Wildfire Smoke Metrology: Protecting Indoor Air Against Catastrophic PM2.5 Infiltration

Wildfire smoke is a complex, toxic aerosol mixture composed of sub-micron carbonaceous ash (PM2.5 and PM0.1), polycyclic aromatic hydrocarbons (PAHs), carbon monoxide, and toxic volatile organic compounds (VOCs). When outdoor AQI spikes above 300, residential air purifiers become vital life-safety equipment.

1. Indoor Infiltration and Building Envelope Dynamics

A typical residential building envelope has an infiltration rate between 0.35 and 1.5 Air Changes per Hour (ACH) through window gaskets, electrical outlets, and chimney flues. When outdoor PM2.5 reaches 250 µg/m³ (Hazardous AQI 300+), an unmitigated home will equilibrate to 70% of outdoor smoke concentrations within 4 hours.

2. Creating a Clean Room Safe Haven Zone

During catastrophic wildfire events, whole-house purifiers often cannot overcome whole-house building leakage. The EPA and CDC recommend establishing a designated Clean Room, typically a master bedroom with minimal external doors and sealed windows:

• Select a room large enough for occupants to sleep comfortably.
• Close all windows and tape painter's tape or damp towels along drafty door thresholds.
• Deploy an air purifier rated for at least 1.5 times the room's floor area to deliver 6+ air changes per hour.
• Keep the purifier running 24/7 on medium or high; cycling the unit on and off causes particle levels to rebound within 20 minutes.