Wildfire Smoke Indoors: MERV 13, HEPA CADR, and What Fails
What actually cleans wildfire smoke indoors: MERV 13 in your HVAC, portable HEPA sized by CADR, sealing. EPA measured the DIY Corsi-Rosenthal box at 401 cfm.
The problem, stated honestly
When wildfire smoke settles over a city, your house is not a sealed container. The EPA reports that during smoke events, indoor PM2.5 levels typically run about 55 to 60 percent of outdoor levels with doors and windows closed — and that this varies enormously between homes, from as low as 30 percent to as much as 100 percent. A leaky house with no filtration is effectively outdoors.
PM2.5 — particles 2.5 micrometers and smaller — is the component public health agencies focus on, because particles that small travel deep into the lungs. Closing the windows is step one and is not sufficient. What follows is ordered by how much air it cleans per dollar.
One federal scale, four state programs. Worth getting straight before the numbers below. The AQI on AirNow, and the EPA guidance this article cites, are federal — the index is defined in 40 CFR Part 58 Appendix G, so an AQI of 155 means the same thing in Georgia as in Oregon. One catch: EPA revised the PM2.5 breakpoints effective 6 May 2024, so a given µg/m³ reading now maps to a different AQI than it did before, and any older µg/m³-to-AQI equivalence you find may predate the change. What is not national is who turns those numbers into instructions. Four states now have enforceable wildfire-smoke rules for outdoor workers: California (Cal/OSHA 8 CCR §5141.1, engaging at AQI 151, permanent since 1 February 2021), Oregon (OAR 437-002-1081, from 1 July 2022, engaging much lower at 35.5 µg/m³), Washington (Ch. 296-820 WAC, effective 15 January 2024 and updated 16 May 2025, where the legal trigger is µg/m³ and the AQI is only an approximation), and now Nevada, whose SB 260 took effect 1 January 2026 — we could not confirm the final implementing regulation text, so check NRS Chapter 618 rather than the tier figures circulating online. No other state has one, federal OSHA has no wildfire smoke standard, and the rest default to EPA advice. And read one mark carefully: CARB certification — Title 17 CCR §§94800–94810, not Title 13 — is a California sales requirement, not a national approval or a federal safety mark. A purifier sold legally elsewhere need not carry it, and its absence outside California tells you nothing.
Start with the HVAC filter: MERV 13
If you have central heating or air conditioning, the filter already in it is the cheapest large-scale air cleaner you own, because the blower is already sized to move the whole house’s air.
The EPA publishes what each MERV rating captures. In the 0.3 to 1.0 micrometer band — the hard band, where smoke particles live — MERV 13 is rated at 50 percent or better. MERV 11 is rated at 20 percent or better in that same band, and MERV 8 is not rated there at all; its ratings start at 20 percent for the coarser 1.0 to 3.0 micrometer range. That gap is the whole argument for MERV 13.
The measured payoff is real but partial. The EPA states that upgrading to MERV 13 or higher and running the fan continuously can decrease indoor PM2.5 by about 50 percent, versus about 24 percent for a typical low-efficiency filter run continuously. Set the thermostat fan to On, not Auto — a filter only works when air is moving through it.
The caveat is genuine. Higher-MERV media has a higher pressure drop. A system with an undersized return duct or an older single-speed blower can lose enough airflow to reduce both comfort and the total volume of air filtered per hour. The EPA’s own guidance is that you may need to consult an HVAC technician to determine the highest efficiency filter your system can handle. The practical workaround, where your filter slot allows it, is a deeper filter: a 4-inch MERV 13 presents far more media area than a 1-inch at the same rating, so it restricts airflow less. EPA’s DIY testing shows the same physics — swapping one 1-inch filter for one 4-inch filter more than doubled clean air delivery, from 111 to 248 cfm.
Check MERV 13 furnace filters on Amazon
The DIY Corsi-Rosenthal box is the best cost-per-CADR option
This is the recommendation we would make even though it earns us nothing.
