A clean air room is one room arranged to keep smoke particles lower than in the rest of a home during a wildfire or other outdoor PM2.5 event. It is a practical exposure-reduction measure—not a sealed bunker, a guarantee of safe air, or a reason to ignore evacuation and heat warnings.
EPA recommends choosing a room that can hold the household comfortably, reducing smoke entry, filtering the air, and avoiding indoor particle sources (EPA clean room guidance). Use this guide to do those steps without creating a different hazard.
Safety Limits Come First
Do not stay in a clean air room when the building itself is unsafe.
- Evacuation: If local authorities order an evacuation, leave by the recommended route. A purifier cannot protect you from flames, rapidly changing fire behavior, or structural danger.
- Extreme heat: Closed windows can make a home dangerously hot. If you cannot keep the room cool, power fails, or the cooling equipment is unsafe to operate, go to a cleaner-air and cooling location. EPA explicitly advises seeking shelter elsewhere when a home cannot stay cool or smoke infiltration remains too high.
- Carbon monoxide and combustion: A HEPA purifier does not remove carbon monoxide (CO). Never use a generator, charcoal grill, camp stove, or other fuel-burning device indoors or in a garage. Keep working CO and smoke alarms. If a CO alarm sounds, move to fresh air and contact emergency services; do not remain inside to troubleshoot the purifier.
- Medical needs: Keep prescribed medication and an asthma or disease action plan accessible. Ask a healthcare professional in advance about symptoms or equipment that require relocation or urgent care.
Check local fire alerts and air-quality advisories before deciding that sheltering at home is appropriate.
Step 1: Choose the Room
Select a room that is:
- large enough for everyone who will use it;
- easy to close off from the rest of the home;
- equipped with safe cooling or connected to an HVAC system you understand;
- away from cooking, smoking, fireplaces, and frequently opened exterior doors; and
- practical for sleeping or spending several hours.
A bedroom with an attached bathroom can work well. Do not block an exit, tape a door shut, or make evacuation harder. “Cleaner air” does not mean airtight: the goal is to reduce outdoor smoke entry while maintaining safe temperature and normal emergency access.
Step 2: Size Particle Filtration Correctly
For commercial purifiers, use the tobacco-smoke clean air delivery rate (CADR), which represents small-particle removal. EPA's consumer sizing table calls for about 65 cfm per 100 square feet with an 8-foot ceiling. For other ceiling heights, calculate from volume:
Room volume (ft³) = length × width × ceiling height
Required CADR (cfm) = room volume × target ACH ÷ 60
For a 12 × 12 × 8 ft room at 5 equivalent air changes per hour:
Room volume = 1,152 ft³
Required CADR = 1,152 × 5 ÷ 60 = 96 cfm
The old shortcut “room volume ÷ 2” is incorrect; it would yield 576 cfm for this example. Use the air purifier size calculator to avoid that mistake. EPA explains how to match CADR to room area in its Guide to Air Cleaners in the Home.
Published CADR is typically measured at the highest fan setting. Choose a unit you can run at a useful speed despite its noise. Keep the intake and outlet clear, and run the purifier continuously during smoke conditions if practical.
If a commercial purifier is unavailable
EPA describes a DIY box-fan air cleaner as a temporary option when a commercial unit of known performance is unavailable or unaffordable—not as a permanent equivalent. Use a 2012-or-newer box fan with a UL or ETL safety marking, follow the fan manufacturer's instructions, use a MERV 13 or higher filter when practical, align the airflow arrow, and maintain a secure seal. Never use a damaged fan or extension cord, and do not leave children unattended with it (EPA DIY air-cleaner safety guidance).
Our DIY air cleaner calculator can compare layouts, but construction quality and actual airflow still determine performance.
Step 3: Set HVAC and Cooling Safely
Cooling and smoke control have to work together.
- Central HVAC: Use recirculation when outdoor smoke is high and close a controllable outdoor-air intake if the system documentation permits. Consider MERV 13 or the highest filter the system can accommodate. Ask an HVAC professional if a higher-efficiency filter could restrict airflow or damage equipment.
- Window or wall unit: Many recirculate indoor air, but some have a fresh-air vent. Close that intake or select recirculation during smoke, following the product instructions.
