What CADR do I need for my room during wildfire smoke?

Use the tobacco smoke CADR, since it reflects small-particle performance. The EPA suggests a tobacco smoke CADR of at least two-thirds the room's floor area in square feet; AHAM's general rule is a CADR roughly equal to the room area. So a 120-square-foot room wants a tobacco smoke CADR of about 80 to 120. Size up for ceilings over 8 feet or for heavier air-change targets.

Is HEPA-type the same as true HEPA?

No. True HEPA captures at least 99.97 percent of particles at 0.3 micrometers, the hardest size to catch. HEPA-type, HEPA-style, and similar labels do not meet that standard and can let more fine smoke through. For smoke, choose a true HEPA unit, ideally a sealed design with published CADR.

Do I need activated carbon for smoke?

HEPA media handles the particles, which is the priority for the health risks authorities track. It does not capture the gases and odor compounds in smoke; those pass through. Activated carbon adsorbs gases and helps with smell. A deeper carbon stage works better and lasts longer than a thin carbon-coated pre-filter.

Are ozone generators or ionizers good for smoke?

Avoid ozone generators. The EPA says no federal agency has approved them for occupied spaces, and ozone can irritate the lungs and worsen asthma. Ionizers can produce ozone as a byproduct; if your unit has one, run it with the ionizer off as a plain HEPA filter. Mechanical filtration through sealed HEPA and carbon is the reliable, byproduct-free option.

Does a DIY box-fan filter actually work?

Yes, as a supplement. EPA research and UC Davis testing found Corsi-Rosenthal boxes can match or beat commercial HEPA units on clean-air delivery at a much lower cost. They use MERV-13 filters, not HEPA, but high airflow compensates. Replace filters often during smoke, and use a newer box fan kept clear of obstructions.

Air guide

Choosing an Air Purifier for Wildfire Smoke

Wildfire smoke is mostly fine PM2.5, with gases mixed in. Choosing well comes down to enough clean-air delivery for your room, a sealed HEPA filter, activated carbon for odors, and avoiding devices that add ozone.

What wildfire smoke actually is

Wildfire smoke is a mix of fine particles and gases. The fraction that drives most health guidance is PM2.5: particles 2.5 micrometers across or smaller. They are small enough to reach deep into the lungs, and the EPA treats PM2.5 as the main pollutant to track during smoke events.

Smoke also carries gases and odor compounds such as benzene, formaldehyde, and acrolein. This matters for filter choice. A particle filter, no matter how efficient, does not capture gases. The two phases of smoke need two different filtration approaches, which is why the sections below cover both HEPA media and activated carbon.

The practical goal indoors during a smoke event is to keep PM2.5 as low as you can in at least one room, while limiting how much new smoke you let in from outside.

Match CADR to your room

Clean Air Delivery Rate (CADR) measures how much filtered air a unit produces, in cubic feet per minute, for a given pollutant. AHAM publishes separate CADR values for tobacco smoke, dust, and pollen. For wildfire smoke, the tobacco smoke CADR is the relevant number, because it reflects performance against small particles.

The EPA's guidance for smoke is to choose a unit with a tobacco smoke CADR of at least two-thirds the room's floor area in square feet. A 120-square-foot room (10 by 12 feet) calls for a tobacco smoke CADR of about 80 or higher. AHAM's broader rule of thumb is a CADR roughly equal to the room area, which gives you more margin.

If your ceiling is higher than 8 feet, size up. There is more air in the room than the floor area implies, so you need a higher CADR to reach the same result. When a listing gives only a room-size rating and no CADR, treat that as missing data rather than a substitute for it.

Aim for enough air changes per hour

Room-size ratings are usually built around a modest number of air changes per hour (ACH), often around 2 to 3. During a smoke event you want more, because smoke leaks in continuously and you are trying to hold concentrations down, not just clean a sealed room once.

A reasonable target during heavy smoke is 4 to 5 air changes per hour in the room you are using as a clean space. The simplest way to get there is to pick a unit whose CADR is generous for the room, or to run two units. Higher fan speeds deliver more clean air, with the tradeoff of more noise.

Pick one room to protect well, ideally a bedroom, rather than spreading one undersized unit across the whole home. Keep windows and doors closed in that room, and run the unit continuously while smoke persists.

Use a sealed HEPA unit, not HEPA-type

A true HEPA filter is defined by a capture standard: at least 99.97 percent of particles at 0.3 micrometers, the hardest size to catch. Labels like HEPA-type, HEPA-style, or 99 percent at a larger particle size do not meet that bar and can let more fine smoke through.