The Corsi-Rosenthal box is a 20-inch box fan taped to a cube of four MERV 13 filters with a cardboard base and a cardboard shroud over the fan face. The EPA tested a range of DIY designs and measured clean air delivery rate for each:
| Configuration | Measured CADR (EPA lab) | Room size at AHAM’s wildfire 1:1 rule | Approx. parts cost |
|---|---|---|---|
| Box fan + one 1” MERV 13 filter | 111 ± 1 cfm | ~110 sq ft | ~$40 |
| Same, plus cardboard shroud | 156 ± 4 cfm | ~155 sq ft | ~$40 |
| Box fan + two 1” filters + shroud | 263 ± 22 cfm | ~260 sq ft | ~$55 |
| Box fan + one 4” filter + shroud | 248 ± 15 cfm | ~250 sq ft | ~$70 |
| Corsi-Rosenthal box (four filters) | 401 ± 31 cfm | ~400 sq ft | ~$75–100 |
| Commercial portable HEPA unit | Check the AHAM Verifide smoke CADR on the box | Match to that number | ~$80–400 |
The Corsi-Rosenthal configuration was the highest performer in EPA’s testing, and 401 cfm exceeds what many residential purifiers deliver at several times the price. Independent academic work supports it too: a Brown University team, publishing in Environmental Science & Technology, deployed the boxes across 17 rooms and measured PFAS concentrations dropping 40 to 60 percent and phthalates 30 to 60 percent, at roughly $100 per unit. The honest downside they documented was noise — about 5 decibels louder during the day and 10 at night.
Three safety points the EPA is specific about. Use a box fan manufactured in 2012 or later; EPA notes that fans built before 2012 were not tested to the current standard and pose known fire risks. Look for a UL or ETL mark. And replace filters often — EPA found DIY cleaners were “almost completely ineffective with dirty filters,” which during a heavy smoke event can mean weeks, not months.
Check 20-inch box fans on Amazon
Check MERV 13 20x20x1 filter 4-packs on Amazon
Sizing a commercial HEPA unit: use the smoke number, and size up
If you want something quiet and presentable, buy on CADR, not on the marketing claim about square footage.
AHAM’s standard guidance is the two-thirds rule: CADR should be at least two-thirds of the room’s floor area in square feet, so a 200 sq ft room wants a CADR of at least 132. But AHAM states that for wildfire smoke the rule changes — cleaners used against wildfire smoke should have a smoke CADR equal to the room size, a 1:1 ratio. That is the number to design around during smoke season.
Two details matter when reading the label. Use the smoke CADR figure, not dust or pollen — smoke is the smallest particle test and gives the lowest number. And note that “HEPA” is defined as 99.97 percent capture at 0.3 micrometers, which is the hardest particle size to catch; filters are more efficient both above and below it, so a true HEPA handles the very fine smoke fraction well. Beware devices marketed as “HEPA-type” that carry no such rating.
Check high-CADR HEPA air purifiers on Amazon
Seal the room, then clean it
Filtration fights infiltration. The EPA’s clean room guidance is to pick one room — ideally a bedroom you can close off, big enough for the household — and run filtration there rather than treating the whole house. Weatherstrip obvious gaps, but never seal anything in a way that makes it hard to get out. On a DIY unit, get a good seal between fan and filters; leakage there is where the airflow you paid for escapes.
What does not work
Ozone generators. The EPA’s position is unambiguous: ozone does not remove particles from the air, and the concentration required to be effective against most indoor contaminants would greatly exceed health standards. The EPA further notes that no federal agency has approved these devices for use in occupied spaces.
Ionizers and “active” air treatment. As we covered in our look at Japanese air purifier technologies, independent evidence for ion-based air treatment is thin. California’s CARB requires electronic air cleaners — a category that explicitly includes ionizers, electrostatic precipitators, PCO and hydroxyl devices, and UV units — to emit no more than 0.050 ppm of ozone to be sold in the state. That is a useful floor, but CARB is clear it certifies ozone emissions and electrical safety only; it is not a measure of whether the device cleans anything.
