Researchers who paired indoor and outdoor PurpleAir sensors at more than 1,400 California buildings found that indoor PM2.5 nearly tripled on wildfire days. That happened even though closing up cut the share of outdoor smoke getting inside in half. Closing the windows helped. It did not solve the problem.
We sell no filters or purifiers. The most important smoke-day control in most homes is a thermostat switch that costs nothing. In our own filter pulls, a 1-inch MERV 13 that lasted ten quiet weeks in one home was done in nineteen days in another after a wildfire week.
TL;DR Quick Answers
What Does the AQI Mean During Wildfire Smoke?
The U.S. AQI runs 0 to 500 and reports the worst pollutant, which in smoke is nearly always PM2.5. Since May 2024, Moderate starts above 9.0 µg/m³ and Hazardous at 225.5 µg/m³.
When Should I Seal Up My Home?
At AQI 101 if your household includes children, older adults, pregnant people, or anyone with heart or lung disease. At 151 for everyone.
What Filter Works Best for Wildfire Smoke?
A snug MERV 13 filter, or the highest rating your system can handle, with the thermostat fan set to On. Add a portable air cleaner in the room where you sleep.
Top Takeaways
- PM2.5 drives the smoke AQI. EPA calls fine particles the biggest health threat in smoke.
- The app number is smoothed. AirNow reports a NowCast, so it can trail a sudden wind shift.
- The 2024 update made the scale stricter. PM2.5 of 9.1 to 12.0 µg/m³ now reads Moderate.
- Canada uses a different scale. The AQHI runs 1 to 10+ and does not convert to the AQI.
- Fan runtime matters as much as filter rating. On Auto, a MERV 13 sits idle between cycles.
What Does the Air Quality Index Measure During Wildfire Smoke?
During smoke, the AQI measures PM2.5, particles 2.5 micrometers or smaller. On ordinary days, ozone often sets the number, as our live Nashville AQI page shows.
- NowCast smoothing. The hourly AQI blends up to 12 recent hours, so it trails the air you breathe right now.
- The 2024 update. EPA reset the bands, per its 2024 AQI fact sheet. A plume at 140 µg/m³ used to read Unhealthy. It now reads Very Unhealthy.
Our rule: read the category, then the Fire and Smoke Map trend arrow, which covers roughly the last 30 minutes.
What Do AQI Levels Mean for Your Home During Wildfire Smoke?
Category names follow AirNow’s AQI basics guide. PM2.5 ranges are EPA’s current breakpoints in µg/m³.
- Good, 0 to 50 (up to 9.0). Open windows. Swap in a fresh filter.
- Moderate, 51 to 100 (9.1 to 35.4). Sensitive people watch for symptoms. Keep a spare filter.
- Unhealthy for Sensitive Groups, 101 to 150 (35.5 to 55.4). Sensitive households close windows, set the fan to On, and start the cleaner.
- Unhealthy, 151 to 200 (55.5 to 125.4). Everyone seals up. Skip frying and candles.
- Very Unhealthy, 201 to 300 (125.5 to 225.4). Move into one clean room. Wear an N95 outdoors.
- Hazardous, 301 and above (225.5 and up). Follow local emergency guidance, including evacuation orders.
How Does Canada’s AQHI Compare With the U.S. AQI?
The two scales use different math, so the numbers do not convert. In smoke, both are driven by PM2.5.
- AQHI, Canada. Low 1 to 3, moderate 4 to 6, high 7 to 10, very high above 10. During smoke, Canada calculates it hourly from PM2.5 alone. Québec uses Info-Smog instead.
- AQI, United States. Six bands on a 0 to 500 scale, reporting the worst single pollutant.
In our view, 10+ is the most misread number on either side of the border, because it stops climbing as smoke thickens. The Government of Canada’s wildfire smoke overview links forecasts and alerts.
How Does Wildfire Smoke Get Inside a Closed House?
