Wildfire Smoke and Indoor Air Quality: How to Protect Your Home During Smoke Events

In 2020, Portland, Oregon recorded the worst air quality of any major city in the world — not due to industrial pollution, but from wildfire smoke blowing in from fires hundreds of miles away. The AQI reached 516 on the 0–500 scale that the EPA describes as "hazardous" at its maximum of 500. California, Washington, Oregon, Colorado, and increasingly the Mountain West and Southeast now experience dangerous smoke events annually.

The health response most public advisories recommend is to "stay indoors." But how protective is your home from wildfire smoke? The answer depends almost entirely on your building's air leakage and whether you have appropriate filtration running.

Why Wildfire Smoke Is Different from Other Indoor Air Pollutants

Wildfire smoke is primarily fine particulate matter — PM2.5 (particles 2.5 micrometers and smaller) — at concentrations that can reach hundreds of times normal background levels during severe events. A "hazardous" AQI day corresponds to roughly 250 µg/m³ of PM2.5; for context, the EPA's 24-hour safe exposure limit is 35 µg/m³.

PM2.5 is the most medically significant air pollutant because particles this size penetrate deep into the lungs and can enter the bloodstream. Health effects at elevated concentrations include:

• Immediate: eye, nose, and throat irritation; coughing; shortness of breath
• Short-term (days to weeks of exposure): reduced lung function, increased asthma attacks, cardiac stress
• Long-term: cardiovascular disease, respiratory disease, and lung cancer risk

Wildfire smoke also contains CO, VOCs, polycyclic aromatic hydrocarbons (PAHs), and heavy metals — making it more complex than industrial PM2.5.

How Much Smoke Gets Into Your Home?

Studies of home smoke infiltration during wildfire events consistently find that, without active filtration, indoor PM2.5 concentrations reach 50–80% of outdoor levels within a few hours of a smoke event. A "sealed" home without filtration is far less protective than most people assume.

The key variable is your home's air exchange rate (how many times per hour the total air volume is replaced with outdoor air). Most US homes have a natural air exchange rate of 0.3–0.5 air changes per hour without any mechanical ventilation. Over an 8-hour smoke event, that means roughly 2.5–4 full air volume replacements with smoky outdoor air.

The factors that matter most for infiltration:

• Building age: Older homes are typically leakier (more infiltration); newer, tighter homes fare better
• Windows and doors: Seal gaps, especially around sliding doors and single-pane windows
• HVAC operation: HVAC systems that draw outdoor air in (fresh air intakes) can actively pull smoke indoors — close fresh air dampers during smoke events
• Fireplace and exhaust fans: All bathroom and kitchen exhaust fans should be off during smoke events (they exhaust indoor air, which is replaced by smoky outdoor air)

Air Purifiers: Your Primary Defense

A properly sized HEPA air purifier running continuously during a smoke event can reduce indoor PM2.5 by 50–90% compared to the indoor-without-purifier baseline. This can mean the difference between indoor air at "hazardous" levels and indoor air at "moderate" or "good" levels during even severe events.

What to Look for in a Smoke-Season Purifier

True HEPA filtration: HEPA filters capture 99.97% of particles at 0.3 microns — well below the 2.5 micron PM2.5 threshold. This is non-negotiable. "HEPA-type" filters are not adequate.

High CADR for smoke: AHAM certifies air purifiers with separate CADR (Clean Air Delivery Rate) values for smoke, dust, and pollen. For wildfire smoke, the smoke CADR is most relevant. Size for 5–6 air changes per hour in the room you'll shelter in, not the manufacturer's "room size" specification, which typically assumes only 4–5 ACH.

Activated carbon for gases: Wildfire smoke contains significant gaseous pollutants (CO, VOCs, PAHs) that HEPA filters don't capture. A substantial activated carbon stage handles these. Look for 2+ lbs of activated carbon media rather than thin carbon-coated mesh.

Filter change indicators: During smoke events, HEPA filters can become saturated faster than normal. A filter change indicator or scheduled replacement is important — a clogged filter reduces effectiveness significantly.

DIY Corsi-Rosenthal Box

During the 2020 smoke season, an engineer named Richard Corsi and a teacher named Jim Rosenthal independently developed what became known as the Corsi-Rosenthal Box: four MERV-13 furnace filters duct-taped to a box fan in a cube configuration. This DIY device costs $60–$80 to build and provides CADR equivalent to a $300–$400 commercial purifier for smoke particles.

It's not a long-term replacement for a quality HEPA purifier — the MERV-13 filters don't perform as well for sub-micron particles and there's no activated carbon stage. But as an emergency supplement during a severe smoke event, multiple Corsi-Rosenthal boxes strategically placed throughout a home can dramatically reduce exposure.

Creating a Clean Room Strategy

During severe smoke events (AQI above 150), the most effective strategy is creating one or two "clean rooms" in your home — spaces with a running air purifier and sealed windows where family members can spend the most time:

1. Choose a bedroom or den that can be sealed from other parts of the house
2. Close all windows and exterior doors in the space
3. Place a running HEPA purifier on high (sized for 2× the room's square footage for maximum ACH)
4. Seal door gaps with towels if needed
5. Turn off window AC units that draw outdoor air (use recirculate mode only if central AC is available)
6. Monitor indoor PM2.5 with an air quality monitor to confirm effectiveness

Even a moderately good HEPA purifier in a closed room can achieve indoor PM2.5 levels significantly below outdoor levels within 1–2 hours of this setup.

After the Smoke Clears: What to Do

Once outdoor air quality returns to good levels (AQI below 50), open windows to ventilate accumulated indoor pollutants from the smoke event. Change or clean HEPA filters that were running during the event — heavily loaded filters can restrict airflow and harbor accumulated smoke particles that can re-release under some conditions.

Surfaces in your home will have accumulated fine smoke particles during the event. Damp-mopping hard floors and damp-wiping surfaces reduces re-suspension of settled particles. Avoid dry sweeping, which suspends settled PM2.5 back into the air.