Wildfire smoke and air quality.
What the AQI number actually means, what a real mask looks like, and how to make one room in your house safe when the smoke arrives.
Reading the AQI
The Air Quality Index runs from 0 to 500. What matters is PM2.5 — particulate small enough to lodge in your lungs and cross into your bloodstream. Thresholds:
- 0-50 (green). Normal outdoor activity.
- 51-100 (yellow). Sensitive groups reduce prolonged outdoor exertion.
- 101-150 (orange). Sensitive groups indoors; everyone else, easy outdoors.
- 151-200 (red). Everyone reduces outdoor activity. N95 outdoors.
- 201-300 (purple). Everyone indoors with filtered air. Skip outdoor exercise entirely.
- 301+ (maroon). Hazardous. Stay inside; run HEPA filtration; consider relocating.
Check AirNow.gov, PurpleAir, or your phone's built-in weather app. PurpleAir is often the fastest local reading because it aggregates residential sensors.
Masks that actually work
Wildfire smoke is PM2.5 — 2.5 micrometers or smaller. Only tight-fitting particulate respirators help:
- N95 or KN95. Correct choice for most people. Must seal to the face — no gap, no facial hair under the seal.
- P100. Reusable half-face respirator with pink cartridges. Best for anyone doing outdoor work in heavy smoke.
- Cloth or surgical masks. No. The weave is too large.
Store N95s in a resealable bag in every go-bag and every vehicle. Kids fit different sizes than adults; check fit before you need it.
Indoor air, in order of cost
Free
- Close all windows and doors. Seal gaps under doors with rolled towels.
- Set HVAC to recirculate (not fresh-air intake).
- Skip cooking that produces smoke — no frying, no gas stove on high.
- No candles, no incense, no vacuuming (agitates settled particulate).
Under $50 — the DIY box-fan filter (Corsi-Rosenthal box)
A standard 20-inch box fan taped to a MERV 13 furnace filter is remarkably effective at cleaning a room. Buy a $20 box fan and a $30 MERV 13 filter. Tape the filter to the intake side (arrows pointing into the fan). Turn on low. This will clean a 300 sq ft room measurably better than most $200 air purifiers.
Even better: a five-filter cube (four sides plus top, wrapped around the fan) — that's the Corsi-Rosenthal box, developed during COVID and equally effective against smoke.
$150-$400 — portable HEPA purifiers
One per bedroom, sized for the square footage. Levoit, Coway, Blueair, and IKEA all sell decent units. Run continuously during smoke events. Change filters after each major event.
$500+ — whole-home upgrades
- Upgrade HVAC filter to MERV 13 (as high as most systems will run without overworking the blower).
- Add a stand-alone in-duct HEPA filter if your HVAC can support the pressure drop.
- Seal leaky ductwork.
Vulnerable household members
Some people are much more sensitive to smoke:
- Anyone with asthma, COPD, or heart disease.
- Pregnant women — smoke exposure is associated with low birth weight.
- Kids under 12 — smaller airways, higher breathing rates.
- Adults over 65.
For these groups, the threshold to stay indoors drops to AQI 100 and the threshold to leave drops to sustained AQI 150 indoors.
When smoke becomes an evacuation
Smoke that hangs over a region for days is a different problem than a fast-moving fire. If indoor AQI stays above 150 despite filtration, or your household has medically sensitive members and indoor AQI stays above 100 for more than 48 hours, plan to relocate to a hotel or family in a clean-air region 100+ miles away. Book while the AQI is still rising — hotels in adjacent regions fill fast.
After the smoke clears
- Replace HVAC filters and air-purifier filters.
- Wipe down hard surfaces with damp cloths (dry dusting resuspends particulate).
- Wash exposed clothing and bedding.
- Anyone with lingering cough, chest pain, or shortness of breath needs to see a doctor — smoke exposure can cause delayed symptoms.
Common questions
- What AQI level is unsafe?
- AQI above 100 is unhealthy for sensitive groups (asthma, COPD, pregnant, elderly, kids). Above 150 is unhealthy for everyone. Above 300 is hazardous — stay inside with filtered air.
- Do cloth masks and surgical masks help with wildfire smoke?
- No. The particulate that hurts you (PM2.5) is smaller than the weave. You need a well-fitted N95, KN95, or P100. Cloth masks help with droplets, not fine particles.
- How far can wildfire smoke travel?
- Thousands of miles. The 2023 Canadian wildfires produced hazardous AQI in New York City for days. Distance from the fire is not protection.
- How do I make my home's air safer?
- Close windows, run HVAC on recirculate with a MERV 13 or better filter, use portable HEPA air purifiers in bedrooms, and add a DIY box-fan filter for large rooms. Seal gaps under doors with rolled towels.
- When should I leave for cleaner air?
- If indoor AQI stays above 150 despite filtration, or above 100 with anyone medically sensitive in the household, plan to relocate to a friend, hotel, or clean-air center 100+ miles away for the duration.
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