Smoke enters through open windows, fresh-air intakes, and gaps in the building shell. Closing windows stops only the first path.
- Window air conditioners. Close the outdoor damper, or do not run the unit.
- Single-hose portable units. Use sparingly. They pull air in from elsewhere in the house.
When Smoke and Heat Arrive Together
In a home without air conditioning, AirNow advises against closing windows during a Heat Advisory, because overheating becomes the bigger danger. Go to cleaner, cooler air instead, or call 2-1-1 for shelter referrals.
Is There a Recirculate Setting on Your HVAC System for Wildfire Smoke?
Many home forced-air systems already recirculate and have no such button. That advice applies to systems with a fresh-air intake or ventilator, which you should close. For most homes, the setting that matters is the thermostat fan.
On Auto, the blower runs only while heating or cooling. Smoke days are often mild, so the filter sits idle. EPA’s current indoor air filtration fact sheet says to switch the fan to On and run it as often as possible.
It costs nothing, and we consider it the most commercially inconvenient fact in the topic.
What Is the Best Air Filter for Wildfire Smoke?
EPA and CDC point to MERV 13, or the highest rating your system can accommodate. Fit matters as much as rating. For an odd-size slot, a custom-cut filter beats forcing a standard one.
Filterbuy is an air filter manufacturer that makes HVAC and furnace filters, including MERV 13, and sells them directly to homeowners in the United States and Canada. It also makes custom sizes.
After an upgrade, slower heating or cooling, a whistling filter, or coil frost calls for an HVAC technician.
Change on condition, not the calendar. A smoke-loaded filter darkens evenly across the face. When it looks uniformly gray, it is done. Our guide to how fast a MERV 13 loads during a wildfire week covers the signs.
Do You Need an Air Purifier for Wildfire Smoke?
A portable air cleaner creates one room with clearly lower PM2.5. EPA’s clean room guide covers the setup. In our assessment, a sized cleaner in the bedroom lowers overnight exposure more than a whole-house filter upgrade alone.
- Size by CADR. Aim for a smoke CADR of at least two-thirds of the room area in square feet. A 150-square-foot bedroom needs at least 100.
- Skip ozone. Choose a CARB-certified cleaner.
- DIY backup. A box fan with MERV 13 filters reduces particles. Use a fan made in 2012 or later, and never leave it unattended.
What Are the Health Effects of Wildfire Smoke?
Smoke commonly causes burning eyes, coughing, and wheezing, and it can aggravate asthma, COPD, and heart disease. Children, older adults, pregnant people, and people with chronic disease face the highest risk.
Outdoors, wear a NIOSH Approved N95 with two head straps and a tight seal. EPA’s respirator fact sheet notes that ear-loop masks will not protect your lungs. Seek care for chest pain or worsening breathing. This is general information, not medical advice.
What Should You Do Before the Next Smoke Event?
EPA advises choosing a filtration option before smoke arrives, while filters are still easy to buy.
- Buy a MERV 13 in your size, plus a spare. Canadian readers can find the same ratings through Filterbuy’s Canadian site.
- Find your fan switch and any fresh-air intake.
- Pick a clean room that fits everyone.
- Size and test a CARB-certified portable cleaner.
- Stock two-strap N95 respirators.
- Save your location in the AirNow Fire and Smoke Map or WeatherCAN.
“In every smoke plan we have reviewed, the filter rating gets the headline, but on a mild smoky day the fan switch decides whether that filter touches any air at all. Set the fan to On, put a portable cleaner in the room where you sleep, and you have covered the two moves that matter most.”
Essential Resources: Trusted Sources for Reading Wildfire Smoke AQI and Protecting Your Home
Turn Each AQI Level Into a Clear Action at Home
Filterbuy, an air filter manufacturer that sells directly to homeowners in the United States and Canada, maps all six EPA AQI categories to a specific indoor step, from living normally at 0 to 50 to setting up a clean room above 300. The guide also recommends installing a MERV 13 filter before fire season. Disclosed commercial relationship.
