Your phone reads 132, the sky is orange-gray, and everyone wants to know if it’s time to close up. Across our field inspections, most families wait for red before acting. This guide explains each wildfire smoke AQI color and why we act indoors at AQI 101.
TL;DR
During a wildfire, the AQI mostly tracks fine particles called PM2.5. At Orange (AQI 101 to 150), close windows, recirculate, and run a snug MERV 13 filter, as the U.S. Environmental Protection Agency (EPA) recommends. Check for a 0.25-inch gap, because smoke that goes around a filter never gets filtered.
Key Takeaways
- Smoke AQI is driven by PM2.5, the EPA’s top particle health concern.
- Since May 2024, Red tops out at 125.4 µg/m³, so heavy smoke reaches Purple sooner.
- Our action threshold is Orange, AQI 101.
- The EPA recommends MERV 13 or higher during smoke.
- A 0.25-inch gap lets smoke bypass the filter, so fit matters as much as rating.
What Does the AQI Measure During Wildfire Smoke?
During a wildfire, the AQI is almost always driven by PM2.5, the fine particles the EPA ranks as the greatest health concern. Smoke can also travel far from the fire, so readings can climb with no flames nearby.
In our field inspections, the outdoor number climbs first and indoor air follows through gaps and loose filter slots. Treat the AQI as an early warning for your indoor air.
Smoke also carries gases that particle filters don’t catch, so odor may linger. See our guide to removing wildfire smoke smell from your house.
What Does Each AQI Color Mean During a Wildfire?
Each color matches a 24-hour PM2.5 range from the EPA’s May 2024 AQI update for particle pollution.
- Green, Good (AQI 0 to 50; PM2.5 0.0 to 9.0 µg/m³). Open windows freely and stock a spare MERV 13 filter.
- Yellow, Moderate (AQI 51 to 100; PM2.5 9.1 to 35.4 µg/m³). Sensitive people may notice symptoms. Check your filter size.
- Orange, Unhealthy for Sensitive Groups (AQI 101 to 150; PM2.5 35.5 to 55.4 µg/m³). Our action threshold. Close up, recirculate, install a MERV 13, and set up a cleaner air room.
- Red, Unhealthy (AQI 151 to 200; PM2.5 55.5 to 125.4 µg/m³). Anyone may feel effects. Stay sealed with the fan running.
- Purple, Very Unhealthy (AQI 201 to 300; PM2.5 125.5 to 225.4 µg/m³). A health alert for everyone. Check the filter every day or two. In our experience, it can turn gray within days.
- Maroon, Hazardous (AQI 301 and above; PM2.5 225.5 µg/m³ and above). An emergency. Shelter in your cleanest room and follow officials.
Why We Treat Orange (AQI 101) as the Indoor Action Threshold
Every protective step takes time, so acting at Orange gets the filter working before PM2.5 reaches Red. Filterbuy’s homeowner guide to the wildfire smoke AQI draws the same line for sensitive groups at AQI 101.
Three reasons drive that recommendation:
- Orange is where health warnings begin. It’s the first band where the EPA flags effects for sensitive groups broadly.
- The upper bands are narrower than before. The EPA says its 2024 update may push some wildfire days into higher categories.
- Smoke rarely arrives on schedule. Readings can climb fast when the wind shifts.
In our field inspections, the cleanest homes closed up at the first orange reading.
What does closing up alone get you? The EPA reports indoor PM2.5 runs 55% to 60% of outdoor levels in a closed, unfiltered house. By our estimate using the 2024 breakpoints:
- Outdoor AQI 150: indoor about AQI 91 to 96 (Yellow).
- Outdoor AQI 200: indoor about AQI 160 to 165 (Red).
- Outdoor AQI 300: indoor about AQI 199 to 211 (Red to Purple).
Closing up buys time, and filtration does the rest.
