Research
Wildfire smoke and the homes that cannot filter it
Filtering a house requires a central system to filter it with. The places that need one most are the places least likely to have one.
Written by HYRE HVAC Research Desk Primary-source research, data analysis and fact checking
The finding
The states with the worst wildfire smoke are among the least able to filter it. California had unhealthy air on 10.7% of monitored days in 2020, and only 73.7% of its homes have ducts for a central filter.
Oregon is at 68.0% and Washington at 58.8%, against 82.0% nationally. In 2023 the worst air moved to the Midwest and East, from fires a thousand miles away.
Can you find out what air filters US homes use?
No, not from any public dataset. This study was first meant to analyze the air filters American homes actually have. It does not, because that page cannot honestly be written.
We checked all 799 variables in the RECS 2020 public use file. There is no filter variable, no MERV rating, no air cleaner and no air purifier. The federal government’s most detailed survey of American household energy use does not ask what is in the filter slot. No other public dataset fills the gap either.
So the question was narrowed to one the evidence can carry. Not what filters homes have, but whether they have the infrastructure that filtration requires.
A MERV 13 filter is useless without a system to put it in and a reason to run that system.
That turns out to be the more useful question anyway, and nobody else publishes it. (To pick a filter for your own system, see the filter selector.)
Which states have the worst smoke exposure?
It depends on the year. EPA publishes county-level air quality summaries. Aggregated to state, and counting days above the "unhealthy for sensitive groups" threshold as a share of monitored days, the picture across 2020, 2023, 2024 is dominated by two very different events.
2020 was a western fire season: California recorded unhealthy air on 10.7% of monitored days, Arizona on 6.9%, Oregon on 3.4%.
2023 was somebody else’s fire season: Canadian wildfire smoke pushed the worst air of the three years into states that had never planned for it: Illinois went from 0.9% of days unhealthy in 2020 to 4.7% in 2023, and Wisconsin from 0.9% to 4.6%.
HyreHVAC analysis: that second pattern is the one worth planning around. Smoke exposure is not a fixed property of a state, and a household in the upper Midwest that regarded filtration as a western problem discovered otherwise over a single week.
Any assessment of who needs filtration based on historical local fire risk will keep being wrong in this direction.
Can homes in smoky states filter their air?
Often not. Filtering a whole house means running a central forced-air system with a good filter in it. Nationally 82.0% of homes have that setup. The three Pacific states fall short.
Washington is at 58.8%, Oregon at 68.0% and California at 73.7%: all below the national figure, and Washington by more than twenty points (see HVAC in Washington). The historical reason is mild summers and cheap hydroelectricity, which produced housing heated by baseboards and cooled by opening a window.
That second habit is the sharper problem. 47.5% of Washington homes and 23.7% of Oregon homes have no cooling equipment of any kind.
On a hot smoky day those households face a real conflict: close the windows and overheat, or open them and breathe smoke (the same households our cooling divide study counts).
A house with air conditioning can stay closed. A house without it cannot.
Nationally, 6.5% of homes have a whole-house fan and 8.7% an attic fan: equipment whose entire function is to pull large volumes of unfiltered outdoor air through the building. On a smoke day it is the worst thing in the house to switch on, and it is normally the cheapest way to cool one.
How do smoke exposure and ductwork compare by state?
