Wildfire Smoke and Your Indoor Air

Last updated September 24, 2026 · Monitor My Air

Wildfire smoke is mostly fine PM2.5 that slips indoors easily. With the right filter, a HEPA unit, and your HVAC on recirculate, you can keep indoor air far cleaner than outside.

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What it is

Wildfire smoke is a dense mix of fine particles and gases, and the biggest health hazard in it is PM2.5, the same deep-lung particles covered elsewhere in this library, but at extreme concentrations. During a bad smoke event, outdoor PM2.5 can climb into the hundreds of µg/m³, dozens of times the healthy target. These particles are small enough to leak through gaps around windows and doors and through ordinary ventilation, so indoor air can degrade fast even with everything shut. The goal during a smoke event is to make your home a clean-air refuge.

The numbers that matter

PM2.5 target: keep indoor levels under 15 µg/m³, and ideally under 5. During heavy smoke, simply staying well below the outdoor reading is a win.
Outdoor AQI: when local air quality index passes 150 (unhealthy), treat it as time to seal up and filter. Above 300 is hazardous for everyone.
MERV-13: the minimum filter rating that meaningfully captures wildfire particles in a home HVAC system.

What NOT to waste money on

Do not buy ozone generators or ionizers for smoke. Ozone is a lung irritant and adding it during a smoke event makes your air worse, not better. You do not need an expensive whole-house air scrubber; a good MERV-13 filter on recirculate plus one or two HEPA units protects most homes. Dust masks and simple cloth masks do little against PM2.5, so if you must go outside, use a properly fitted N95 or better rather than a bandana. And remember that a monitor showing you the bad numbers does not clean anything. The filters and the recirculate setting are what actually protect your lungs.

What professionals know

Wildfire smoke PM2.5 behaves like other fine particulate but arrives in overwhelming concentration and with a substantial ultrafine fraction, plus gas-phase components (carbon monoxide, VOCs, and irritant aldehydes) that HEPA filtration does not remove. This is why ventilation strategy flips during smoke: normally you ventilate to lower CO2 and VOCs, but during heavy smoke you deliberately recirculate and filter, accepting a temporary CO2 rise as the lesser harm. Once outdoor air improves, ventilate to flush accumulated indoor gases.

Filtration physics favors you here. MERV-13 captures a large share of PM2.5, and HEPA captures 99.97 percent at the most-penetrating 0.3 micron size. Effectiveness scales with clean air delivery rate versus room volume; target 4 to 5 air changes per hour in the clean room. The Corsi-Rosenthal box (four MERV-13 filters plus a box fan) delivers high CADR at low cost and has been validated in peer-reviewed testing, making it a legitimate option, not a mere hack.

On monitoring during smoke, low-cost optical PM2.5 sensors tend to over-read at very high humidity and extreme concentrations, so pair indoor readings with official data. EPA's AirNow Fire and Smoke Map integrates regulatory monitors and humidity-corrected PurpleAir sensors and is the authoritative source for outdoor conditions. For personal protection outdoors, only a well-sealed N95/FFP2 or higher provides real PM2.5 protection. Sources: US EPA, CDC/NIOSH, California Air Resources Board wildfire guidance.

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