How to Read the AirNow Fire and Smoke Map During a Wildfire
The EPA and U.S. Forest Service map combines official monitors, thousands of corrected sensors, fires, and smoke plumes.
The standard AirNow dial can mislead. AirNow’s guidance for wildfire smoke events notes it may show ozone or a distant monitor instead of smoke near you. To get a local reading:
- Enter your street address.
- Select the nearest monitor or sensor.
- Read the NowCast AQI color.
- Check the trend indicator to see where conditions are heading.
- Recheck in the morning, midday, and evening.
The map is the first screen we open when assessing a home during smoke. If readings disagree, plan around the higher one.
What Is the Best Air Filter for Wildfire Smoke?
The EPA recommends an HVAC filter rated MERV 13 or higher during smoke, and most systems can handle one if you replace it often. MERV 13 is also our year-round minimum.
Per an EPA Indoor airPLUS filtration bulletin, MERV 13 removes at least 50% of the smallest particles tested, versus roughly 20% for MERV 11. After testing hundreds of filters, that is where we set our floor.
Denser filters add static pressure, so older systems should get a technician’s check first. See our MERV filter pressure drop chart.
- Let the filter’s color set the schedule. Replace it when it turns dark brown or black, per the EPA.
- Keep spares on hand in your exact size.
- Run the fan. EPA’s Indoor AirPlus wildfire guidance suggests running it as often as possible during smoke.
- Skip ionizers and ozone generators. The EPA does not recommend them for smoke. Choose a CARB-certified mechanical filter such as HEPA.
Why Filter Fit Matters More During a Smoke Event
A MERV 13 filter only works if air goes through it. EPA wildfire guidance for homes calls for a snug fit with no bending or crushing, since gaps let smoke bypass the filter.
The most common mistake we see is buying by the old filter’s label. Measure the slot and follow our step-by-step furnace filter installation guide. If you fall between sizes, filter makers such as Filterbuy make custom sizes.
How to Protect Your Home From Wildfire Smoke
Start these steps at Orange:
- Close up the house. Shut windows and set the system to recirculate.
- Filter continuously. Run a snug MERV 13 with the fan on.
- Set up a cleaner air room. Follow the EPA steps to create a cleaner air room at home. Use an air cleaner with a smoke CADR of at least two-thirds of the room’s square footage (80 for a 10×12-foot room).
- Skip indoor smoke sources like candles, aerosols, vacuuming, and stove cooking.
- Watch the heat. Keep the AC on. Swamp coolers and single-hose portable ACs can pull smoke inside.
- Air out when the map improves, as the EPA advises.
Homeowners tell us the fan setting is the step they forget. Tape a “Fan: On” note to the thermostat.
Wildfire Smoke Health Effects: Who Should Act First?
Smoke can cause coughing, wheezing, stinging eyes, headaches, and chest pain. The CDC lists children, pregnant people, and people with asthma, COPD, diabetes, or heart disease as higher risk.
Call a doctor or 911 for chest pain or worsening symptoms. Outdoors, the CDC recommends an N95. Move the most sensitive person into the cleaner air room first, since the American Lung Association notes symptoms can appear 24 to 48 hours later.
Filterbuy filter team reports the AQI often spikes hours before families notice smoke indoors.
“After years of pulling smoke-darkened filters, I’ve learned the AQI on your phone changes before the air in your hallway does. That lag gives you time to act.”
Essential Resources for Reading Wildfire Smoke AQI and Protecting Your Home
Listed in the order you’ll need them:
Choose the Right Filtration Setup for Smoky Days
The EPA’s Indoor Air Filtration fact sheet compares three options: MERV 13 HVAC filters, portable air cleaners, and DIY box-fan cleaners. It also shows how to size a portable unit: its smoke CADR should be at least two-thirds of the room’s square footage.
