Seasonal · PM2.5 defense
Wildfire smoke indoors: the clean room playbook
Smoke's harm rides on PM2.5 — particles small enough to reach deep into lungs and, for fine fractions, the bloodstream. Indoors is only safer than outdoors if you make it so: unmanaged, indoor levels track outdoor smoke closely. This is the EPA's clean-room approach, ordered by impact, with the filtration specifics filled in.
The playbook, by impact
- Seal the envelope. Close windows and doors; in extended events, damp-towel obvious gaps. If you have a fresh-air intake or an HRV/ERV, set it to recirculate or close the intake — during smoke, "fresh" air isn't.
- Make the HVAC a whole-house filter. Best filter the system tolerates — MERV 13 if it runs it comfortably — and set the fan from AUTO to ON so air passes the filter continuously, not just during heating/cooling calls. Expect to replace the filter promptly afterward; smoke loads media fast.
- Create one genuinely clean room. Pick the room where you sleep, close it off, and run a portable cleaner sized to it — a sealed HEPA unit or a CR box (peer-reviewed smoke CADRs of 184–233 CFM cover roughly a 285–360 sq ft room). Clean-air delivery in the room you occupy 8+ hours beats diluted effort across the whole house.
- Stop making particles indoors. No frying, broiling, candles, incense, or vacuuming without a HEPA vacuum while smoke persists — indoor sources stack on top of infiltrating smoke.
- Protect the trips outside. A well-fitted N95 or KN95 handles PM2.5; cloth and surgical masks don't seal. Keep car ventilation on recirculate with the cabin filter in good shape.
- Watch the people smoke hits hardest. Asthma, COPD, heart disease, pregnancy, kids and older adults: symptoms during smoke events (chest tightness, wheezing, unusual fatigue) are a talk-to-your-clinician matter, not a push-through matter.
AQI thresholds, translated to action
| AQI | Band | Indoor response |
|---|---|---|
| 51–100 | Moderate (Yellow) | Sensitive individuals: have the clean-room gear ready; everyone else: normal life, watch the forecast. |
| 101–150 | Unhealthy for sensitive groups (Orange) | Sensitive groups run the full playbook now. Everyone: close up, fan to ON. |
| 151–200 | Unhealthy (Red) | Everyone runs the playbook. Clean room active, portable cleaner on high, indoor particle sources off. |
| 201+ | Very unhealthy / hazardous (Purple+) | Maximize everything; minimize trips out; N95 when you must; consider relocating sensitive household members if it persists. |
The one product category to refuse
Ozone generators marketed as air purifiers — sometimes as "activated oxygen" or "energized air." Ozone is itself a lung irritant; the EPA's position is blunt that it both harms health at concentrations that do anything and doesn't remove particles anyway. During a smoke event it adds a second respiratory insult to the first. Adjacent caution: some ionizer-type purifiers emit ozone as a byproduct — if a device's mechanism isn't "fan pushes air through a physical filter," look for certified low ozone emissions (CARB certification in California is the useful proxy) or skip it. Boring mechanical filtration is the entire trick.
"It still smells like smoke"
Particles and smell are different problems. MERV 13 and HEPA capture the particles doing the health damage; the campfire odor is gas-phase VOCs that pass through particle media. Meaningful odor removal needs pounds of activated carbon (some larger purifiers carry real beds), and even then it's mitigation, not elimination. Health priority during events is unambiguous: particles first. The smell fades after; the PM2.5 exposure doesn't un-happen.
Common questions
Window AC during smoke — on or off?
Most window units mainly recirculate room air, with a small fresh-air vent — close that vent (many have a lever) and it's fine to run. Evaporative ("swamp") coolers are the exception: they pull large volumes of outdoor air through by design, the wrong move in smoke — use other cooling if you can.
How do I know my setup is working?
A consumer PM2.5 monitor (widely available from ~$40–$200, or check the crowd-sourced PurpleAir map for your area) turns guessing into numbers. The satisfying version: watch your clean room's reading drop after the cleaner runs 30–60 minutes while outdoor numbers stay ugly.
After the smoke clears, then what?
Air out the house, inspect and likely replace HVAC filters (smoke events count double against any interval), rebuild or at least inspect CR box filters, wipe horizontal surfaces (settled particles resuspend), and restock — the next event shouldn't find you shopping with everyone else.
Sources
- US EPA, Wildfires and indoor air quality (clean-room guidance)
- US EPA / AirNow, AQI bands and fire & smoke map
- US EPA, ozone generators sold as air cleaners (health position)
- DIY air cleaner performance during wildfire events, Indoor Environments (2024)
General preparedness information, not medical advice. People with respiratory or cardiac conditions should follow their clinician's smoke-event guidance, which outranks anything on a website.