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Choose a room that fits everyone, close its windows and doors without blocking an exit, keep it safely cool, and filter its air. A cleaner-air room can reduce smoke exposure while you are indoors, but it cannot guarantee smoke-free air or make an unsafe home safe. The U.S. Environmental Protection Agency (EPA) recommends spending as much time as practical in one during a wildfire smoke event, as long as it is safe to stay home.
What a cleaner-air room does—and what it cannot do
A cleaner-air room is a space in your home where you take practical steps to limit wildfire smoke entering and remove airborne particles. It may be useful even if you cannot make the whole house cleaner. The goal is to reduce exposure, not to create a sealed or guaranteed smoke-free space. The EPA explains the approach in its home guidance on creating a cleaner-air room.
There are two parts to the setup: reduce smoke coming into the room, then filter the air that is already there. You also need to avoid adding indoor particles and make sure the room stays comfortable enough to use. Those needs can conflict: closing windows helps limit smoke entry, but a home that becomes dangerously hot is not a safe place to remain. A cleaner-air room is not a substitute for local emergency instructions or evacuation.
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Set up the room in this order
1. Choose a room that works for everyone
Pick a room large enough for the people who will use it, with as few windows and doors as practical. A bedroom with an attached bathroom may be convenient if you need to spend extended time there. Comfort and safe access matter too: do not choose a room that makes it difficult to leave in an emergency. Plan to keep the route out clear.
Think about how the room will work for the whole household, not only how small or enclosed it is. People need enough space to sit or sleep, operate the filtration equipment, and reach the door. Keep necessary exits usable; limiting air exchange should never mean barricading the room.
2. Close openings, but do not block your exit
Close the room’s windows and doors when smoke is present outdoors. Keep doors closed when practical to limit air exchange with other parts of the home, but do not obstruct an emergency exit. Avoid running an exhaust fan or range hood in the room: exhausting indoor air can draw replacement air from elsewhere in the home, potentially bringing in smoke.
Closing openings reduces one route for smoke to enter, but it does not make a home airtight. Smoke may still move indoors, which is why filtration and reducing indoor particle sources are part of the plan.
3. Manage outdoor-air intake and temperature
Use fans, window air conditioning, or central air conditioning as appropriate to keep the room cool. If your air-conditioning or ventilation system has a fresh-air intake or outdoor-air option, close it or select recirculation during smoky periods if the system allows it. System controls differ, so consult the equipment instructions if you are unsure which setting recirculates indoor air.
Do not keep windows closed if the home becomes too hot to stay in safely. When outdoor air quality improves, opening windows can help ventilate the home; if conditions remain smoky and the room cannot be kept cool, follow local guidance and move to a safer, cooler place. The choice is not simply “windows closed at all costs”: staying safe from heat takes priority over maintaining the room setup.
4. Choose a filtration route
Use a portable room air cleaner, compatible central HVAC filtration, or—if a commercial portable cleaner is unavailable or unaffordable—a temporary DIY box-fan cleaner. The options can be combined when they are compatible. The table below compares what each requires; details for selecting and operating each option follow.
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| Option | Best fit | What to check | Main limitation |
|---|---|---|---|
| Portable room air cleaner | You want filtration in one room, and can choose a unit sized for its floor area. | Tobacco-smoke CADR, low or no ozone emissions, noise, and replacement-filter expense. | It only addresses the air in the room where it is used; filter costs and noise may affect continued use. |
| Central HVAC filter | Your home has forced-air heating or cooling and the system can accommodate a higher-efficiency filter. | Whether the equipment supports MERV 13 or higher; check with the manufacturer or an HVAC technician. | A filter that is not compatible with the system is not an appropriate upgrade. |
| DIY box-fan cleaner | A portable cleaner is not available or affordable, and you can assemble and operate the unit safely. | Fan certification and age, filter fit and airflow direction, secure attachment, and supervision. | Performance varies with design and assembly; EPA presents it as a temporary alternative. |
There is no single best route for every home. A portable cleaner offers a room-focused option with a standardized product rating, but its purchase price, noise, and replacement filters matter. Central filtration can circulate air through an existing forced-air system, but only if the equipment can use the chosen filter. A DIY unit can be a less expensive fallback when a commercial portable unit is unavailable or unaffordable, but it takes correct parts, assembly, and supervision, and its performance is less standardized. Compare ongoing filter expense as well as the initial purchase or assembly cost; the EPA guidance does not set a universal price for any of these options.
