Why Wildfire Smoke Is Different from Normal Dust
Regular household air pollution is mostly large particles: dust, pollen, pet dander. These range from 5 to 100 microns in size. Your nose and throat catch most of them before they reach your lungs.
Wildfire smoke is mostly PM2.5, fine particles smaller than 2.5 microns. Many smoke particles are as small as 0.1 microns. At that size, they skip your nose and throat. They go deep into your lungs and can reach your blood. That is why wildfire smoke causes heart attacks and strokes, not just coughing.
Smoke also carries toxic gases that normal outdoor air does not. Carbon monoxide, formaldehyde, and benzene all come from burning trees, homes, and cars. These gases have no smell at low levels. But they still cause harm.
Key Point
Normal dust is big. Your body filters most of it. Wildfire smoke particles are tiny enough to enter your blood. That is why the EPA rates PM2.5 as the most dangerous common air pollutant.
The AQI Scale: All 6 Levels Explained
The Air Quality Index (AQI) runs from 0 to 500. It takes readings from several sources. Then it turns them into one number. During wildfires, PM2.5 almost always drives the AQI.
| AQI Range | Level | What to Do |
|---|---|---|
| 0–50 | Good | No limits. Windows open, outdoor exercise fine. |
| 51–100 | Moderate | Fine for most. Very sensitive people may want to limit hard outdoor exercise. |
| 101–150 | Unhealthy for Sensitive Groups | Close windows. Run purifier on high. Children, older adults, and those with heart or lung disease should stay indoors. |
| 151–200 | Unhealthy | Everyone should limit outdoor time. Wear N95 outside. Seal gaps. Run purifier on max. |
| 201–300 | Very Unhealthy | Stay indoors. Wear N95 outside. Leave if your home cannot be sealed well. |
| 301+ | Hazardous | Health alert for all. Avoid going outside. Leave if told to, or if you cannot keep indoor air clean. |
Track current AQI at airnow.gov or your local weather app. For real-time indoor readings, see our guide to the best air quality monitors.
PM2.5 vs PM0.1, The Two Particles That Matter
The AQI uses PM2.5 as its benchmark. But wildfire smoke has particles even smaller than that: PM0.1, also called ultrafine particles.
PM2.5
Smaller than 2.5 microns. The standard measure used by the EPA and AQI.
A True HEPA filter catches 99.97% of PM2.5 particles. This is the main target for air purifiers.
PM0.1 (Ultrafine)
Smaller than 0.1 microns. More common in wildfire smoke than in city air.
Harder to filter. Even HEPA filters miss some at this size. These particles go deepest into your lungs.
Most home air monitors only track PM2.5 and larger. They do not show PM0.1 levels. So your monitor can look clean while tiny particles are still in the air. During a fire event, assume PM0.1 is high when PM2.5 reads above 35 ug/m3.
Do not wait for the AQI to look bad. During fire season, keep windows closed and purifiers running even when outdoor AQI reads moderate.
When to Close Windows, and How Long It Takes
The threshold is AQI 100. Once outdoor AQI crosses 100, close all windows and doors. Switch your HVAC to recirculate. Do not wait until you can smell smoke. By then, your indoor PM2.5 is already rising.
How fast do windows help or hurt? A study from Berkeley Lab found that indoor PM2.5 follows outdoor levels with a 1-2 hour lag. That means:
- Closing windows: Indoor PM2.5 takes 1-4 hours to fall. It depends on how tight your home is and whether a purifier is running.
- Opening windows: Indoor levels rise within 30-60 minutes on a bad smoke day. Do not open windows to air things out during a smoke event, even for a few minutes.
Rule of Thumb
Close windows when AQI hits 100. Open them only when AQI has been below 50 for at least 2 hours in a row. Check airnow.gov before opening.
Pre-Event Prep, What to Do Before Fire Season
The best time to prepare is before smoke arrives. Once AQI spikes, stores sell out of N95s and purifiers within hours. Do this before fire season starts:
- Seal gaps and weatherstrip doors. Most smoke gets in through gaps around doors, windows, attic hatches, and fireplace dampers. A tight home holds clean air much longer. Focus on exterior doors first. Add door sweeps if you can see any gaps.
- Upgrade your HVAC filter to MERV 13. A standard MERV 8 filter catches about 20% of PM2.5. A MERV 13 catches around 85%. Check that your HVAC can handle the extra load. Some older systems cannot. If yours struggles, MERV 11 is a good step up.
- Test your air purifier now. Run it on high for an hour and check the filter. If it is past due, swap it before smoke season. A clogged filter has a fraction of the output of a clean one.
- Stock NIOSH-approved N95 masks. Buy enough for every person in your home, plus a few extras. Store them in a sealed bag away from heat and moisture. Check the NIOSH website to confirm the brand is approved before buying.
- Set up an air quality monitor. It shows your actual indoor PM2.5 instead of guessing from outdoor AQI. Indoor readings can be much better or worse than outdoor. The real number tells you when to act.
During a Smoke Event, Room by Room
When outdoor AQI is above 100, treat your home like a clean room. Every open gap matters.
Bedroom (top priority)
You spend 8 hours here breathing deeply during sleep. Put your best purifier here. Run it on high or max. Keep the door closed so the clean air stays in one room.
