Creating a Clean Air Room at Home

When the AQI hits red or purple, you don't need to filter your whole house. You need one room where the air is dramatically cleaner than outside, where the family can sleep, work, and ride out the smoke event. EPA calls this a "clean room," and setting one up takes an hour and around $100 to $300.

Why it matters

During wildfire smoke events, indoor PM2.5 typically reaches 30% to 70% of outdoor levels in an unprotected home. A properly set up clean room can drop indoor PM2.5 below 10% of outdoor levels: from "very unhealthy" to "good," in the same building. The EPA's clean room guidance is the go-to reference for this.

Pick the right room

Look for:

• Small to medium size (100 to 300 square feet). Smaller is easier to keep clean.
• Few windows and exterior doors.
• A door that closes tightly and seals well.
• Comfortable for the time you'll spend there (bedroom is usually best).
• Bonus: attached bathroom, so you don't have to leave for basics.

For a household, the primary bedroom often wins. For a family with kids, the kid's bedroom or a finished basement room works.

What to do

Seal the room

• Close windows and exterior doors. Lock them so the seal is tight.
• Use weatherstripping or a rolled towel along the bottom of the door.
• Tape over any obvious gaps with painter's tape (gaps around old window frames, vents you don't need).
• Close fireplace flues. Tape the damper if needed.

Add the right filtration

You have two good options and one excellent option:

1. Portable HEPA purifier sized for the room. CADR should be at least two-thirds of the room's square footage. A 200 sq ft bedroom needs CADR 135 or higher. Run it 24/7 on at least medium during smoke events.
2. Corsi-Rosenthal box built with a 20-inch box fan and four MERV 13 filters. About $65, more clean air per dollar than commercial units. See the separate build guide.
3. Both, ideally. Stacking a HEPA and a CR Box pulls indoor PM2.5 down faster, which matters at the start of a smoke event.

If your central HVAC has a MERV 13 filter and is in good shape, run the fan continuously to add a third layer.

Avoid making it worse

Inside the clean room, don't:

• Cook (no air fryers, hot plates, or stoves).
• Burn candles or incense.
• Use a wood-burning anything.
• Vacuum without a HEPA-bagged vacuum.
• Smoke or vape.

These all add particles to the room you just spent an hour cleaning.

Plan for longer events

• Stock water, snacks, books, devices, chargers.
• Have a way to track outdoor AQI without opening windows (phone, monitor, app).
• Replace HEPA and box-fan filters more often during smoke season. Heavy use means 2 to 4 month replacement instead of the usual 6 to 12.
• If smoke lasts more than a few days, plan brief errand trips with the rest of the house left closed up, so you don't undo the seal.

What to avoid

• Choosing a kitchen or living room as the clean room. Cooking and high traffic make sealing pointless.
• Running an undersized purifier on low. CADR matters, and so does fan speed.
• Opening the door for long stretches. Smoke from the rest of the house leaks in fast.
• Air "fresheners" or essential oil diffusers. These add VOCs, which are not what you want.

Quick checklist

• One small to medium room with a door that closes tight.
• Windows shut, weatherstripping on the door, gaps taped.
• HEPA purifier with CADR equal to at least two-thirds of room sq ft, or a Corsi-Rosenthal box, or both.
• No cooking, candles, smoking, or fireplace use inside the room.
• Replace filters more often during heavy smoke (every 2 to 4 months).
• Water, snacks, charger, and a way to monitor AQI.

Sources

• Create a Clean Room to Protect Indoor Air Quality During a Wildfire, US EPA
• How to Create a Clean Room at Home (Factsheet), AirNow
• Strategies to Reduce Exposure Indoors, US EPA
• Creating Clean Air Spaces During Wildland Fire Smoke Episodes, NIH PMC
• Wildfires and Indoor Air Quality (IAQ), US EPA
• Winners of the Cleaner Indoor Air During Wildfires Challenge, US EPA