Air purifier for wildfire smoke: sizing, setup and limits

An air purifier for wildfire smoke should be a filter-based unit with a smoke CADR matched to one room, run in that closed room on its highest setting for as long as the smoke lasts. Set up that way, a portable air cleaner can lower indoor fine particles, which the EPA’s wildfire guide names as the principal pollutant of concern in smoke. It will not take out every gas in the smoke, and it cannot make a leaky house smoke-free. The measurements below describe air, not health, and anyone with a heart or lung condition should follow their clinician’s plan and local health agency advice first.

What is in smoke, and what a filter can reach

The EPA guide says fine particles, PM2.5, are “the principal air pollutant of concern for public health” in wildland fire smoke. EPA defines PM2.5 as particles “generally 2.5 micrometers and smaller,” against a human hair of about 70 micrometers.

Smoke also contains gases. A particle filter does not capture them. The EPA guide says air cleaners that remove VOCs and other gases “will have filters with activated carbon, or chemisorbents such as impregnated alumina or permanganate,” and adds that those filters “will generally be quite expensive and often have a relatively short lifespan.” EPA’s Technical Summary also notes that carbon monoxide “is not readily captured by adsorption or chemisorption.” Treat carbon as a possible help with smell and some gases, and treat the particle filter as the part that does the measured work.

What size unit to buy for a smoke event

There is a real disagreement between two sources, so both are given here.

• EPA: “Choose a PAC with a clean air delivery rate (CADR) for smoke that is at least two-thirds of the room area in square feet.”
• AHAM, the appliance trade group that runs the CADR program: “For wild fire smoke, AHAM recommends a Smoke CADR equal to the size of the room in square feet.”

Both rules assume an 8-foot ceiling. AHAM and EPA both say that a higher ceiling calls for a unit rated for a larger room. If the ceiling is 10 feet, the air volume is a quarter larger, so a simple way to adjust the 12-by-15 room is to multiply by 10 and divide by 8: EPA’s 120 becomes 150 and AHAM’s 180 becomes 225.

The EPA guide also gives a volume target: portable units should ideally provide “a filtered airflow at least four to five times the room volume per hour.” For the 12-by-15 room with an 8-foot ceiling, the volume is 1,440 cubic feet. Five times that per hour is 7,200 cubic feet, or 120 cfm, which matches EPA’s two-thirds figure. Choose the larger number if you can, because labels are measured at maximum speed and real rooms leak. More on the method is in the guide to matching an air cleaner to room size.

Use the smoke number on the label, not the dust or pollen number. EPA’s Technical Summary says “tobacco smoke CADRs should be used as the most conservative estimate” for small particles such as PM2.5.

Which kind of air cleaner to choose

EPA’s wildfire fact sheet says mechanical air cleaners and filters, “especially higher-efficiency filters, effectively trap particles from smoke and do not produce ozone.” Filters labeled HEPA are the usual choice, though EPA’s Technical Summary says there is “no widely accepted definition of HEPA performance in consumer products,” so the smoke CADR is a better check than the word alone.

EPA says “Electronic air cleaners are not recommended for smoke removal.” It also advises CARB-certified air cleaners because “they produce little or no ozone.” California certification concerns ozone emissions and electrical safety; CARB says it “does not reflect air cleaner effectiveness or ‘health safety’.” Never run a device built to make ozone. The Pediatric Environmental Health Specialty Units say plainly, “Never use an ozone-generating air cleaner,” and the reasons are set out in the page on ozone generators sold as air cleaners. Cigarette and cigar smoke indoors is a different problem with a continuing source; see what an air purifier can do about tobacco smoke.

How to set up a clean room

EPA describes a clean room as one “set up to keep levels of smoke and other particles as low as possible.” Do this before the smoke arrives if you have warning.

