Why Indoor Air Gets Smoky
Most wildfire smoke advice reduces to two words: stay inside, which leaves out the part that determines whether staying inside actually helps.
Missoula Public Health, which has been publishing near-daily smoke updates through this summer's fire season, is more specific. Its July 28 update explains that when it is smoky outside, fine particles enter homes "through windows, doors, outdoor air intakes, and cracks." Closing a door does not create a seal.
The particles in question are PM2.5, meaning particulate matter 2.5 microns or smaller. Wildfire smoke particles run roughly 0.4 to 0.7 microns, small enough to pass deep into the lungs and into the bloodstream, and small enough that ordinary dust filters do little.
That is why the department's guidance centers on filtration rather than only on closing up. It recommends a portable air cleaner with a HEPA filter, or a higher-grade MERV furnace filter rated 13 or above, which it describes as very efficient at removing smoke from indoor air.
The department reports air quality across Missoula County running from Moderate to Unhealthy for Sensitive Groups, with a Montana Department of Environmental Quality air quality alert covering the county and many others in the state.
The Morning Flush
The most useful piece of advice in the update is also the most counterintuitive, and it exists because many households face a genuine conflict.
If a home has no air conditioning, keeping windows shut through a hot smoky stretch is not sustainable. People open windows at night to cool the house, which pulls in smoke that often settles heaviest overnight and in the early morning.
The department's alternative is what it calls a morning flush. Rather than an evening or all-night airing, open all the windows in the morning and use fans to draw in cool air. Then close the windows and blinds, and run a HEPA air cleaner or a homemade fan filter at full power to remove the smoke that came in with the cool air.
The logic is a deliberate trade. Accept a short, controlled intake of smoke at the coolest point in the day, then spend the rest of the day filtering it out with the house sealed.
One clarification the guidance implies but does not spell out: this is advice for homes without air conditioning. A household with working air conditioning should keep windows closed, run the system on recirculate rather than drawing outside air, and upgrade to a MERV 13 filter if the system can handle it.
Building a Filter for About Thirty Dollars
The cost barrier to clean indoor air is lower than most people assume, and the department points directly at the workaround.
A DIY fan filter is a box fan with a MERV 13 or higher furnace filter attached to the intake side, so the fan pulls air through the filter. Both components are available at any hardware store, and the combination costs a fraction of a commercial air cleaner. Public health agencies have promoted versions of this design for years, and it measurably lowers indoor particle levels.
A few practical notes. The filter must sit on the intake side, with the airflow arrow printed on the filter pointing toward the fan. Filters clog quickly during heavy smoke and need replacing more often than the packaging suggests. And a box fan filter should be used while someone is home and awake rather than left running unattended overnight, since box fans are not designed for continuous duty.
Missoula residents have one additional option that will not apply elsewhere. The department notes that Northwestern Energy residential customers who buy an Energy Star certified air cleaner can submit for a $100 rebate. Readers in other areas should check whether their own utility offers something comparable, because many do and few advertise it.
Prioritize one room. Filtering a bedroom well is more achievable than filtering a whole house, and it protects the hours when exposure is longest.
What the Alert Covers Right Now
The smoke over western Montana this week is mostly not Montana's.
The department reports several wildfires in Oregon producing smoke that is being carried toward Missoula County on southwesterly winds, including the Rowe Creek Complex near Bend at close to 300,000 acres and 33 percent contained, the Shingle Fire at over 63,000 acres, and the Coleman Creek Fire in eastern Oregon at 140,000 acres.
Locally, the Elder 1 Fire south of Frenchtown has grown to 510 acres, and the North Fork Lost Horse Fire southwest of Hamilton was mapped at 105 acres with minimal recent growth.
The Montana DEQ alert ran until Friday morning as of the department's update, and smoke conditions change with wind direction, so current numbers matter more than any published forecast. Readers anywhere can check conditions at the EPA's Fire and Smoke Map, which combines regulatory monitors with low-cost sensors and gives a far more local picture than a city-level AQI reading.
Air quality categories are worth knowing. Moderate means unusually sensitive people should consider reducing prolonged outdoor exertion. Unhealthy for Sensitive Groups means people with heart or lung disease, older adults, children, and pregnant people should limit it.
Who Should Be Most Careful
Smoke is not equally risky for everyone, and the guidance should be scaled accordingly.
The groups with the most to gain from indoor filtration are people with asthma or COPD, people with heart disease, older adults, infants and young children, pregnant people, and anyone recovering from a respiratory illness. For these groups, PM2.5 exposure can trigger asthma attacks, worsen heart failure, and prompt emergency visits.
People with asthma should keep rescue inhalers accessible and confirm that controller medications are up to date before a smoke episode, not during one. Anyone whose symptoms escalate despite staying indoors should contact a clinician rather than waiting it out.
Seek urgent care for chest pain, severe shortness of breath, confusion, or an asthma attack that does not respond to a rescue inhaler. These warrant emergency evaluation, not a next-day appointment.
For healthy adults, the sensible response is reducing outdoor exertion when readings climb, since heavy breathing multiplies the dose inhaled. A run in Unhealthy air delivers far more particulate than a walk.
N95 respirators help outdoors when properly fitted. Cloth masks and surgical masks do not filter PM2.5 meaningfully, and running a home HVAC fan without an adequate filter simply circulates smoke.
The confirmed facts are an active air quality alert covering Missoula County and much of Montana, with smoke largely transported from Oregon fires. Those most affected are people with heart or lung conditions, older adults, young children and pregnant people. The reasonable actions are a morning flush if there is no air conditioning, then sustained filtration in one room. The central uncertainty is how long wind patterns keep transporting smoke inland. The next expected development is the department's next update and the DEQ alert's expiration or extension.
Frequently Asked Questions
What is a morning flush? Opening all windows in the morning to draw in cool air, then closing up and running an air cleaner at full power to remove the smoke that entered.
Should everyone do this? No. It is guidance for homes without air conditioning. If you have working air conditioning, keep windows closed and run it on recirculate.
How do I build a DIY fan filter? Attach a MERV 13 or higher furnace filter to the intake side of a box fan, with the filter's airflow arrow pointing toward the fan.
Is a box fan filter safe to leave running overnight? Use it while someone is home and awake. Box fans are not built for continuous unattended operation.
Do masks help? A properly fitted N95 helps outdoors. Cloth and surgical masks do not meaningfully filter PM2.5.
Where can I check my own air quality? The EPA Fire and Smoke Map at fire.airnow.gov gives a more local reading than a city-wide AQI figure.
Who is most at risk from smoke? People with asthma, COPD or heart disease, older adults, infants and young children, and pregnant people.
