A wildfire-smoke plume is not one thing. It is a mixture of fine and ultrafine particles, vapors, semi-volatile organics, and gas-phase species. By mass, PM2.5 dominates: a heavy smoke day will push outdoor PM2.5 from a background of 10 µg/m³ to 200–500 µg/m³, occasionally higher. The EPA Wildfire Smoke Course is the canonical reference.
On the gas side, smoke carries carbon monoxide (very high near the fire, dilutes quickly downwind), benzene, formaldehyde, acrolein, and a long tail of VOCs the SEN66 will see as a sustained VOC-index elevation. NOx rises moderately. None of those gas-phase species reach acutely toxic levels at the distances most people experience smoke from, but they irritate eyes, nose, and lungs at the levels reported during heavy events.
The correct indoor response is counterintuitive: close everything. Cracking a window on a wildfire-smoke day pulls smoke directly inside. Run a HEPA purifier (or several DIY Corsi-Rosenthal box-fan filters), set HVAC to recirculate, and stay indoors. The dashboard will suppress its usual "open a window" suggestion when outdoor AQI is in red or worse.
Recovery: once outdoor PM2.5 drops below 35 µg/m³ for several hours, ventilate to clear residual indoor smoke. Rain is the great ally, see the precipitation article. Wildfire seasons in much of North America now extend May through October; in coastal British Columbia and California, year-round monitoring is becoming routine.
References
- EPA - Wildfire smoke course www.epa.gov
- AirNow - Wildfires & smoke www.airnow.gov
- CDC - Wildfire smoke health information www.cdc.gov
- Reid et al. - Critical review of wildfire smoke health effects doi.org