How to choose and run a portable air cleaner
For a portable unit, compare its tobacco-smoke clean air delivery rate (CADR) with the room’s floor area. EPA’s rule of thumb is a tobacco-smoke CADR of at least two-thirds of the room’s area in square feet. For example, EPA says a 10-by-12-foot room (120 square feet) needs a tobacco-smoke CADR of at least 80. This is a sizing recommendation, not a guarantee of a particular indoor particle level or health outcome. See the EPA’s indoor air filtration guidance.
- Measure the room. Multiply its length by its width in feet to get floor area. For example, 10 × 12 = 120 square feet.
- Check the tobacco-smoke CADR. Use the device’s stated tobacco-smoke rating, not a rating for another particle category. For a 120-square-foot room, apply EPA’s two-thirds rule: 120 × 2/3 = 80 CADR.
- Check ozone emissions. Choose a unit certified by the California Air Resources Board to limit ozone emissions. EPA includes this as a selection criterion for cleaner-air rooms.
- Consider whether you will keep using it. Noise at the intended fan setting and the cost of replacement filters affect whether a unit is practical for extended use. Check the filter replacement requirements and expense before choosing.
- Run it continuously at the highest fan setting that is practical. Higher fan settings may be noisy, so choose a setting you can tolerate while spending time in the room. Keep the air cleaner operating during smoky periods and follow the manufacturer’s operating and maintenance directions.
A portable cleaner is a room-level measure: place and run it in the room where people are spending time rather than assuming it will clean the whole home. The EPA’s current materials do not make CADR a promise that indoor air will reach a particular concentration; the actual result depends on the home and conditions.
When comparing models, use the room’s floor area and tobacco-smoke CADR as the starting point, then weigh the features that affect whether you can run the unit as recommended. A unit that is difficult to tolerate because of noise, or whose replacement filters are too costly to maintain, may be less practical for continuous use. Check that filters are available and review the maker’s replacement instructions before buying. The cited guidance does not prescribe one brand, purchase price, or replacement interval for every device.
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When central HVAC filtration is an option
If your home has forced-air heating or cooling, EPA recommends a filter marked MERV 13 or higher when the system can use it. MERV is a filter-efficiency rating; the rating alone does not establish that a given filter is suitable for your particular equipment. Before upgrading, check the equipment manufacturer’s instructions or ask an HVAC technician whether the system can accommodate that filter.
During smoky periods, run the system fan as often as practical so air circulates through the filter. Check the filter frequently: heavy smoke can mean it needs replacement earlier than usual. Follow the system and filter manufacturers’ directions for inspection and replacement. A filter that is incompatible with the system is not a safe shortcut; do not assume that every forced-air system can take MERV 13 or higher.
This option depends on having forced-air equipment; a home without that kind of system cannot use a central HVAC filter as its room-cleaning method. Even in a home with forced air, compatibility comes before the higher rating. Do not choose a filter solely by its MERV number or treat MERV 13 as a universal size specification—the equipment manufacturer or an HVAC professional can confirm what the system supports.
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How to make a temporary DIY box-fan cleaner
EPA describes a DIY air cleaner as a potentially effective, less expensive option when portable cleaners are unavailable or unaffordable, but treats it as a temporary alternative rather than a permanent replacement for tested commercial equipment. Its illustrated basic design uses one 20-by-20-inch high-efficiency filter attached to a 20-inch box fan. A filter marked MERV 13 or higher is preferred when using this approach. EPA’s instructions and design details are in its clean-room guidance.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems- Use an appropriate fan. EPA calls for a box fan made since 2012 that bears a UL or ETL mark. Do not use a fan that does not meet those criteria.
