Wildfire Smoke and Indoor Air Quality: Monitoring, Filtration, and Protection

Volume I  ·  May 2026  ·  675 words

Wildfire smoke penetrates buildings. Even in a closed home, indoor PM2.5 concentrations during a nearby wildfire can reach 50–200 µg/m³ — levels associated with measurable increases in respiratory and cardiovascular emergency department visits within 24 hours. An air quality monitor provides the data to decide when to run filtration, when to seal the building, and when to leave.

Wildfire Smoke Composition

Wildfire smoke is predominantly fine particulate matter (PM2.5) — particles smaller than 2.5 µm that penetrate deep into the alveoli. The particle size distribution peaks at 0.3–0.5 µm, well within the detection range of laser particle counters. Smoke also contains carbon monoxide, VOCs (benzene, formaldehyde, acrolein), and polycyclic aromatic hydrocarbons (PAHs), but the dominant acute health risk is PM2.5. Monitoring PM2.5 captures the primary exposure pathway.

PM2.5 from wildfire smoke is optically different from ambient urban PM2.5 (sulfates, nitrates, road dust). The mass scattering efficiency of smoke particles is higher — they scatter more light per unit mass. A laser particle counter calibrated against urban aerosol may overestimate smoke PM2.5 by 10–30%. The reading is still useful for relative changes and trends, but the absolute concentration should be interpreted with this bias in mind.

Health Thresholds

Indoor Filtration

A portable HEPA air purifier is the most effective intervention. The Clean Air Delivery Rate (CADR) specifies the volume of filtered air delivered per minute — a CADR of 300 CFM (cubic feet per minute) for smoke means the purifier can filter the air in a 300 ft² room approximately 5 times per hour (assuming 8-foot ceilings). Size the purifier to provide ≥ 4 air changes per hour for the room:

Required CADR (CFM) = Room area (ft²) × Ceiling height (ft) × 4 ÷ 60

Recommended purifiers: Coway Airmega AP-1512HH (CADR 246 CFM smoke, ~$200) for rooms up to 300 ft², Blueair Blue Pure 211+ (CADR 350 CFM smoke, ~$300) for larger spaces.

DIY Box Fan Filter

A 20-inch box fan with a MERV 13 furnace filter taped to the intake side provides approximately 50–100 CFM CADR at a cost of $40. This is 25–50% of the performance of a dedicated HEPA purifier but at 15–20% of the cost. The Lasko 20" box fan and 3M Filtrete 20×20 MERV 13 filter is the standard configuration, validated by university and EPA testing.

Sealing the Building

During smoke events, close all windows and doors. Set HVAC systems to recirculate (disable fresh air intake if your system has one). Place a damp towel at the base of exterior doors to reduce infiltration. These measures reduce but do not eliminate smoke penetration — a typical US home experiences 0.5–2 air changes per hour even with windows closed. Filtration is still required.

Do not rely on an air quality monitor alone. A monitor showing indoor PM2.5 of 50 µg/m³ is telling you the filtration is insufficient — act on the reading by increasing filtration or considering evacuation. The monitor's value is in closing the feedback loop between outdoor conditions, building envelope performance, and filtration capacity.