, switch that feature off, or pick a filter-only unit, before you leave it on while you're out.

Is It Safe to Leave Air Purifier on When Not Home?

Leave it running while you're out if it's a filter-only unit plugged straight into the wall. Purifier maker Intellipure calls running a unit 24 hours a day, 7 days a week the best way to improve your air in most situations. It also warns against extension cords because of an overheating fire risk. Look for UL 2998 zero ozone certification on the box.

In your house, run it all day. Use a high speed while you're out and a quieter one while you're home. Intellipure says running it while you're away may be the best choice if noise bothers you.

Ozone is the one thing to check before you leave a unit running. EPA says to avoid air cleaners that intentionally produce ozone. Ionizers and plasma units can generate high amounts of ozone. California requires an ARB Certified label, which means less than 0.050 ppm of ozone. Anywhere else, check CARB's list of certified air cleaning devices. Several Dyson air purifier models sit on that list as mechanical types.

The best air purifier to leave on is one that fits your room and makes no ozone. Pick the branch that matches your room.

• Your bedroom fits within 358 sq ft at 4.8 air changes per hour, and you sleep with the unit running: this one lists that same 358 sq ft rating and reads 35.5 dBA in sleep mode from 3 ft. Get the Levoit Core with AirSight Plus.
• Your room is larger than 358 square feet, such as a living room: this one lists 606 sq ft at 4.8 air changes per hour. Get the Levoit Core with light sensor.
• The box lists ions, plasma or PlasmaWave and you'd rather skip them: Coway removed the ionizer from this model, its page lists 1,800 sq ft at 1 air change per hour, and its MegaScan sensor reads PM1, PM2.5 and PM10. Get the Coway Airmega Mighty MegaScan.
• You smoke indoors, or the room smells of old cigarette smoke: for the smell, EPA says carbon works best in large amounts, so you want a deep bed, and this unit holds 24 lb of it in a 3 inch bed and moves 400 CFM. Get the AllerAir 5000 DS.
• You can see mold or smell mustiness: a purifier filters airborne spores and odors, but it won't fix the cause, which is moisture. Dry the damp area, scrub mold off hard surfaces, and keep humidity no higher than 50%. If the moldy patch is larger than about 10 square feet, call a mold remediation professional.

Both Levoit air purifier picks list CARB marks on their pages. The Core with AirSight Plus also lists ENERGY STAR, a label for energy efficiency, not for how well a unit cleans air. If you are still choosing a unit for the whole home, the best air purifier guide is the place to start.

Skip the Blueair Blue Max for unattended running. Its HEPASilent ionization can't be switched off and gives off small amounts of ozone. The reviewer also calls it pretty useless for VOCs.

How Long to Run Air Purifier in Bedroom

Run it all night, and start it on top speed before bed. A purifier filters air the moment its fan runs. At top speed, the Levoit Core with AirSight Plus cleared incense smoke from a 728 cubic ft room in 24 minutes, and the Coway Airmega Mighty MegaScan took 23. At speed 2, the Coway needed 38 minutes. Start high, then drop to a quiet speed for the night.

Size the unit to the bedroom first. EPA says to look for at least 4.8 air changes per hour for the room size and a CADR at least two-thirds of the room's square footage. Levoit rates the Core with AirSight Plus at 231 CFM, and Air Purifier First gives it 260 CFM. Either figure meets the two-thirds rule for a 300 sq ft bedroom.

For a 400 sq ft bedroom, take the Levoit Core with light sensor. Levoit rates it at 391 CFM, and Air Purifier First gives it 410 CFM. Both figures clear the 260 cfm that EPA's sizing lists for a 400 sq ft room.

Replace filters on schedule. A dirty, overloaded filter won't work well, and a longer run time won't fix that. If you are still shopping for a sleeping space, the best air purifier for bedroom guide is written for you.

Should Air Purifier Be on Floor?

Put it wherever air can move freely around it, in the room you spend the most time in. Keep it away from curtains and anything that blocks airflow. If it blows a draft on you, redirect the airflow away from you. Read the manual for placement.