- Match the filter to the fan design. A typical design uses a 20-by-20-inch filter with a 20-inch fan, but the filter’s thickness and fit must match the intended setup. The dimension is not a universal HVAC filter size.
- Align the airflow arrow. Point the filter’s airflow arrow in the direction indicated for the fan design, so air passes through the filter in the intended direction.
- Secure the filter to the fan. EPA’s illustration shows attachment options including clamps, duct tape, or bungee cords. Secure it so the filter stays in place during operation; do not assume a loose or poorly fitted filter will perform like the illustrated design.
- Follow the manufacturers’ safety directions and supervise operation. Use the fan and filter according to their instructions. CDC says never to leave a DIY box-fan filtration unit unattended; see its wildfire safety guidance.
The dimensions in EPA’s basic example are a starting design, not a guarantee that every nominally 20-inch fan and filter will fit together. Check the fan and filter design, thickness, and airflow direction, and secure the assembly before switching it on. If the parts do not match or cannot be attached securely, do not assume that a loose arrangement will work the same way.
EPA reports that multiple studies found well-built DIY cleaners can perform similarly to commercial portable cleaners at reducing airborne particles. That finding does not make every homemade build equivalent: performance varies with design and assembly, while commercial units have more standardized performance testing. Treat the DIY unit as a temporary option and do not assume it has a known CADR unless that performance has been established for the specific design.
Reduce particles made inside the home
Filtration has to contend with particles generated indoors as well as smoke that enters from outside. During the smoke event, avoid activities and products that add particles to the room:
- Do not smoke or vape indoors.
- Avoid burning candles or incense, using aerosol sprays, or operating gas-, propane-, or wood-burning devices in the room.
- Limit frying, broiling, high-heat cooking, and prolonged cooking where practical.
- Avoid vacuuming unless the vacuum has a HEPA filter.
- Use a damp cloth or mop to remove settled dust rather than stirring it up.
These steps reduce sources within the room; they do not replace closing openings or running filtration. If you need to use an exhaust fan or range hood elsewhere in the home, remember that exhaust can draw air into the building from other areas. Avoid running one in the cleaner-air room while relying on it to stay closed off.
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For the same reason, keep combustion and cooking activities away from the room where people are sheltering when practical. If cooking cannot be avoided, reducing high-heat, frying, broiling, or prolonged cooking helps avoid adding as many particles indoors. For cleaning, damp-wiping or mopping captures settled dust without stirring it into the air the way vacuuming can; if vacuuming is necessary, EPA’s advice is to use a vacuum with a HEPA filter.
Use the room during the smoke event, then ventilate when conditions improve
Spend as much time in the room as practical while outdoor smoke is affecting your area. EPA’s guidance says that, as long as it is safe to stay indoors at home, anyone can benefit from spending time in a cleaner-air room during a wildfire smoke event. Keep the room’s cooling and filtration arrangements operating as appropriate, and continue reducing indoor particle sources.
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When outdoor air quality improves—even temporarily—air out the room or home to clear remaining smoky air. The right time to open windows depends on current local conditions, not on a fixed number of hours or a particular point in the season. Check current AirNow and state or local air-quality alerts during an event, and follow local emergency and evacuation instructions.
Once outdoor air is better, ventilation can help clear smoky air that remains indoors. Reassess conditions as they change rather than assuming that one clear period means the event is over. If air quality worsens again, close openings and resume the cleaner-air-room steps as appropriate.
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Closing a home does not necessarily keep all wildfire smoke outside. In a summary of Barn et al. (2022), EPA reports that during wildfire events, indoor PM2.5 in closed homes without portable air cleaners averaged around 55% to 60% of outdoor PM2.5, with a range of about 30% to 100% across homes. This is a study-based estimate across homes, not a prediction for a particular room, building, or smoke event. It also does not tell you what a specific cleaner will achieve. EPA’s overview is in Strategies to Reduce Exposure Indoors.