Source: www.epa.gov/wildfires/indoor-air-filtration
Keep Smoke Out and Know When to Leave
The CDC’s safety guidelines cover sealing your home against smoke, choosing a well-fitting respirator, and recognizing symptoms that need medical attention. They also explain how to evacuate safely when local officials give the order.
Source: www.cdc.gov/wildfires/safety/how-to-safely-stay-safe-during-a-wildfire.html
Protect Family Members With Asthma, COPD, or Heart Disease
The American Lung Association outlines lung-health steps for before, during, and after a smoke event. It notes that breathing symptoms can appear 24 to 48 hours after exposure.
Source: www.lung.org/clean-air/emergencies-and-natural-disasters/wildfires
Spot Smoke Risk Days Before It Reaches You
The National Weather Service Fire Weather page posts Red Flag Warnings, fire weather outlooks, and wildland fire potential forecasts. Use it to install a fresh filter before your local AQI starts to climb.
Source: www.weather.gov/fire
Supporting Statistics
Federal research backs our Orange-level action standard.
- Closed windows alone leave more than half the smoke inside.
- Indoor PM2.5 stays at 55% to 60% of outdoor levels with windows shut.
- A high-efficiency HVAC filter with the fan on can cut it by about 50%.
Source: U.S. EPA, Strategies to Reduce Exposure Indoors
- Fire is a leading source of fine particles in the U.S.
- Wildland fire produces 44% of primary PM2.5 emissions nationwide.
- Wildfires alone produce 29%.
Source: U.S. EPA final rule on particulate matter standards, Federal Register (2024)
- Neighborhood smoke data has doubled since 2020.
- The AirNow Fire and Smoke Map now uses roughly 15,000 corrected PM2.5 sensors.
- That is up from roughly 7,500 at its 2020 launch.
Source: U.S. EPA, AirNow Fire and Smoke Map sensor briefing (2024)
Final Thought & Opinion
Your phone shows the outdoor AQI. Your home sets the indoor one.
Our take:
- Act at Orange. Red is too late to start.
- Fit equals rating. Air that goes around a filter never gets filtered.
- Measure the slot. Two minutes finds the gap the label hides.
- Stock up early. The EPA warns filters get scarce once smoke hits.
Start with the tape measure this week, while the sky is still clear.
Frequently Asked Questions
What AQI Level Is Unhealthy During Wildfire Smoke?
AQI 101 (Orange) for sensitive groups and 151 (Red) for everyone. We recommend acting at 101.
Is a MERV 13 Filter Good Enough for Wildfire Smoke?
Yes. The EPA recommends MERV 13 or higher for smoke.
Can a Small Gap Around My Air Filter Let Smoke In?
Yes. In our experience, a 0.25-inch gap is enough to create an air bypass. Measure the slot and buy the exact size.
Why Does My Phone’s AQI Differ From the AirNow Fire and Smoke Map?
Many apps use a distant monitor or report ozone. The map uses sensors closer to your address.
How Often Should I Replace My Air Filter During Wildfire Smoke?
When it looks dark brown or black. In heavy smoke, check it every day or two.
Should I Run My HVAC Fan All the Time During Wildfire Smoke?
Yes. The EPA recommends switching the thermostat fan from Auto to On while the system recirculates.
Know What Each AQI Color Means, Then Take Indoor Action at Orange
Make Orange your signal to act. Measure your filter slot today, then browse our air filter guides by size and MERV rating to have a snug MERV 13 ready.
Glossary
Air Quality Index (AQI): The EPA’s 0-to-500 outdoor air quality scale with six color categories.
PM2.5: Fine particles 2.5 microns or smaller; the main pollutant in smoke.
AirNow Fire and Smoke Map: An EPA and U.S. Forest Service map of PM2.5, fires, and smoke plumes.
MERV rating: The ASHRAE Standard 52.2 filter rating; higher numbers capture smaller particles.
Air bypass: Air that flows around a filter instead of through it.
Cleaner air room: The EPA term for a closed, filtered room for sheltering from smoke.