Living Room / Main Space
Add a second purifier if you have one. If not, close off unused rooms and move the bedroom purifier here during the day. Keep HVAC in recirculate mode so it filters air through the MERV 13 filter.
Kitchen
Avoid the range hood during smoke events. It pulls air out and draws smoky air in through gaps. Cook with the hood off, or use an oven.
Bathroom
Turn off bathroom exhaust fans. Like range hoods, these exhaust air and pull smoke in through leaks elsewhere in the home. Keep use brief and the door closed.
Wear an N95 mask if you must go outside. Surgical masks and cloth masks do not filter PM2.5.
When to Leave vs Shelter
Shelter in place works when your home can keep indoor AQI below 150. If indoor PM2.5 keeps climbing with windows closed and a purifier running, leave. And leave if official orders are issued. No purifier replaces leaving when the fire is close.
Post-Event Cleanup, Getting Your Home Back to Normal
Do not open windows the moment outdoor AQI drops. Smoke particles settle on carpets, curtains, fabric furniture, and bedding. They float back up whenever you move around. Follow this sequence:
- Wait for clean air. Outdoor AQI must stay below 50 for at least 2 hours in a row before you open windows. Check airnow.gov.
- Keep purifiers running for 2-3 more days. Even after outdoor air clears, settled particles keep floating back up. Do not turn off the purifier just because you cannot smell smoke.
- Replace your HVAC filter. It took a heavy load during the event. A clogged filter cuts airflow and loses power. Change MERV 13 filters after any major smoke event, even if they are not due yet.
- Vacuum with a HEPA vacuum. A normal vacuum blows fine particles back into the air. Use a sealed HEPA vacuum. Do carpets, furniture, and mattresses.
- Wipe hard surfaces with a damp cloth. Dry dusting kicks particles into the air. Damp-wipe countertops, shelves, windowsills, and window frames where particles collect.
- Wash all fabric that was exposed. Curtains, bedding, and throw pillows absorb smoke particles. Wash them in warm water.
- Check your air purifier filter. If it looks dark gray or brown, replace it even if it is not due. A loaded carbon filter will still smell smoky when running.
Monitoring Tools Worth Having
We may earn a commission at no extra cost to you.
These three products show you what is in your air and help you filter smoke during fire season.
Airthings Wave Plus, ~$229
Tracks PM2.5, CO2, humidity, temperature, VOCs, and radon, all in one puck-shaped device. Wave your hand to check current levels without getting up. Works with the Airthings app for charts and alerts. The best all-around indoor air monitor for wildfire season.
See Airthings Wave Plus on Amazon →IQAir HealthPro Plus
The top air purifier for high-risk homes. Its HyperHEPA filter catches particles down to 0.003 microns, well below standard HEPA. If someone in your home has asthma, COPD, or a weak immune system, get this one before smoke season.
See IQAir HealthPro Plus at IQAir.com →FilterBuy MERV 13 Bulk Packs
Switching your HVAC filter to MERV 13 is one of the best changes you can make before fire season. FilterBuy sells MERV 13 filters in bulk across standard sizes. Buy enough to swap your filter before and after a smoke event.
See FilterBuy MERV 13 on Amazon →For full air purifier picks, see our guide to the best air purifiers for smoke and our roundup of the best air quality monitors.
Common Questions
What AQI is dangerous for wildfire smoke?
AQI above 100 means you should act. Close windows, run your purifier on high, and limit time outside. This is most urgent if you have asthma, heart disease, or are over 65. At AQI 150, everyone is at risk. Not just sensitive groups. At AQI 200 and above, stay indoors. Wear an N95 outside. Leave if your home cannot be sealed well.
Should I run my HVAC during wildfire smoke?
Switch your HVAC to recirculate mode. This keeps it on without pulling in outside air. If your system has no recirculate setting, set the fan to 'on' instead of 'auto'. This keeps indoor air moving through your filter. Upgrade to a MERV 13 filter first. Never run a whole-house fan or attic fan during smoke. These pull smoky air in through every gap and crack.
Do N95 masks work for wildfire smoke?
Yes. N95 masks filter at least 95% of airborne particles, including PM2.5. They must fit tight. A loose seal defeats the purpose. Surgical masks and cloth masks do not filter fine particles well. Look for NIOSH-approved N95 masks. KN95 masks offer similar protection from a good source. But they are not NIOSH-approved.
How long does wildfire smoke stay in the air?
It depends on how far away the fire is and the weather. Local fires can cover an area for days. Smoke from far-away fires can travel on wind and linger for days. After the fire is out, ground smoke usually clears within 1-3 days once wind shifts. Check airnow.gov rather than guessing. Smoke is not always visible, but AQI monitors catch it.
What is PM2.5 and why does it matter?
PM2.5 is a measure of tiny particles smaller than 2.5 microns wide. That is about 1/30th the width of a human hair. These particles get past your nose and throat. They go deep into your lungs. From there, they can enter your blood. Long-term exposure is linked to heart disease, lung cancer, and stroke. During wildfires, PM2.5 can spike to 10-20x normal levels in hours. An indoor air monitor shows your real exposure.