1. Pick the room where you spend the most time, usually a bedroom, and one everyone can fit in if needed.
2. “Close all windows and doors in the room, but don’t do anything that makes it hard to get out.” EPA’s filtration page likewise says to use the air cleaner “in a room with doors and windows closed, when it is safe to do so.”
3. Place the air cleaner where air can flow freely around it, away from curtains and furniture.
4. Run it “continuously on the highest fan setting if you can.”
5. Stop making particles indoors. EPA’s list includes smoking, vaping, gas or wood stoves, aerosol sprays, frying or broiling, candles, incense, and vacuuming without a HEPA filter.
6. Use a portable air conditioner with a single hose “sparingly in smoky conditions,” as EPA puts it.
7. Check the filter. EPA’s fact sheet says to “check the filter often and change it when it gets dirty or starts to smell.”

If you cannot stay cool at home, or heavy smoke keeps getting in, EPA’s clean-room page says to consider seeking shelter elsewhere. Heat is its own danger, so do not seal yourself into a hot room to escape smoke.

If you have central air, EPA advises a filter marked MERV 13 or higher and running the system fan as often as you can, provided your system can take that filter. The trade-offs are in the guide to whole-house air purifiers and in-duct filters. If you cannot get or afford a portable unit, EPA treats a box fan with a filter as a temporary option; the page on DIY box fan air filters gives the test results and cautions.

How to read the Air Quality Index

AirNow’s scale is the quickest way to decide how seriously to take a smoke day. Local readings can change fast, so check them often.

AirNow describes orange this way: “Members of sensitive groups may experience health effects. The general public is less likely to be affected.” At purple it says “Health alert: The risk of health effects is increased for everyone,” and at maroon, “Health warning of emergency conditions: everyone is more likely to be affected.”

Indoor air is not the same as the outdoor number. A consumer PM2.5 monitor may show whether the clean room’s reading is falling, but treat it as a rough guide: South Coast AQMD’s sensor program says “At this time, no program exists to certify the performance of sensors.”

Respirators, and a conflict about children

For time outdoors or in a home that stays smoky, a respirator does a different job from an air cleaner. The EPA guide says that correctly fitted and used N95 respirators “will significantly reduce the wearer’s exposure to smoke particles,” and that N95 or P100 respirators “remove only particles, not gases or vapors.” NIOSH adds that “Facial hair can cause tight-fitting respirators to leak.”

Sources differ on children, so read them together. A 2020 factsheet from the Pediatric Environmental Health Specialty Units says “even N95 masks, are not made to fit children and will not” protect them. The June 2026 EPA guide says “NIOSH does not currently approve respirators for use by children,” and that for ages 2 to 7 “it is less likely to find an N95® respirator that will fit their face well.” NIOSH’s 2025 bulletin says masks and respirators “should not be worn by children younger than 2 years old,” and that respirators “are designed primarily to be used by adults in workplaces.” The later federal sources leave room for a well-fitted respirator on children aged 2 and older, while the 2020 factsheet does not. Ask your child’s clinician or local health agency, and keep the priority on cleaner indoor air for children: the factsheet warns they “are especially at risk.”

What the evidence shows, and what it does not

EPA’s Technical Summary reports that most studies of HEPA or other high-efficiency portable units found particle exposure reductions “on the order of approximately 50 percent or higher.” A 2024 randomized cross-over trial by Lu and colleagues, in 29 homes sampled over two consecutive 24-hour periods with the HEPA filter installed versus removed, measured a mean PM2.5 reduction of 78.8% in the room with the unit and 57.9% in a second room. Those are measurements of air in homes, not of people breathing heavy wildfire smoke. The lower second-room figure is one reason to place the unit where you actually spend your time.

The sources for this page do not include a trial showing that a home air cleaner lowers illness during a smoke event. EPA’s Technical Summary says of purifier studies generally that “most of the health improvements were relatively small in magnitude and, when multiple outcomes were measured, typically only a fraction of health outcomes or biomarkers of health outcomes were improved.” A purifier cleans air. It does not treat or cure anything, and it is not a reason to skip a clinician’s advice or an evacuation order.