When outdoor air is polluted, reduce exposure in this order: close windows and doors, set heating or cooling equipment to recirculate and close its outdoor-air intake if the system allows it, run compatible filtration, stop indoor particle-producing activities, and check local instructions. Use AirNow and your local health department for current conditions. If closing the home makes it dangerously hot, move to a cooler safe location rather than trading particle exposure for heat illness.
The most detailed U.S. Environmental Protection Agency (EPA) household advice concerns wildfire smoke and fine particles (PM2.5). Those steps can reduce smoke exposure, but they are not a universal treatment for gases or every pollutant. The correct response depends on the pollutant named in your local advisory.
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What to do in the first 10 minutes
- Check the advisory. Look up your local air-quality conditions through AirNow or local health authorities. Follow any evacuation, shelter-in-place, or industrial-incident instructions before applying household filtration advice.
- Stop outdoor air from entering. Close every window and exterior door. If your heating, ventilation, and air-conditioning (HVAC) system has an outdoor-air damper or “fresh air” control, close it temporarily if the manufacturer permits. Set the thermostat or fan control to recirculate.
- Start filtration. Install a properly fitting MERV 13 filter, or the highest rating your central system can safely handle. Turn the fan on as often as is feasible. If you do not have forced-air HVAC, place a portable air cleaner in the room where people spend the most time.
- Remove indoor sources. Do not smoke or vape indoors. Extinguish candles and fires, and postpone high-temperature cooking that creates smoke or particles.
- Check temperature and symptoms. Use air conditioning or fans that do not draw outdoor air. If the home becomes dangerously hot, go to a cooler safe location and follow emergency guidance.
Should you open windows when outdoor air quality is bad?
Usually, no. During a smoke or outdoor-particle episode, closed windows and doors reduce the amount entering your home. They do not make the building airtight: particles can pass through cracks, gaps, and normal air leakage. The EPA explains, “Outdoor and indoor air are constantly mixing together,” which is why indoor concentrations never track outdoor air perfectly.
When keeping windows closed is the right choice
- Keep them closed when your local advisory identifies unhealthy smoke, dust, soot, or other outdoor particles.
- Keep the HVAC system on recirculation when that setting is available.
- Do not use a window fan or whole-house fan that pulls contaminated outdoor air inside.
When ventilation becomes useful again
General EPA ventilation guidance says to avoid outdoor-air ventilation when outdoor pollution or pollen is high, then bring outdoor air in when outdoor quality and weather conditions permit. Ventilate after the advisory improves, not according to a fixed clock. Open windows briefly, or reopen the HVAC fresh-air intake, when outdoor readings are clearly better and indoor sources have been controlled.
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How to use a central HVAC system during smoke
A forced-air system can filter air throughout the home, but only if the filter and equipment are compatible. EPA recommends MERV 13 or higher during smoke when the system can accommodate it. A higher rating is not automatically safer for the equipment: a filter that is too restrictive can reduce airflow or stress a blower.
Install the filter correctly
- Find the filter slot, usually at the return-air grille or inside the furnace or air-handler cabinet.
- Read the arrow on the frame and point it in the direction of airflow.
- Confirm the filter dimensions and rating match the equipment manufacturer’s requirements.
- Seat it snugly. Do not bend, crush, or leave gaps around the frame.
- If the manual does not state whether MERV 13 is supported, ask the manufacturer or an HVAC professional before installing it.
Run the fan without overloading the system
Run the HVAC fan as often as feasible while the outdoor air is polluted. “On” runs the blower continuously; “auto” runs it only during heating or cooling. Continuous operation can improve filtration but uses more electricity and may be impractical in a system that cannot maintain airflow with a higher-efficiency filter. Stop and seek professional help if you notice unusual noise, weak airflow, overheating, or a filter that bows inward.
Will a portable air purifier help with wildfire smoke?
