An AQI of 151 means outdoor PM2.5 has passed 55.4 micrograms per cubic meter, the point where the EPA rates the air unhealthy for everyone. What you breathe inside depends mostly on how tightly the house is closed and what filter sits in the return.
Official sources report the outdoor wildfire smoke AQI. This guide covers the indoor changes they leave out.
TL;DR Quick Answers
Wildfire smoke raises the AQI through PM2.5. Above 100, close the house and run the HVAC with a MERV 13 filter if your system can handle it. Then put a correctly sized air cleaner in one room.
- What the number means: The air quality index runs from 0 to 500, and PM2.5 sets it during smoke events.
- When to act indoors: Sensitive groups move inside at 101, and everyone does at 151. Filterbuy, a U.S. air filter manufacturer, explains how to run a MERV 13 filter for wildfire smoke season and switch back when the air clears.
- Where to check smoke today: Start with the AirNow Fire and Smoke Map. Our live Kentucky wildfire and smoke map page pairs a state map with indoor action.
Top Takeaways
- PM2.5 drives the smoke AQI. Since May 6, 2024, Good tops out at 9.0 micrograms per cubic meter, and Unhealthy starts at 55.5.
- Closing up helps, but it isn’t enough. Indoor PM2.5 still nearly tripled on wildfire days in a study of more than 1,400 California buildings.
- MERV 13 is the smoke filter, as long as your blower can handle the added resistance.
- Size air cleaners by smoke CADR: at least two-thirds of the room’s square footage, or the full square footage in heavy smoke.
How Do You Read the Wildfire Smoke AQI Today?
Open the AirNow Fire and Smoke Map and read the nearest official monitor first, then the corrected sensors around it. We treat the sensors as the early warning, because the map’s 30-minute trend shows a plume moving in before the hourly NowCast catches up. Filterbuy team has found the AQI usually spikes hours before a family notices smoke indoors.
Do this now: Bookmark the map and turn on smoke alerts in the AirNow app.
What PM2.5 Levels Match Each AQI Category?
The EPA revised the PM2.5 AQI breakpoints in its final particulate matter rule, effective May 6, 2024. We act one band early, since indoor levels take time to fall.
- Good (AQI 0 to 50): PM2.5 0 to 9.0. Ventilate normally and install a MERV 13.
- Moderate (AQI 51 to 100): PM2.5 9.1 to 35.4. Stage the air cleaner in the room you plan to use.
- Unhealthy for Sensitive Groups (AQI 101 to 150): PM2.5 35.5 to 55.4. Close windows, switch the fan from AUTO to ON, and start the air cleaner.
- Unhealthy (AQI 151 to 200): PM2.5 55.5 to 125.4. Cut indoor sources like frying and candles.
- Very Unhealthy (AQI 201 to 300): PM2.5 125.5 to 225.4. Stay in your cleaner air room and check the filter for loading.
- Hazardous (AQI 301 and up): PM2.5 225.5 and higher. Follow local officials, and go to a clean air shelter if you can’t keep the house clean and cool.
How Does Wildfire Smoke Get Inside Your Home?
Smoke enters through open windows, door gaps, and anything that pulls outdoor air in, including fresh-air intakes, ERVs, swamp coolers, and exhaust fans, the path we see overlooked most.
- Mechanical ventilation: The EPA advises turning off mechanical ventilation temporarily during a smoke event and restarting it once the smoke clears.
- The limit: A sealed house heats up fast. Without working AC, AirNow recommends a clean air shelter or another cooled building with good filtration.
Do this now: Find your fresh-air intake or ERV control and label it.
Which MERV Filter Works for Wildfire Smoke?
Per the EPA’s MERV rating chart, MERV 13 captures at least 50 percent of particles from 0.3 to 1.0 micron, where much of the smoke mass sits. MERV 8 has no minimum there.
Filterbuy, a U.S. air filter manufacturer based in Talladega, Alabama, recommends MERV 13 for wildfire smoke in its AQI guide because it catches fine PM2.5 that basic furnace filters miss. On systems with tight static pressure, we treat MERV 13 as a smoke-season setting, the same seasonal approach Filterbuy recommends.
The limit: A 1-inch MERV 13 can starve the blower on an undersized return. If it struggles, ask a technician about a 4-inch media cabinet.
- Fan setting: Switch the fan to ON. As Filterbuy notes, most home systems already recirculate, so fan runtime is what changes.
- Odor: MERV covers particles only. For smoke smell, add an activated carbon filter, such as Filterbuy’s Odor Eliminator line.
Do this now: Measure the filter slot correctly and order two MERV 13 replacements.
How Big Should an Air Purifier for Wildfire Smoke Be?
The EPA sets a floor of two-thirds of the room’s square footage, so a 120-square-foot room needs a smoke CADR of 80. For heavy smoke, AHAM recommends a smoke CADR equal to the room’s square footage. We size sleeping rooms to the full number.
- Ozone: Buy only CARB-certified air cleaners.
- Placement: Set the unit mid-room and run it on high continuously.
- Budget fallback: A box fan with a MERV 13 filter cuts smoke. In EPA chamber tests, a cardboard shroud raised its smoke CADR about 40 percent, and a four-filter Corsi-Rosenthal box reached about 401. DIY units nearly stop working with dirty filters.
Which Smoke Setup Fits Your Home?
- Central air that handles MERV 13: Run it with the fan on ON, plus one bedroom air cleaner.
- Restricted return or renters: Use the highest MERV the system allows, and let a portable air cleaner carry the load.
- Ductless or no central air: The EPA points these homes to portable air cleaners.
- Swamp cooler or single-hose portable AC: Both pull smoky air inside, so switch to fans.
How Do You Set Up a Cleaner Air Room?
