Wildfire Smoke: How to Set Up Your HVAC to Keep It Out

Quick answer

During wildfire smoke, keep windows and doors closed, set your HVAC to recirculate or close its fresh air intake, and use a high-efficiency filter that fits snugly. With a good filter, set the fan to On instead of Auto. A portable air cleaner sized for the room adds protection. Follow local officials' advice.

Key facts

Windows and doors	Keep closed during smoke
HVAC setting	Recirculate, or close the fresh air intake
Filter	MERV 13, or as high as your system allows (EPA)
Fan	On, not Auto, with a high-efficiency filter (EPA)
Portable air cleaner	Sized for the room; no ozone
Air quality updates	AirNow.gov or the Oregon Smoke Information blog

How does wildfire smoke get inside my house?

Smoke gets in through open windows and doors, through fans and HVAC systems that pull in outdoor air, and through small cracks and gaps around the house. EPA lists all three. Even with everything shut, some smoke can leak in.

EPA says the biggest health threat in smoke is fine particles. They can reach your eyes and lungs indoors as well as outdoors. The Oregon Health Authority says smoke can hurt your eyes, irritate your respiratory system, and worsen chronic heart and lung disease.

Your HVAC system can work for you or against you here. Set up right, it filters the air you breathe. Set up wrong, it can pull smoky air inside.

What should I set my HVAC system to during wildfire smoke?

Set your system to keep outdoor air out and to move indoor air through a good filter. These steps come from EPA and the Oregon Health Authority:

1Keep windows and doors closed.
2If your system has a fresh air intake, set it to recirculate or close the outdoor air damper.
3If you have a high-efficiency filter installed, set the fan to On instead of Auto, so air passes through the filter more often.
4Use AC and fans to stay cool. The Oregon Health Authority says to keep the fresh air intake closed and the filter clean while the AC runs.
5If you have a window AC, close its outdoor air damper. If you can't, EPA says not to use it.
6When air quality improves, even for a while, air out the house and run the kitchen and bathroom exhaust fans.

Not sure whether your system has a fresh air intake? EPA suggests finding out before smoke season. A technician can check and show you how it works.

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Sources listed for outside facts

What filter should I use for wildfire smoke?

Use a high-efficiency filter. EPA suggests MERV 13, or as high a rating as your system fan and filter slot can accommodate. The filter should fit snugly so air doesn't leak around it. Don't bend or crush a filter to make it fit.

EPA says you may need an HVAC technician to find the highest-efficiency filter your system can handle. If you're not sure, ask before smoke season, not in the middle of it. Keep a spare filter on hand.

During smoke events, check the filter often. EPA says to change it when it looks dirty. A filter that loads up with smoke particles also slows airflow, which makes your system work harder on hot days.

Do portable air cleaners help with wildfire smoke?

Yes. EPA says portable air cleaners can remove smoke particles from indoor air. A central HVAC filter cleans the air throughout the area the system serves, while a portable unit cleans one room. Using both together gives more effective filtration.

When you choose and use one:

Pick one sized for the room you'll use it in (EPA and the Oregon Health Authority).
The Oregon Health Authority suggests a HEPA filter or an electrostatic precipitator (ESP).
Make sure it doesn't produce ozone (EPA).
Place it where people spend the most time, and run it as often as you can on the highest speed (EPA).
Can't get one? EPA describes a DIY air cleaner made by attaching a furnace filter to a box fan. EPA notes that commercial units with a known clean air delivery rate are typically preferred.

What else keeps smoke levels down inside?

Avoid adding more particles to your indoor air while it's smoky. EPA and the Oregon Health Authority suggest skipping these:

Candles, incense, fireplaces, and wood stoves
Gas stoves, and frying or broiling food
Smoking and vaping
Aerosol sprays
Vacuuming, unless the vacuum has a HEPA filter
Hard exercise during the smokiest times

EPA also suggests setting up a clean room: one room you keep as smoke-free as you can, with a portable air cleaner running, for times when smoke is heavy.

Who is most at risk from wildfire smoke?

The Oregon Health Authority says people with heart or lung disease — such as asthma, COPD, emphysema, angina, or congestive heart failure — are at higher risk from smoke. Older adults, children, and people who are pregnant are also at higher risk. People who work or live outdoors may face more exposure.

For people with chronic disease, the Oregon Health Authority suggests keeping more than five days of medication on hand, having a written asthma plan if you have asthma, and calling your health care provider if your condition gets worse in smoke.

What can a technician check before smoke season?

Before smoke season, a technician can help you get your system ready to keep smoke out. If your system can't take a higher-MERV filter, a portable air cleaner in the rooms you use most can help fill the gap. A pre-season check covers the things EPA says to learn ahead of time:

Whether your system has a fresh air intake, and how to close it or set recirculate.
What filter size you need, and the highest MERV your system can handle.
Whether the filter fits snugly, with no gaps for air to slip past.
How to set your thermostat fan to On during smoke.
The filters on ductless heads and any other indoor units.
Whether your AC is ready for hot, smoky days with the windows shut.

What should I do after the smoke clears?

After the smoke clears, check your HVAC filter and change it if it looks dirty. Then air out the house by opening windows or your fresh air intake, and run your exhaust fans.

You might wonder about duct cleaning. EPA recommends cleaning air ducts only as needed — for example, if the ducts are clogged with excessive dust and debris, or particles are actually coming out of the supply vents.

For ash cleanup, EPA suggests wearing an N95 respirator, misting surfaces with water before sweeping, and following up with wet mopping. EPA says children, older adults, and people with heart or lung disease should not help with cleanup.

Common questions

Should I run my AC during wildfire smoke?

Yes, if you have one. The Oregon Health Authority says to run your AC with the fresh air intake closed and the filter clean. Keep windows and doors shut. If a window AC's outdoor air damper won't close, EPA says not to use it.

Does my furnace filter help with wildfire smoke?

Yes, a high-efficiency HVAC filter can remove smoke particles from indoor air, according to EPA. EPA suggests MERV 13, or as high as your system can accommodate, with a snug fit. Check it often during smoke and change it when it looks dirty.

Should my thermostat fan be on On or Auto during smoke?

Set it to On if you have a high-efficiency filter installed. EPA says running the fan more often moves more air through the filter while air quality is poor. Switch back to Auto once the smoke clears.

Do dust masks protect against wildfire smoke?

No. The Oregon Health Authority says paper comfort or dust masks won't protect your lungs from smoke. It says a NIOSH-approved particulate respirator, such as an N95, that fits well can help. People with health conditions should talk with their health care provider first.

Should I get my ducts cleaned after wildfire smoke?

Only if they need it. EPA recommends cleaning ducts only as needed, such as when they're clogged with excessive dust and debris or particles come out of the vents. Changing a dirty filter is the first step.

Where can I check air quality during smoke season?

EPA points to AirNow.gov for local air quality. In Oregon, the Oregon Health Authority also points to the Oregon Smoke Information blog for current conditions and health advisories. Watch for local news and health warnings, too.

Does a ductless mini-split keep smoke out?

Most ductless mini-splits recirculate indoor air rather than bring in outdoor air, which helps. Their built-in filters usually aren't high-efficiency filters, though. A portable air cleaner in the same room adds filtration.