Wildfire smoke and indoor air: what to do

Keeping wildfire smoke out of your house takes four things: shut the house before the smoke reaches you, filter the air already inside with MERV 13 or better, set up one cleaner air room, and stop adding particles of your own. Do none of them and indoor PM2.5 sits at about 55% to 60% of the outdoor level, and that is the EPA's figure for a house with the windows and doors already closed.

That last part is what catches people out on a smoke day. You shut everything at 8am, the sky goes orange by noon, and the house still smells like a campfire at bedtime. Closed windows were never the whole job, because wildfire smoke reaches indoor air through gaps you cannot see and cannot tape shut.

The rest of this post is where the smoke gets in and what each fix is worth in µg/m³. If you would rather watch the number than guess at it, that is what a kit for homeowners does.

Key Takeaways

With the windows and doors closed and no air cleaner running, the EPA puts indoor PM2.5 at about 55% to 60% of the outdoor level during a smoke event.

The EPA calls outdoor PM2.5 unhealthy from 55.5 µg/m³ up, so an outdoor reading of 150 µg/m³ still leaves roughly 85 µg/m³ inside a closed house.

A MERV 13 or higher filter with the HVAC fan set to On, plus one room purifier sized to the room, is the pair that moves the indoor number.

In EPA field testing, a box fan with a MERV 13 filter cut PM2.5 by about 56% in 90 minutes in a large room and about 99% in 60 minutes in a small one.

Sealing the house raises CO₂. A monitor tells you which of the two problems you currently have, and removes neither.

What wildfire smoke does to indoor air

Wildfire smoke is mostly PM2.5: particles under 2.5 micrometers across, light enough to stay in the air for hours and small enough to follow air through any gap that lets air through. That is why smoke behaves differently from pollen or road dust. A window screen stops nothing, and a closed window slows it down without stopping it.

The EPA's air quality index breakpoints put 24 hour outdoor PM2.5 on this scale:

0.0 to 9.0 µg/m³: good
9.1 to 35.4 µg/m³: moderate
35.5 to 55.4 µg/m³: unhealthy for sensitive groups
55.5 to 125.4 µg/m³: unhealthy
125.5 to 225.4 µg/m³: very unhealthy
225.5 µg/m³ and up: hazardous

Those are outdoor numbers on a 24 hour average, and the EPA has not set an equivalent standard for the air inside a house. They are still the nearest reference anyone has, which is why an indoor reading only means something next to the outdoor reading at the same address and the same hour. The air outside is measured and the air you breathe isn't, and on a smoke day that gap is the whole problem.

Run the infiltration figure through that scale and the size of the problem shows up. The EPA's wildfire smoke course states that indoor PM2.5 runs around 55% to 60% of outdoor levels during a smoke event with doors and windows closed and no portable air cleaner going. An outdoor day at 150 µg/m³ therefore leaves something near 85 µg/m³ in the living room, which is still in the unhealthy band. That is why closing the house is the first of the four steps rather than the only one.

How wildfire smoke gets into indoor air

Smoke gets in three ways, and only the first one is obvious.

Open windows and doors. The EPA calls this natural ventilation. It is the fastest route in and the easiest to close.
The ventilation you run on purpose: a furnace or air handler with a fresh air intake, a bathroom fan, a range hood. Each one pulls outdoor air into the house while it runs.
Infiltration. Air moves through joints, cracks, and the gaps around closed windows and doors. You cannot switch this one off, and it is what keeps the indoor line climbing after the house is shut.

A monitor makes the third route visible. Outdoor PM2.5 climbs first, the indoor line follows an hour or two behind at a lower height, and both fall later and slower. Measured Air reports PM2.5 room by room next to the outdoor AQI at your own address, so the two lines sit on the same chart instead of in two different apps.

Close the house before the smoke arrives

Shut the windows and doors, and do it while the outdoor number is still low. A house that gets closed at 60 µg/m³ starts the day with less smoke in it than one closed at 200 µg/m³, and everything after this step is spent removing what is already inside.

Then deal with the ventilation you control:

Set the HVAC to recirculate and close the fresh air intake if the system has one.
Set the fan to On rather than Auto, so the air keeps passing through the filter between heating and cooling cycles.
Keep range hoods and bathroom fans to short runs. Both pull outdoor air in through whatever gaps the house has.
Do not seal an exit. The EPA is explicit about this: close the room up, but not in a way that makes it hard to get out.

Closing the house trades one reading for another. A shut house with the fan on recirculate stops exchanging air with outdoors, and the CO₂ you exhale has nowhere to go. Past roughly 1,000 ppm most people start calling a room stuffy, and a closed bedroom on a smoke night gets there easily. So bedroom CO₂ climbs while PM2.5 stays down, and knowing which number is high is the difference between opening a window and leaving it shut.

Filter the air already inside the house

Closing the house stops more smoke from coming in. Filtering is what takes out what is already there, and there are three ways to do it.

Start with the HVAC filter. The EPA recommends a filter rated MERV 13 or higher during smoky periods, because MERV 13 is the rating that catches particles in the size range smoke sits in. Check compatibility first: a denser filter restricts airflow, and not every system is built for one, so the EPA suggests asking an HVAC professional if you are unsure.

A HEPA room purifier works on one room at a time, and size matters more than features. The EPA's rule is a clean air delivery rate, or CADR, of at least two-thirds the room's floor area in square feet. A 10 by 12 foot bedroom is 120 square feet, so you want a CADR of 80 or more. Pick a model certified by the California Air Resources Board, which means it produces little or no ozone, and run it on the highest setting you can stand.

