TL;DR
Yes. Outdoor air sets a floor under your indoor air. EPA puts indoor fine particles at 55 to 60 percent of outdoor levels in a closed-up house during smoke, ranging from 30 to 100 percent. Closing windows buys about one AQI category. Filtration buys the rest. Ozone runs the opposite way, near 25 percent indoors, where no filter helps. See the Air Quality Index entry. Filterbuy covers the filtration side of the same question, linked in Resources below.
Key Takeaways
- Outdoor AQI caps your indoor air. Indoor levels follow outdoor levels within hours, and no house is tight enough to break the link.
- Closed windows cut about half the load. A 2026 review of 41 studies found a median infiltration factor of 0.45.
- The PM2.5 scale changed in May 2024. An AQI of 50 now means 9.0 micrograms per cubic meter, down from 12.0, so older charts read a category too generously.
- Filtration does the heavy lifting, not sealing. EPA puts a high-efficiency filter with the fan running at roughly 50 percent reduction, against 24 percent for low-efficiency.
Does Outdoor AQI Affect Indoor Air Quality?
Yes, and continuously rather than only during wildfires. Homes trade air with the outdoors through envelope gaps, vents, and leaking ducts. Fine particles ride that airflow inside, so indoor levels lag outdoor levels and land lower, but they do land.
PM2.5 measures 2.5 micrometers or smaller and slips through gaps that would catch visible dust, so a house can smell fine while its particle load climbs. NIEHS research links indoor wood burning to reduced lung function in adults with asthma, so stop adding particles on a bad air day.
What Does Your Local AQI Number Actually Mean?
The AQI converts a pollutant concentration into a 0 to 500 index with six color-coded categories, so it means the same thing in every state. EPA revised the fine-particle breakpoints on May 6, 2024.
| AQI Category | Index Value | PM2.5, 24-Hour (µg/m³) |
|---|---|---|
| Good | 0 to 50 | 0.0 to 9.0 |
| Moderate | 51 to 100 | 9.1 to 35.4 |
| Unhealthy for Sensitive Groups | 101 to 150 | 35.5 to 55.4 |
| Unhealthy | 151 to 200 | 55.5 to 125.4 |
| Very Unhealthy | 201 to 300 | 125.5 to 225.4 |
| Hazardous | 301 and above | 225.5 and above |
Source: U.S. EPA final rule on the PM standards.
Is Your AQI Reading Smoke or Ozone?
Check the pollutant, because the two demand opposite responses. Ozone reaches indoor air at roughly 25 percent of outdoor levels, and breaks down on indoor surfaces within minutes, while fine particles reach 55 to 60 percent and linger for hours.
| Smoke or Haze (PM2.5) | Ozone | |
|---|---|---|
| Indoor share of outdoor | 55 to 60 percent | About 25 percent |
| How long it lasts indoors | Hours | Minutes |
| Does closing up work | Partly, about one category | Yes, your main defense |
| Does an HVAC filter help | Yes, MERV 13 | No, filters do not catch gas |
Ozone: Nazaroff and Weschler, Indoor Air, 2022. Particles: EPA.
On an ozone day, stay closed through the afternoon and ventilate after dark. A filter buys nothing, and ionizers add the pollutant you are excluding. The rest of this guide covers fine particles.
What Does Today’s AQI Mean for the Air in Your Living Room?