| State | Unhealthy days 2020 | 2023 | 2024 | Homes with ducts | Homes with no cooling |
|---|---|---|---|---|---|
| California | 10.7% | 4.9% | 6.1% | 73.7% | 27.6% |
| Arizona | 6.9% | 3.3% | 4.7% | 92.6% | 5.8% |
| Illinois | 0.9% | 4.7% | 0.5% | 84.2% | 4.0% |
| North Dakota | 0.0% | 4.6% | 2.1% | 72.0% | 7.1% |
| Wisconsin | 0.9% | 4.6% | 0.7% | 82.4% | 7.6% |
| Iowa | 0.2% | 3.4% | 0.0% | 91.6% | 2.8% |
| Oregon | 3.4% | 1.9% | 1.9% | 68.0% | 23.7% |
| District of Columbia | 0.3% | 3.3% | 1.1% | 75.2% | 6.7% |
| Nevada | 3.3% | 1.0% | 1.6% | 88.3% | 8.0% |
| South Dakota | 0.1% | 3.0% | 1.0% | 84.4% | 8.0% |
| Texas | 0.9% | 2.1% | 3.0% | 90.4% | 5.0% |
| Minnesota | 0.1% | 2.8% | 0.4% | 79.6% | 5.7% |
| Missouri | 0.4% | 2.8% | 0.4% | 90.3% | 4.8% |
| Michigan | 1.3% | 2.7% | 0.4% | 87.6% | 13.4% |
| New Mexico | 1.2% | 1.1% | 2.7% | 78.7% | 15.0% |
| Colorado | 2.3% | 1.1% | 2.6% | 81.9% | 18.3% |
| Connecticut | 1.2% | 2.5% | 2.5% | 66.7% | 10.4% |
| Idaho | 2.3% | 0.6% | 2.5% | 80.3% | 19.4% |
| Washington | 2.5% | 0.8% | 0.3% | 58.8% | 47.5% |
| Indiana | 0.4% | 2.3% | 0.2% | 94.2% | 5.7% |
| Utah | 1.5% | 2.2% | 2.0% | 93.2% | 8.1% |
| Delaware | 0.2% | 2.0% | 0.4% | 92.5% | 3.0% |
| New Jersey | 0.2% | 2.0% | 0.9% | 76.0% | 3.5% |
| Ohio | 0.5% | 1.9% | 0.5% | 87.5% | 6.5% |
| Maryland | 0.1% | 1.8% | 0.6% | 91.2% | 8.6% |
The 25 states with the worst single year of the three. Unhealthy days are the share of monitored days at or above "unhealthy for sensitive groups". Housing figures are RECS 2020.
AQI does not attribute fine particulate to wildfire. A high-PM2.5 day may be smoke, traffic, industry or wood burning, so these are exposure figures rather than smoke figures.
What helps during a wildfire smoke event?
A MERV 13 filter captures a large share of fine particulate and costs a few dollars more than the MERV 8 in the slot now.
The one check worth making first is whether the system can take the extra resistance to airflow.
An older furnace with undersized returns may not, and a technician can tell you in a single visit.
A filter only cleans air that passes through it. On a smoke day the fan needs to run continuously rather than only when there is a call for heating or cooling. Most thermostats have a "fan on" or circulate setting and almost nobody uses it.
A portable HEPA cleaner sized for the room is the realistic option in a home with no central system, which is most of the Pacific Northwest housing stock.
One unit in a hallway does very little; one in the bedroom does a great deal, because that is where the hours are. An indoor air quality assessment can size them.
Whole-house and attic fans move large volumes of unfiltered outdoor air through the building by design. They are excellent on a clear cool evening and actively harmful on a smoke day. This is the single most common mistake in a house that has one.
A closed house in a heatwave is its own hazard, and the households facing that trade-off are disproportionately the ones without air conditioning. Where the choice is genuinely between heat and smoke, cooling the house is the intervention that makes filtration possible at all.
How did we measure it, and what are the limits?
EPA AirData annual AQI by county for 2020, 2023, 2024. Days at or above "unhealthy for sensitive groups" (including unhealthy, very unhealthy and hazardous days) summed across all monitored days in each state.
EIA RECS 2020 public use microdata. Ductwork is inferred from central heating or cooling equipment; there is no direct duct variable. Whole-house and attic fans are the HOUSEFAN and ATTICFAN variables.
This is the most important limitation on the page. EPA does not attribute fine particulate to a source, so a high-PM2.5 day may be wildfire smoke, traffic, industry or residential wood burning.
The 2020 western and 2023 eastern patterns are consistent with the documented fire seasons, but this dataset cannot prove attribution and no claim of attribution is made.
Air quality monitors are concentrated in populated counties, and a state figure is the share of monitored days rather than of population-weighted exposure. Rural exposure is under-observed, and in fire-prone rural areas it is likely understated.
Two of the three were chosen because they were severe. This describes the exposure households have recently experienced, which is what drives filtration decisions, but it is not a long-run risk estimate.
A ducted home may have a MERV 4 filter, a leaking return pulling attic air, or a system nobody runs in June. Ductwork is the precondition for whole-house filtration, not evidence of it, and as noted above no public dataset records what is actually in the filter slot.
Questions
Does my HVAC system filter wildfire smoke?
Which states have the worst smoke exposure?
What MERV rating do I need for wildfire smoke?
What if my home has no ducts?
Should I run my whole-house fan when it is smoky?
How many American homes cannot filter their air at all?
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HYRE HVAC Research Desk
Primary-source research, data analysis and fact checking
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Data as of EPA AQI 2020, 2023, 2024; RECS 2020 microdata. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
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Getting filtration into a house that has none
Whether the answer is a filter upgrade, a ducted system or portable units comes down to what is already in the building.