A box fan and a furnace filter is the cheap version, and the EPA has measured it. Its testing of DIY air cleaners in a chamber published a CADR for each build: 111 for a box fan with one 1 inch MERV 13 filter, 156 once a cardboard shroud was added, 248 with a single 4 inch filter and a shroud, and 401 for a four-filter box. In a real home during a smoke event, a DIY cleaner cut PM2.5 by about 56% after 90 minutes in a larger room and by about 99% within 60 minutes in a smaller one.

Two cautions on the DIY version, both from the same EPA research. Use a box fan made in 2012 or later with a UL or ETL mark, because those have thermal cutoffs and fused plugs. And change the filter often: the testing found DIY cleaners almost completely ineffective once the filter was dirty, which during a bad week can mean days rather than months.

Set up one cleaner air room

Getting a whole house down to single digit PM2.5 during a smoke week is not realistic, and it is not necessary either. The EPA's advice is to pick one room and make that one good.

Choose a room big enough for everyone in the house to be comfortable in. A bedroom with an attached bathroom is the usual pick.
Close its windows and doors and keep them closed.
Put the filtration in that room: the purifier or the DIY build, sized to that room's square footage rather than to the house.
Spend time in it. A room you set up and then sit somewhere else all evening does nothing for you.
Put a monitor in it, because a cleaner air room with no reading in it is still a guess.

The purifier is running and the room feels better. Feeling better and sitting at 12 µg/m³ instead of 60 µg/m³ are different claims, and the reading is the one you can check later.

Stop adding particles indoors

These are the sources you control, and on a smoke day they stack on top of whatever came in from outside. The EPA's cleaner air room guidance says to avoid:

Smoking and vaping
Gas, propane and wood-burning stoves
Candles and incense
Frying or broiling food
Aerosol sprays
Vacuuming without a HEPA filter, which stirs settled particles back into the air

Dust with a damp cloth instead of a dry one. Cook something that does not need a hot pan, or accept the spike and let the filter work it back down. A monitor in the kitchen shows how long that takes in your house, which is usually longer than people expect.

When indoor air is clear enough to open the windows again

Open up when the outdoor reading drops below the indoor reading and stays there for a few hours. That is the whole rule, and it needs two numbers to work. Outdoor air clears from the top down and from upwind, so the sky can look fine while the number is still in the moderate band, and a house that has been shut for three days can be the dirtier of the two.

History is what makes that comparison worth anything later. Measured Air keeps every reading, so you can hold this smoke week against the last one and see whether the filter you bought in between made a difference. A monitoring run can also end in a written report covering what the air did over the period, and you can run it again after a fix for a before-and-after.

One limit, plainly: a monitor does not remove a single particle. It tells you whether the filter did the work and when you can open the windows. The four steps above are what change the number.

Frequently asked questions

Does closing the windows keep wildfire smoke out of the house?

Only partly. With doors and windows closed and no air cleaner running, the EPA puts indoor PM2.5 at about 55% to 60% of the outdoor level during a smoke event, because air still moves through joints, cracks and the gaps around closed windows. Closing up is the first step, and filtration is the one that finishes the job.

What PM2.5 level is safe indoors during wildfire smoke?

There is no EPA standard for indoor PM2.5, so the outdoor scale is the closest reference: the EPA calls 0.0 to 9.0 µg/m³ good and 9.1 to 35.4 µg/m³ moderate on a 24 hour average. Aim to keep the indoor number in the single digits and well under whatever is happening outside. A reading describes the air, not a particular person's health.

Do air purifiers work for wildfire smoke?

Yes, for PM2.5, and size decides how well. A HEPA room purifier removes the fine particles smoke is made of, and the EPA's sizing rule is a CADR of at least two-thirds the room's floor area in square feet. It does nothing for CO₂, so a closed room with a purifier running can still go stuffy overnight.

Does a MERV 13 filter help with wildfire smoke?

Yes. The EPA recommends MERV 13 or higher in the HVAC system during smoky periods, with the fan set to On so air keeps moving through it. Check that your system can handle the airflow restriction before you put one in, and change it more often than the box says during a smoke week.

Where should I put an air quality monitor during a smoke event?

In the room you sit in, at about head height when seated, away from windows, doors and vents. The bedroom and the room where people spend the evening settle the most decisions. The other reading that matters is the outdoor one at your address, which is what makes the indoor number mean something.

How long does wildfire smoke stay in a house after the outdoor air clears?

Longer than the outdoor air takes to clear, and the exact answer depends on the house. Without filtration, the indoor level falls only as fast as air exchanges through the same gaps the smoke came in through. With a correctly sized purifier running, EPA field testing showed about 99% removal within 60 minutes in a small room and about 56% in 90 minutes in a larger one.

The short version

Four numbers cover most of what you need. Outdoor PM2.5 turns unhealthy at 55.5 µg/m³. A closed house with no filtration holds about 55% to 60% of whatever is outside. MERV 13 is the filter rating the EPA names for smoke. And a room purifier needs a CADR of two-thirds the room's square footage to keep up.

The order matters: close the house early, filter what is already in it, make one room the good one, and stop adding particles while you wait. Then watch two lines instead of one, because wildfire smoke in indoor air is only a problem relative to what is happening outside, and the moment to open the windows again is the moment the outdoor line drops under the indoor one.

Every fix here trades something. Sealing the house drives CO₂ up. Running the fan costs electricity. A dirty filter is the same as no filter. A reading tells you which trade you are currently making.

Order a kit sized to your rooms, put a monitor in the room you will sit out the next smoke day in, and stop guessing whether the air in your house is fine.

Wildfire smoke and indoor air: how to keep it out