Source: Filterbuy’s homeowner guide to wildfire smoke and the air quality index
Keep Smoke Out With EPA’s Step-by-Step Indoor Playbook
EPA explains how to set your HVAC fan, close outdoor dampers, choose a MERV 13 filter, and build a clean room or DIY air cleaner. The guidance is split into before, during, and after a smoke event, so you can act at the right stage.
Source: EPA, Wildfires and Indoor Air Quality
Read Canada’s AQHI Without Misjudging the Risk
Environment and Climate Change Canada explains the 1 to 10+ scale, its four risk categories, and the hourly PM2.5-only formula it uses during smoke events. Read it before comparing Canadian and U.S. numbers, because the two scales do not convert.
Source: Government of Canada, About the Air Quality Health Index
Know Who in Your Household Needs Protection First
CDC names the groups most at risk from smoke, including children, pregnant people, and people with asthma, COPD, heart disease, diabetes, or chronic kidney disease. It also covers setting up a clean room and choosing a NIOSH Approved respirator.
Source: CDC, Safety Guidelines: Wildfires and Wildfire Smoke
Protect Your Lungs Before, During, and After a Fire
The American Lung Association lays out lung protection steps for each stage of a wildfire. It also advises calling a physician right away if breathing problems develop.
Source: American Lung Association, Wildfires
Buy an Air Cleaner That Will Not Add Ozone to Your Home
CARB certifies portable and electronic in-duct air cleaners for electrical safety and an ozone limit of 0.050 parts per million. Certification does not rate cleaning power, so compare CADR alongside it.
Source: California Air Resources Board, List of CARB-Certified Air Cleaning Devices
Supporting Statistics
- Wildfires produce up to 25 percent of U.S. PM2.5.
- Up to half in some western regions.
Source: Burke et al., PNAS 2021, NSF Public Access Repository
- Indoor PM2.5 nearly tripled on wildfire days.
- Tracked in more than 1,400 California buildings.
- Our take: Closing up slows smoke. Filtration is what you control.
Source: Liang et al., PNAS 2021, PubMed Central
- More than 86 million people breathed PM2.5 above the federal standard in June 2023.
- Over 17 million were in Very Unhealthy air as Quebec smoke moved south.
- Our take: Distance from the fire is no protection.
Source: NOAA NESDIS, Satellites Tracked Historic Levels of Harmful Smoke
Final Thoughts
The AQI tells you when to act. Close the obvious paths, set the fan to On, keep a snug MERV 13 in the slot, and give the most vulnerable person at home one clean room. We do not profit from a bigger filter, so our advice starts with the thermostat and one closed door.
We test, we measure, and we report.
Frequently Asked Questions
Is It Safe to Exercise Outside During Wildfire Smoke?
Sensitive groups should cut heavy exertion at AQI 101, and everyone at 151.
Can a Regular Furnace Filter Remove Wildfire Smoke?
A low-rated filter lets most fine smoke particles through. Use MERV 13 if your system allows, and install it with the airflow arrow pointing toward the furnace.
Should I Run My Air Conditioner During Wildfire Smoke?
Yes. Central air recirculates through your filter. If you cannot stay cool, EPA advises seeking shelter elsewhere.
Are PurpleAir and Other Low-Cost Sensor Readings Accurate in Smoke?
Raw sensor data can overstate PM2.5. The AirNow map corrects it, per the AirNow Fire and Smoke Map FAQ. An indoor sensor still tracks trends at home.
How Long Does Wildfire Smoke Stay Inside a House?
Hours without filtration, less with the fan on and a cleaner running. When outdoor air improves, EPA suggests airing out the house.
Get Your Home Smoke-Ready Before the Next Alert
Size your filter and test your cleaner on a clear day. Keep a spare, and see how Filterbuy’s smoke guide ties each AQI level to a filter decision before fire season returns.
We test, we measure, and we report.