Most masks, indoors. A NIOSH-approved N95 filters at least 95 percent of PM2.5 if it seals to your face, but the EPA’s guidance is to use respirators only after implementing more effective measures — staying indoors, reducing activity, running filtration. Cloth, surgical and paint masks do not prevent breathing in smoke. Indoors, cleaning the air beats wearing a mask in it.
Particle filters, against gases. Nothing above removes the gaseous fraction of smoke. HEPA and MERV media capture particles; the VOCs and gases pass through. That is why a room can read low on a PM2.5 monitor and still smell like a fire. Gas removal needs a real activated carbon bed, and the thin carbon layers in consumer units saturate fast.
Summary
Order of operations: MERV 13 in the HVAC with the fan on continuous, a Corsi-Rosenthal box or a portable HEPA sized at 1:1 smoke CADR to room area for the room you actually occupy, then sealing. The DIY box wins on cost-per-CADR by a wide margin — EPA measured 401 cfm for under $100 in parts — and no affiliate commission changes that. Watch AirNow for the AQI, replace filters far more often than you think, and treat ionizers and ozone generators as noise.
More on filtration technology and how to read purifier specs in our guide to Japanese air purifiers, and more gear breakdowns at gadget-hub.jp/en.
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Frequently Asked Questions
- Q: At what AQI should I start closing up the house?
- A: The AirNow AQI scale runs Good (0-50), Moderate (51-100), Unhealthy for Sensitive Groups (101-150), Unhealthy (151-200), Very Unhealthy (201-300), and Hazardous (301+). At 101-150, people with asthma, heart or lung conditions, older adults and children should limit outdoor exertion. At 151+ the EPA says everyone may experience effects. Practically, close windows and start filtration before the number climbs, because clearing a house that has already filled with smoke takes hours.
- Q: Is the DIY Corsi-Rosenthal box really as good as a store-bought purifier?
- A: In the EPA's own laboratory testing, a Corsi-Rosenthal box built from a box fan and four MERV 13 filters produced a clean air delivery rate of 401 plus or minus 31 cfm — the highest of every DIY configuration EPA tested, and higher than many residential purifiers deliver. It costs roughly 75 to 100 dollars in parts. The tradeoffs are noise, bulk, and appearance, not performance. Brown University researchers measured about a 5 decibel daytime increase in rooms running them.
- Q: Will a MERV 13 filter damage my furnace or AC?
- A: It can restrict airflow if the system was not designed for it. Higher-MERV media has a higher pressure drop, and a system with an undersized return or an older single-speed blower can lose enough airflow to hurt performance. The EPA advises that you may need to consult a professional HVAC technician to determine the highest efficiency filter that will work with your system. A deeper 4-inch MERV 13 filter generally restricts airflow less than a 1-inch filter at the same rating.
- Q: Do ionizers or ozone generators help with wildfire smoke?
- A: No. The EPA states plainly that ozone does not remove particles from the air, and that ozone concentrations would have to greatly exceed health standards to be effective against most indoor pollutants. The EPA also notes that no federal agency has approved ozone generators for use in occupied spaces. California's CARB requires electronic air cleaners sold in the state to emit no more than 0.050 ppm of ozone, but that certification covers ozone and electrical safety only — it is not a measure of effectiveness.
- Q: Does a HEPA filter remove the smell of smoke?
- A: Not by itself. HEPA and MERV filters capture particles, not gases. Wildfire smoke also contains gaseous compounds and VOCs that pass straight through a particle filter, which is why a room can measure low PM2.5 and still smell like a campfire. Removing gases requires a substantial activated carbon bed, and the thin carbon layers in most consumer purifiers saturate quickly and need frequent replacement.