- Portable air conditioner: A single-hose unit can depressurize the room and draw smoky air through leaks; EPA advises using it sparingly in smoky conditions. A properly installed dual-hose design generally reduces that pressure problem, but installation and seals still matter.
- Evaporative cooler: These normally bring in outdoor air. EPA recommends either using them sparingly or, only if safely accessible and compatible, filtering the intake as described in its guidance.
- Fans: A circulating fan can improve comfort and mixing but does not remove smoke unless air passes through a filter.
Never disable a required ventilation, combustion-air, or life-safety system without qualified guidance. If safe cooling is not possible with smoke controls in place, relocate to a cooling center, cleaner-air shelter, or another suitable building.
Step 4: Reduce Smoke Entry Without Creating a Trap
Close windows and exterior doors and limit unnecessary trips through them. Use ordinary weatherstripping or removable gap sealing where smoke clearly enters, but preserve exits and do not permanently alter rented property without permission.
Avoid running exhaust fans longer than necessary because they can depressurize the space and pull smoke through leaks. Do not use incense, candles, or smoke to “find leaks”—they add particles to the room. A low-cost PM2.5 sensor can show whether a change improves the trend, though consumer readings have uncertainty.
When outdoor air becomes cleaner, ventilate the room if it is safe to do so. EPA recommends taking advantage of temporary improvements by opening windows or the HVAC fresh-air intake and using kitchen or bathroom exhaust fans to refresh indoor air.
Step 5: Stop Indoor Particle and Combustion Sources
During a smoke event, the clean air room should be free of activities that add particles or gases:
- no smoking or vaping;
- no candles, incense, wood fires, or ozone-producing devices;
- avoid frying, broiling, and other smoky cooking;
- avoid aerosol sprays;
- damp-mop or wipe rather than stirring up dust; and
- vacuum only when needed with equipment designed to limit particle exhaust.
Fuel-burning appliances require special caution. A particle purifier cannot make an improperly vented stove, heater, fireplace, or generator safe. Follow appliance instructions and CO-alarm guidance.
Step 6: Monitor the Room and the Event
Use several signals rather than one indoor sensor number:
- Check official current conditions and forecasts through AirNow or your local air agency.
- Watch local fire, heat, power, and evacuation alerts.
- If you have an indoor PM2.5 monitor, place it away from the purifier outlet, cooking, and open doors. Compare trends over time rather than assuming perfect accuracy.
- Confirm that the purifier has airflow and that the filter is not heavily soiled, wet, damaged, or releasing odors.
- Check room temperature and the household's symptoms, medications, and ability to leave.
If smoke persists for days, filters may load faster than their normal replacement schedule. Inspect them and follow manufacturer instructions. Keep a spare filter when extended smoke is forecast.
A Clean Air Room Checklist
Before smoke season
- identify alerts and evacuation routes;
- select a room and measure it;
- calculate the required smoke CADR;
- obtain filters, prescribed medication, and replacement supplies;
- test smoke and CO alarms; and
- understand HVAC and air-conditioner settings.
When smoke arrives
- confirm that staying home remains safe;
- close outdoor-air openings and select recirculation where appropriate;
- start particle filtration early;
- avoid indoor particle and combustion sources;
- monitor smoke, heat, power, and local instructions; and
- be ready to relocate or evacuate.
For a broader event plan, use the wildfire smoke planner.
About Cigarette-Equivalent Estimates
Do not estimate a clean room's benefit by applying the purifier's filter percentage to the outdoor AQI. Measure indoor PM2.5 if possible. Our calculator can interpret a U.S. PM2.5 AQI or direct PM2.5 concentration; it must not convert an overall AQI driven by ozone, CO, PM10, sulfur dioxide, or nitrogen dioxide as if it were PM2.5.
The Berkeley Earth comparison uses 22 µg/m³ over 24 hours as one cigarette-equivalent for population-level risk communication. It is not inhaled dose, cigarette smoke mass, a personal diagnosis, or a count of cigarettes “prevented.” Read the methodology and limitations before using that output.
Bottom Line
A clean air room combines a suitable room, correctly sized particle filtration, reduced smoke entry, and fewer indoor sources. It is useful only while the home remains safe. Heat, power loss, CO or combustion hazards, medical needs, and local evacuation orders always take priority over keeping windows closed.