A portable air cleaner can lower particle concentrations in one room when its clean-air delivery rate (CADR) is adequate for that room and the device is operated correctly. It supplements closed windows and source control; it does not justify opening windows during a smoke event.
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For a cleaner-air room during wildfire smoke, EPA’s rule of thumb is a tobacco-smoke CADR of at least two-thirds of the room area in square feet. Use this calculation:
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minimum smoke CADR (cubic feet per minute) = room area (square feet) × 2/3
For example, a 300-square-foot bedroom calls for a smoke CADR of at least 200 cubic feet per minute. Measure the room, use the published smoke CADR rather than a vague “covers up to” claim, and choose a larger unit or multiple units for open-plan spaces, high ceilings, or rooms with doors that remain open.
Choose low-ozone equipment
Choose a cleaner that produces little or no ozone. EPA points to California Air Resources Board (CARB) certification as one way to identify devices meeting a low-ozone standard. Avoid products whose main method is intentional ozone generation.
Place and operate it for real-world cleaning
- Put the unit in the occupied room, with unobstructed intake and exhaust space.
- Run it continuously or at the highest tolerable setting while smoke is present; lower settings clean more slowly.
- Keep doors and windows closed during the episode.
- Replace filters according to the manufacturer’s schedule, sooner if they are visibly loaded or airflow drops.
- Use a bedroom cleaner overnight if that is where a sensitive person sleeps, while keeping noise and electrical-safety limits in mind.
Central HVAC, portable cleaners, and DIY box-fan units
Each approach solves a different coverage problem. The useful comparison is not simply “which is strongest,” but which one fits your home, can run safely for long periods, and has independently stated particle-cleaning performance.
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| Approach | Area served | What to verify | Strengths | Limitations |
|---|---|---|---|---|
| Compatible central HVAC filter | Potentially the whole home through the forced-air ducts | MERV rating supported by the equipment, correct dimensions, snug fit, and adequate airflow | Uses existing distribution; can filter several rooms while the fan runs | Not useful in rooms not connected to the system; high-efficiency filters may reduce airflow or increase operating cost |
| Portable air cleaner | One room or a defined zone | Tobacco-smoke CADR sized to at least two-thirds of room area; low or no ozone emissions | Known room-level performance; can target a bedroom or other occupied space | Does not clean the entire house; requires filter replacement, electricity, and space |
| DIY box-fan and filter setup | Primarily one room | Stable construction, appropriate filter fit, safe electrical operation, and unobstructed airflow | Possible temporary, lower-cost fallback when commercial units are unavailable or unaffordable | Performance varies; noise, leakage, and safety depend on the build; standardized CADR may not be available |
EPA prefers commercial cleaners with known performance when they are available and affordable. Two or more room cleaners can sometimes provide equivalent coverage and may cost less to operate than running a central system continuously with a high-efficiency filter. That depends on the home’s layout, equipment, and electricity use, so compare the area served and the ability to run each option continuously.
How to avoid making indoor air worse
Stop combustion and smoke sources
During wildfire smoke, do not smoke indoors, burn candles, use a fireplace, or operate other combustion sources. These activities add particles to a home that is already receiving some outdoor pollution.
Change cooking methods
High-temperature cooking can create substantial indoor particles. Postpone frying, broiling, searing, and other smoky methods until outdoor air improves. If food must be prepared, use the least smoky method available and keep the range hood exhausting outdoors only when the hood and building design do not bring contaminated outdoor air into the home. A recirculating hood can reduce some cooking particles but does not replace a properly sized air cleaner.
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Do not use an ozone generator
EPA advises against ozone generators in occupied spaces. Ozone can irritate the respiratory system and, at concentrations that do not exceed public-health standards, is generally ineffective at controlling indoor air pollution. A device marketed as an “air sanitizer” is not a substitute for particle filtration unless its particle-cleaning performance and ozone emissions are clearly established.
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How much protection should you expect?
