Wildfire Smoke: Protecting Indoor Air When You Can't Leave
You do not need to be near a fire to be affected by one. Smoke routinely travels hundreds of miles, and the fine particulate matter it carries, PM2.5, is small enough to reach deep into the lungs and cross into the bloodstream. For most people during most smoke events, the relevant question is not whether to evacuate, but how to make indoor air survivable for several days.
What the Numbers Mean
Air Quality Index values are easier to act on when you treat them as thresholds rather than a continuous scale:
- 0–50 (Good): No action needed.
- 51–100 (Moderate): Unusually sensitive people may want to limit prolonged exertion outdoors.
- 101–150 (Unhealthy for Sensitive Groups): Children, older adults, pregnant people, and anyone with heart or lung conditions should stay inside.
- 151–200 (Unhealthy): Everyone should limit outdoor exertion.
- 201+ (Very Unhealthy / Hazardous): Stay indoors with filtration running. Avoid outdoor activity entirely.
Building a Clean Room
Pick one interior room, ideally with few windows and a door that closes properly. Then:
- Run a HEPA air purifier sized for the room's square footage.
- Close windows and doors, and seal obvious gaps with weather stripping or a rolled towel.
- Set central HVAC to recirculate rather than pulling in outside air, and fit it with a MERV 13 filter if the system supports one.
- Avoid anything that adds particulates indoors: candles, gas stoves, vacuuming without HEPA, wood stoves.
A box fan with a taped-on MERV 13 filter is a well-documented budget substitute for a purifier. It is less efficient and noisier, but it meaningfully lowers indoor PM2.5.
Masks Do Not All Work the Same
Cloth and surgical masks do very little against PM2.5. An N95 or P100 that seals properly against the face does. Facial hair breaks the seal, and a mask worn loosely provides a fraction of its rated protection.
Watch Conditions, Not the Calendar
Smoke conditions can shift within hours as winds change. Community reports on Blaze Sentry often register a change in visibility and smell before regional monitoring stations reflect it, which can buy you time to close up and switch on filtration.