Pick a bedroom big enough for the household.
- Close windows and doors, and stop any ventilation that pulls in outdoor air.
- Run a correctly sized air cleaner on high the entire time smoke is present.
- Keep the room cool with AC or fans.
- Skip cooking, candles, and sprays, and wipe dust with a damp cloth.
- Put a PM2.5 monitor in the room and confirm the reading drops within the first hour.
What Are the Health Effects of Wildfire Smoke?
Smoke can cause cough, wheezing, chest pain, and fatigue, and it worsens asthma, COPD, and heart failure. We plan every setup around the most sensitive person in the house.
- Higher-risk groups: People with asthma, COPD, or heart disease, children, older adults, and pregnant people, according to CDC wildfire safety guidance.
- Multi-day smoke: Several days of heavy smoke slows recovery, per AirNow.
- When to get help: Call 911 for chest pain or severe shortness of breath.
How Do You Know Your Indoor Air Is Clean?
Measure it. A PM2.5 monitor in your cleaner air room shows whether filtration keeps up. Our target is 9.0 micrograms per cubic meter or lower. Low-cost sensors tend to over-read heavy smoke, so compare readings from the same sensor type and log them twice a day.
“In every smoke event we’ve tracked, the cleanest homes shared three habits set before the AQI crossed 101: the fresh-air intake was closed, the thermostat fan was switched to ON, and one right-sized air cleaner was already running in the room where everyone sleeps. The outdoor number tells you when to act, and the monitor in that room tells you whether it worked.”
Essential Resources for Wildfire Smoke AQI
Turn Every AQI Number Into a Household Action Plan
Filterbuy’s homeowner guide maps each of the six AQI categories to a specific indoor step, from moving sensitive groups inside at 101 to setting up a clean room at 201 and above. It also explains why a MERV 13 filter catches the fine PM2.5 that basic furnace filters miss.
Source: Filterbuy, Wildfire Smoke and the Air Quality Index: A Homeowner’s Guide
Know Who in Your Home Needs Protection First
The CDC health advisory on wildfire smoke names the highest-risk groups, including people with asthma, COPD, or heart disease, children, older adults, and pregnant people, and lists the symptoms that call for medical care.
Source: CDC Health Advisory, Wildfire Smoke Exposure Poses Threat to At-Risk Populations
Pick the Right HVAC Filter and Air Cleaner for Your Space
The EPA consumer guide to air cleaners and air filters shows how to choose a furnace filter rated MERV 13 or as high as your system allows, and how to size a portable air cleaner for the room it serves.
Source: EPA, Air Cleaners and Air Filters in the Home
Build One Room Where Smoke Stays Lowest
The EPA cleaner air room guide walks through six setup steps, including sizing an air cleaner at a smoke CADR of at least two-thirds of the room’s square footage and building a DIY box fan filter.
Source: EPA, How to Create a Cleaner Air Room at Home
Get Your Home Smoke-Ready Before Fire Season
The EPA wildfire smoke preparation checklist covers spare HVAC filters, a right-sized air cleaner, an air sensor, N95 respirators, and a plan for pets.
Source: EPA, Prepare for Wildfire Smoke
Supporting Statistics on Wildfire Smoke Indoors
1. 145 Million Americans Breathed Unhealthy Particle Pollution at Least Once
The American Lung Association’s State of the Air 2026 report found 145.0 million people lived where fine particle pollution reached unhealthy or worse on at least one day from 2022 through 2024, with wildfire smoke a key factor.
Source: American Lung Association, State of the Air 2026
2. Indoor PM2.5 Nearly Tripled on Wildfire Days, Even With Windows Closed
UC Berkeley researchers tracking more than 1,400 California buildings found indoor PM2.5 nearly tripled on wildfire days.
Source: Liang et al., PNAS 2021, via NOAA
3. A HEPA Air Cleaner Cut Indoor Smoke by 48 to 78 Percent
University of Washington researchers found HEPA air cleaners lowered indoor PM2.5 by 48 to 78 percent in Seattle homes during a 2020 wildfire episode.
Source: Xiang et al., 2021, via PubMed
Final Thoughts and Opinion
Opinion: The purchase that protects you most happens in spring. Filters and air cleaners can sell out once smoke arrives, and Filterbuy warns that shipping slows during a regional event. We’d rather see one fully equipped cleaner air room than a half-measure in every room.
Frequently Asked Questions
Is It Safe to Run My AC During Wildfire Smoke?
Yes. Close any outdoor air intake first.
Can a MERV 13 Filter Damage My HVAC System?
The EPA notes most systems handle MERV 13 if the filter is replaced often. Watch for signs of a clogged or restricted filter.
When Can I Open the Windows After Wildfire Smoke?
Once the map shows Good or Moderate air, flush the house and restart ventilation.
Do Masks Help Against Wildfire Smoke?
Cloth masks won’t protect you, according to AirNow. A well-fitted N95 respirator can.
When Should I Change My Filter After a Smoke Event?
Check it once the smoke clears and replace it if it looks gray, then return to your normal furnace filter change schedule.
Take the Next Step: Read the AQI, Then Keep Smoke Out of Your Home
Check the AirNow Fire and Smoke Map today so you know the AQI at your address before smoke arrives. Then measure your HVAC filter slot and put two MERV 13 filters on the shelf, so keeping smoke out of your home takes minutes when the number climbs.
Glossary
- Air Quality Index (AQI): The EPA’s 0 to 500 outdoor air scale.
- PM2.5: Particles 2.5 microns or smaller, the main threat in smoke.
- NowCast: AirNow’s hourly method that weights recent hours.
- MERV: The ASHRAE 1 to 16 HVAC filter rating.
- CADR: An air cleaner’s tested clean air output.