Do not assume that closed windows or a particular filter produces a guaranteed percentage reduction. EPA reports that, during wildfire-smoke events, indoor PM2.5 levels are generally about 55% to 60% of outdoor levels when doors and windows are closed and portable air cleaners are not in use. The agency also reports a much wider range, about 30% to 100% of outdoor levels across homes. Construction, leakage, HVAC operation, room size, indoor sources, and filter use all change the result.
Those figures apply to typical indoor PM2.5 during wildfire smoke under the stated conditions. They do not establish performance for every gas, chemical vapor, pollen episode, or home, and they are not a promise for a particular product.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep filtration from creating a heat hazard
Closed windows can make a home hotter, especially during a summer smoke event or a power outage. Use air conditioning on recirculation, if available. Fans can improve comfort when the room is already cooler than the person, but a fan does not remove smoke and should not be used to pull polluted outdoor air through an open window.
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- Move to a cooler, cleaner-air building if one is available and local authorities consider travel safe.
- Follow local emergency instructions for cooling centers, shelters, or evacuation.
- Check children, older adults, pregnant people, and anyone with heart or lung disease more frequently for heat or breathing problems.
- Seek medical advice promptly for severe shortness of breath, chest pain, confusion, fainting, or worsening symptoms.
Exposure reduction is not a reason to remain in a dangerously hot room. Balance particle control with temperature, power reliability, and personal health.
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Extra precautions for people at higher risk
Children, older adults, pregnant people, and people with heart or lung disease can be more affected by smoke and fine particles. They should spend as much time as practical in the cleaner room, avoid exertion during the advisory, and follow current local health guidance and their clinician’s advice. Keep prescribed medicines and essential medical equipment available before conditions worsen.
What to do when outdoor air improves
- Confirm that local air-quality information shows improvement, not just a temporary drop at one monitoring station.
- Turn off or reduce portable cleaners only after the advisory and indoor conditions allow it.
- Open windows or restore the HVAC outdoor-air intake to flush indoor air when outdoor air is cleaner and weather conditions permit.
- Continue avoiding indoor smoke sources until the home has been aired out.
- Inspect the HVAC and portable-unit filters. Replace a filter that is loaded, damaged, or past its service interval.
Troubleshooting common situations
There is no central HVAC system
Close windows and doors, then use a correctly sized portable air cleaner in the room where people spend the most time. A DIY box-fan filter can be a temporary fallback, but commercial units with published CADR provide more standardized performance when they are obtainable.
The HVAC filter does not fit tightly
Do not force, bend, or crush it. An ill-fitting filter lets air bypass the media. Confirm the exact filter size and cabinet requirements with the equipment manufacturer or an HVAC professional; a correctly fitting lower rating is safer than an incompatible higher rating.
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Odor is not a reliable measurement of PM2.5, and a lack of odor does not prove that particles are absent. Keep the room closed to outdoor air, remove indoor sources, verify that the cleaner is running at an effective setting, and check for gaps around doors, windows, and ductwork. If the advisory concerns a gas or chemical vapor rather than smoke particles, particle filtration alone may not address it; follow the pollutant-specific instructions from local authorities.
The power fails
Stop using equipment that cannot operate safely. Keep windows closed if outdoor conditions remain unhealthy, but monitor indoor heat. Move to a cooler safe location if necessary, and follow emergency guidance rather than improvising with unsafe wiring or combustion devices indoors.
Which advice applies beyond wildfire smoke?
Closing windows during high outdoor particle pollution, avoiding outdoor-air ventilation until conditions improve, and using compatible filtration are broadly useful exposure-reduction steps. The numeric CADR rule, the MERV 13 recommendation, and the indoor-to-outdoor PM2.5 figures are specifically framed by EPA guidance around wildfire smoke and particulate pollution. They should not be presented as a complete response to radon, carbon monoxide, volatile chemicals, industrial releases, or other gases. For those hazards, use the local advisory’s instructions and the appropriate detector, evacuation, or shelter guidance.
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