Equally important is whether the unit is sealed. Air follows the path of least resistance. If gaskets and seams let air slip around the filter, some smoke bypasses the media no matter how good the filter is. Sealed-system designs route all the air through the HEPA media. This is harder to verify from a spec sheet, which is one reason third-party CADR testing is useful: it measures the whole machine, not just the filter on its own.

For smoke, prioritize a unit that pairs a true HEPA filter with verified CADR over one that advertises an exotic technology without published particle performance.

Add activated carbon for gases and odor

HEPA media captures particles. It does not remove the gases and volatile organic compounds that give smoke its smell and contribute to irritation. Those molecules pass straight through a particle filter. Capturing them requires activated carbon, which traps gases by adsorption onto its surface.

Carbon performance scales with the amount of carbon present. A thin carbon-coated pre-filter does little for sustained smoke; a deeper carbon stage holds up better. Carbon also saturates over time and needs replacement, so factor it into running cost.

Be clear-eyed about limits: for the health risks authorities focus on during smoke events, reducing PM2.5 is the priority, and carbon is a useful addition for odor and gases rather than a replacement for particle filtration.

Avoid ozone generators, be cautious with always-on ionizers

Do not buy an ozone generator to clean smoke. The EPA states that no federal agency has approved ozone-generating air cleaners for use in occupied spaces, and that ozone can cause chest pain, coughing, shortness of breath, and throat irritation, and can worsen asthma. Ozone is the same irritant gas found in outdoor smog. Using one during a smoke event adds a pollutant on top of the smoke.

Ionizers and devices marketed with terms like plasma or ions are a separate caution. Some produce ozone as a byproduct. If you own a unit with an ionizer, you can usually run it as a plain HEPA filter with the ionizer switched off. The EPA notes ion generators can produce ozone at levels above what is considered harmful under some conditions.

The reliable approach for smoke is mechanical filtration: move air through sealed HEPA media and carbon. It does not generate byproducts and its performance can be measured.

The DIY box-fan filter as a supplement

The Corsi-Rosenthal box is a DIY air cleaner built from a 20-inch box fan, four or five MERV-13 furnace filters arranged in a cube, a cardboard top, and tape. It costs roughly 50 to 150 dollars in materials and takes about 15 minutes to assemble. The EPA studied DIY air cleaners as part of its ASPIRE research into reducing indoor smoke exposure.

Testing at UC Davis and elsewhere found these boxes can deliver clean air at rates comparable to, and sometimes better than, commercial HEPA units, at a far lower cost per unit of clean air. MERV-13 is not HEPA, but it captures a large share of smoke particles, and the high airflow of a multi-filter cube compensates.

Two cautions from the research. Performance collapses with dirty filters, so replace them often during a smoke event. And use a newer box fan kept clear of obstructions. Treat a DIY box as a strong supplement, for example in a second room, rather than a reason to skip a properly sized unit in your main clean room.

FAQ

What CADR do I need for my room during wildfire smoke?: Use the tobacco smoke CADR, since it reflects small-particle performance. The EPA suggests a tobacco smoke CADR of at least two-thirds the room's floor area in square feet; AHAM's general rule is a CADR roughly equal to the room area. So a 120-square-foot room wants a tobacco smoke CADR of about 80 to 120. Size up for ceilings over 8 feet or for heavier air-change targets.
Is HEPA-type the same as true HEPA?: No. True HEPA captures at least 99.97 percent of particles at 0.3 micrometers, the hardest size to catch. HEPA-type, HEPA-style, and similar labels do not meet that standard and can let more fine smoke through. For smoke, choose a true HEPA unit, ideally a sealed design with published CADR.
Do I need activated carbon for smoke?: HEPA media handles the particles, which is the priority for the health risks authorities track. It does not capture the gases and odor compounds in smoke; those pass through. Activated carbon adsorbs gases and helps with smell. A deeper carbon stage works better and lasts longer than a thin carbon-coated pre-filter.
Are ozone generators or ionizers good for smoke?: Avoid ozone generators. The EPA says no federal agency has approved them for occupied spaces, and ozone can irritate the lungs and worsen asthma. Ionizers can produce ozone as a byproduct; if your unit has one, run it with the ionizer off as a plain HEPA filter. Mechanical filtration through sealed HEPA and carbon is the reliable, byproduct-free option.
Does a DIY box-fan filter actually work?: Yes, as a supplement. EPA research and UC Davis testing found Corsi-Rosenthal boxes can match or beat commercial HEPA units on clean-air delivery at a much lower cost. They use MERV-13 filters, not HEPA, but high airflow compensates. Replace filters often during smoke, and use a newer box fan kept clear of obstructions.

Sources

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