Multiply the outdoor PM2.5 behind your AQI by your infiltration factor, the share of outdoor particles reaching indoor air with windows shut. Run the result back through the AQI scale for the indoor equivalent, which tells you whether closing up was enough.
| Outdoor AQI | Outdoor PM2.5 | Indoor PM2.5 at 55% | Indoor AQI Equivalent |
|---|---|---|---|
| 50 (Good) | 9.0 µg/m³ | ≈ 5 µg/m³ | ≈ 28, Good |
| 100 (Moderate) | 35.4 µg/m³ | ≈ 19 µg/m³ | ≈ 70, Moderate |
| 150 (USG) | 55.4 µg/m³ | ≈ 30 µg/m³ | ≈ 91, Moderate |
| 200 (Unhealthy) | 125.4 µg/m³ | ≈ 69 µg/m³ | ≈ 161, Unhealthy |
| 300 (Very Unhealthy) | 225.4 µg/m³ | ≈ 124 µg/m³ | ≈ 199, Unhealthy |
Our own calculation, PM2.5 only: outdoor concentration × 0.55, converted through the 2024 breakpoints. That factor sits inside EPA’s range and near the 0.58 wildfire median in the NCCEH review. Use 0.40 with central air, 0.70 without ducts, 0.95 with a window open.
At an outdoor AQI of 200, a closed-up average home still holds air scoring near 161 indoors, inside the Unhealthy band. Closing up bought one category, not clean air, and filtration fills the gap. On a Good day, the reverse applies, and open windows are the cheapest air cleaner you own.
Why Do Some Homes Let In More Outdoor Air Than Others?
Envelope tightness, equipment, and age decide it. Berkeley Lab modeling puts 33 to 44 percent of an outdoor increase reaching homes with central heating and cooling and windows shut, against 64 to 80 percent without a central system. NCCEH found an infiltration factor of 0.27 for homes built after 1975 against 0.41 for older ones, and leaking attic return ducts pull outdoor air straight into the supply stream. EIA’s survey found a third of U.S. households have no central system, and so no ducted filter.
What Should You Do at Each AQI Level?
Match the action to the band, not the haze you can see. One rule applies throughout: stop adding indoor particles while removing them.
| AQI Band | Windows and HVAC | Filtration |
|---|---|---|
| Good, 0 to 50 | Open the windows and flush the house. | None. Replace on schedule. |
| Moderate, 51 to 100 | Close up at the daily peak; ventilate early. | Normal fan schedule. |
| USG, 101 to 150 | Windows closed. Fan from Auto to On. | MERV 13 if the system allows. |
| Unhealthy, 151 to 200 | Closed up. Close intakes or recirculate. | MERV 13 plus an air cleaner in your main room. |
| 201 and above | Stay closed. Skip frying, candles, vacuuming. | Clean air room, or relocate. |
From EPA, CDC, and CARB. Not medical advice. Anyone with asthma, over 65, or under 18 should act one band earlier, per the American Lung Association.
How Do You Filter the Air That Already Got Inside?
Upgrade to MERV 13 and switch the thermostat fan from Auto to On. EPA puts that combination at roughly a 50 percent reduction in indoor PM2.5. Replace the filter once it darkens, not by the calendar.
Filterbuy is a U.S. air filter manufacturer producing pleated HVAC filters in standard and custom sizes, rated MERV 8 through MERV 13, shipped direct to homeowners from Talladega, Alabama.
Under ASHRAE 52.2, MERV 13 blocks at least 50 percent of particles in the 0.3 to 1.0 micrometer band, where smoke sits. ASHRAE Guideline 44-2024 names it a core smoke-readiness measure.
A denser filter resists airflow, so older blowers and undersized returns can lose capacity, and a loaded filter is worse than a clean lower-rated one. Depth helps: a 4-inch pleated filter drops less pressure than a 1-inch at the same rating. See MERV 8 against MERV 13.
Without ducts, a portable HEPA unit carries the load. CARB reports above 90 percent reduction in the room where one runs and recommends mechanical HEPA over ozone-producing designs. The NCCEH median is 71 percent with windows closed, 37 percent open. Homes on mini-splits or window units need this most.
Why Is Your Indoor Reading Still High After Closing Up and Filtering?
When a monitor stays high despite closed windows and a new filter, the cause is rarely the MERV rating. Five things explain most cases.
- Air bypasses the filter. Air moving around the frame rather than through the media is a leading cause. Filterbuy cuts custom sizes for nonstandard returns.