The practical implication is that room closure alone may leave substantial indoor smoke. Filtering the air and reducing indoor particle sources add useful layers of protection, but no one figure guarantees the conditions in your home. Portable-cleaner CADR is a device-sizing measure; it should not be turned into a promised indoor concentration or health result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When to leave rather than stay in the cleaner-air room
A cleaner-air room is only an option when staying indoors remains safe. If the home becomes too hot to occupy safely, power fails and you cannot keep the space safe, smoke keeps entering, or local officials issue evacuation directions, do not treat the room as a reason to stay. Follow local warnings and seek a safer, cooler place or shelter as directed. EPA’s wildfire clean-room guidance addresses the limits of using a clean room when a home cannot be kept cool.
Conditions, equipment availability, and local instructions vary and can change during an event. Check current AirNow and state or local air-quality alerts rather than relying on a past reading or general seasonal expectation. If the situation is unsafe, prioritize leaving for a safer location over improving filtration.
Frequently asked questions
How big should a portable air cleaner be for my room?
Use the tobacco-smoke CADR rating and EPA’s rule of thumb: the rating should be at least two-thirds of the room’s floor area in square feet. A 120-square-foot room, for example, calls for a tobacco-smoke CADR of at least 80. This is a sizing recommendation, not a promise of a specific indoor smoke level.
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Can a DIY box-fan cleaner replace a portable air cleaner?
EPA describes a DIY cleaner as a potentially effective, less expensive option when a portable cleaner is unavailable or unaffordable, and recommends it as a temporary alternative. Well-built DIY designs have performed similarly to commercial units in multiple studies, but results vary with design and assembly. Use a suitable, marked fan, a well-matched filter, secure attachment, and never leave the DIY unit unattended.
Can I install a MERV 13 filter in any furnace or central air system?
No. EPA recommends MERV 13 or higher only when the forced-air system can use it. Check the equipment manufacturer’s instructions or ask an HVAC technician before upgrading; the filter rating by itself does not establish compatibility.
Should I keep the windows closed if the room is getting too hot?
No cleaner-air setup is a reason to remain in a home that is too hot to occupy safely. Use appropriate cooling and recirculation where available, but if you cannot keep the home safely cool, follow local guidance and seek a safer place. Open windows to ventilate when outdoor air quality improves.
When should I air out the room?
When outdoor air quality improves, even temporarily, open windows or otherwise air out the room or home to clear remaining smoky air. Check current AirNow and state or local alerts rather than relying on a fixed time or past reading.
Frequently Asked Questions
How big should a portable air cleaner be for my room?
Use the tobacco-smoke CADR rating and EPA’s rule of thumb: at least two-thirds of the room’s floor area in square feet. For a 120-square-foot room, that is a tobacco-smoke CADR of at least 80. It is a sizing recommendation, not a promise of a specific indoor smoke level.
Can a DIY box-fan cleaner replace a portable air cleaner?
EPA describes a DIY cleaner as a potentially effective, less expensive temporary option when a portable cleaner is unavailable or unaffordable. Performance varies with design and assembly. Use an appropriate marked fan, a correctly matched and secured filter, follow safety directions, and never leave the unit unattended.
Can I install a MERV 13 filter in any furnace or central air system?
No. Use MERV 13 or higher only if your forced-air system can accommodate it. Check the equipment manufacturer’s instructions or ask an HVAC technician before upgrading.
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Should I keep windows closed if the room is getting too hot?
No. A cleaner-air room is not a reason to stay in a home that is too hot to occupy safely. Use cooling and recirculation where available, and follow local guidance to find a safer place if you cannot keep the home cool.
When should I air out the room?
When outdoor air quality improves, even temporarily, ventilate the room or home to clear remaining smoky air. Check current AirNow and state or local alerts rather than relying on a fixed time or an old reading.
The Bottom Line
Choose a room that fits everyone, close its openings without blocking the exit, manage outdoor-air intake and heat, and filter the air using a correctly sized portable cleaner, compatible HVAC filter, or safely assembled temporary DIY unit. Reduce indoor particle sources and check current local air-quality and emergency guidance. If the home cannot be kept cool or safe, leave for a safer place rather than relying on the room.
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