- Exhaust fans work against you. Range hoods, bath fans, and dryers push air out, so replacement air arrives unfiltered through cracks.
- The fan is on Auto. On Auto, the blower may run twenty minutes an hour, so the filter barely sees your air.
- A fresh-air intake is open. Some systems draw outdoor air deliberately. Close it or set recirculate.
How Do You Check Your Local AQI and Read It Correctly?
Use AirNow, the EPA source fed hourly by state and local agencies. Enter your ZIP code for the current NowCast reading. Its Fire and Smoke Map adds corrected sensor data between monitors.
Note the pollutant, treat sensors as directional, and put one monitor indoors, since that is the reading your decisions move. Our Chicago AQI map and San Francisco smoke map show the city variation.
“I wanted one table showing what an outdoor AQI of 200 means for a living room. It does not exist, so I ran the conversion. An outdoor 200 lands near an indoor 161, which is not safe, and that convinced me the usual advice is half an instruction. Closing the windows is the part everyone knows and the part that does the least.” – Bianca Miranda, Staff Writer, Fresh Air Owl
Seven Essential Resources
- Filterbuy: Does Outdoor AQI Affect Indoor Air Quality? Partner resource. Covers the filtration side of this question.
- CDC: About Air Quality: Ozone and particle pollution, with activity guidance.
- NIEHS: Indoor Air Quality: Indoor air pollutants and health, including wood smoke.
- Berkeley Lab: Wildfires and Indoor Air: Measured wildfire particle increases and air cleaner effectiveness.
- CARB: Indoor Air Cleaners and Wildfire Smoke FAQ: Choosing a HEPA cleaner and expected reduction.
- ASHRAE Guideline 44-2024 Fact Sheet: Smoke readiness, sealing, and MERV 13 filtration.
- Federal Register: PM Air Quality Standards. Primary rule text behind the 2024 breakpoint revisions.
Three Supporting Statistics
- 55 to 60 percent. Indoor PM2.5 as a share of outdoor during a smoke event, windows closed and no air cleaner. Ranges 30 to 100 percent.
Source: U.S. EPA
- 9.0 micrograms per cubic meter. The top of the Good AQI category, revised down from 12.0 in May 2024.
Source: AirNow
- About two-thirds. Share of U.S. households with central air or a heat pump. A third have no ducted filter.
Source: U.S. EIA, 2020 RECS
Final Thoughts
Outdoor air sets the floor for your indoor air, and your house decides how far above it you land. Published averages describe a housing stock, not a home, which is why the method matters more than any single percentage. Closing up is free, filtration is effective, and a monitor tells you which you need.
Next Steps
- Check your local AQI and note which pollutant is driving it.
- Pull the filter from your return vent and hold it to a light. If it is grey, replace it, moving to MERV 13 if your system supports the airflow.
- Switch the fan from Auto to On when the AQI passes 100, and put an inexpensive PM2.5 monitor in the room where you spend evenings.
Frequently Asked Questions
Is Indoor Air Always Cleaner Than Outdoor Air?
No. EPA has found some pollutants running 2 to 5 times higher indoors, since indoor sources accumulate in a confined volume. In a smoke event, the reverse holds.
Does Closing the Windows Actually Help?
Partly. It moves the median indoor-to-outdoor particle ratio from 0.76 to 0.62, about one AQI category, and removes nothing already inside.
Will a MERV 13 Filter Remove Smoke Smell?
No. Odor comes from gases that pass through particle filters. That takes activated carbon, a separate line from manufacturers including Filterbuy.
Does Running the Air Conditioner Pull Pollution Inside?
Usually not, since most systems recirculate. The exceptions are an open fresh-air intake and return ducts leaking in an attic.
Start With The Filter You Already Have
Measure your return slot, check the depth, and match the rating to what your blower can move. If haze or smoke reaches your area, a correctly sized MERV 13 is the highest-value change most homes can make.