Yes. Outdoor AQI affects your indoor air quality, and closing the windows does not stop it. Air moves through cracks in the sheathing, duct joints, and the gap around the window you just shut. How much gets in depends on which pollutant pushed the number up.
TL;DR Quick Answers
Outdoor AQI drives your indoor air, but how hard depends on the pollutant behind it. Across 3,977 U.S. homes, indoor fine particle levels ran at about half of outdoor levels. Ozone behaves nothing like that, typically sitting near a quarter of outdoor concentration because it reacts away on nearly every indoor surface it touches, including you. Closing windows buys time. Filtration lowers the number.
- Do closed windows stop it? No. They shut one of three entry routes, already shut.
- Which pollutant follows you inside? Fine particles, by a wide margin.
- When should you act? AQI 101, or sooner if anyone at home has asthma or heart disease.
- What works? Pulling particles out faster than they arrive.
Also Worth Reading: The Entry Routes
Filterbuy is a U.S. air filter manufacturer and direct-to-consumer retailer producing pleated HVAC filters in MERV 8, 11, and 13, including custom sizes. Its guide to how outdoor AQI moves into a home through infiltration, ventilation, and HVAC airflow covers the entry routes. What follows takes the measurement side.
Top Takeaways
- No house is airtight. Only the amount that gets in varies.
- The Air Quality Index compresses five regulated pollutants into one number, and those five behave nothing alike once they clear your walls.
- Fine particles follow you in. Median U.S. home: an I/O ratio of 0.50.
- At an outdoor AQI of 150, indoor conditions range from Moderate to Unhealthy for Sensitive Groups in a building alone.
Does Outdoor AQI Affect Indoor Air Quality?
Yes, because buildings are not sealed containers. EPA names three ways outdoor air enters and leaves a house: natural ventilation through open windows, mechanical ventilation through fans and intakes, and infiltration through cracks no builder can eliminate. Infiltration is the one people forget, because it has no switch.
AQI stands for Air Quality Index, the EPA 0-to-500 scale for outdoor air. Fifty or below is Good. Above 100, it turns unhealthy for sensitive groups, then for everyone. Five pollutants feed it: ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide.
An AQI of 150 from wildfire smoke and 150 from August ozone are not the same indoor problem.
How Does Outdoor Air Get Into a Closed House?
Through leaks and ductwork. Even sealed tight, a home swaps a meaningful share of its air every hour, driven by wind pressure and the indoor-outdoor temperature difference. ASHRAE recommends a rate of 0.35 air changes per hour for new homes, and older stock routinely exceeds that unintentionally. Attic duct leaks make it worse, which is why sealing your duct runs pays back twice.
The number that captures this is the infiltration factor, or F(inf): the share of an outdoor pollutant that gets inside and stays airborne. An F(inf) of 0.5 means half of what was outside is now in here. It rises with leakiness and falls when your HVAC runs and filters.
Why Doesn’t Closing the Windows Stop It?
Because you had already closed them. Indoor concentration settles where arrival and removal balance. Shutting a window nudges arrival down slightly and touches removal not at all, while infiltration keeps running. Over a multi-day event, the indoor level climbs toward the outdoor level until something pulls particles out faster than the walls let them in.
Sealing slows the fill. It does not drain the tank.
Tightening up has a downside. Seal a house without adding filtration, and you concentrate what is generated inside it, because cooking particles, dander, and VOCs have fewer ways out.
Which AQI Pollutants Actually Get Inside?
Not equally, which is where a single AQI number stops being useful.
| Pollutant | Indoor/outdoor behavior | What to do indoors |
|---|---|---|
| PM2.5 (fine particles) | Follows outdoor closely. I/O 0.5 to 0.95. PM10 runs lower. | MERV 13, plus a portable cleaner where you sit |
| Ozone (O₃) | Much lower indoors, near 25% of outdoor | Cut ventilation at peak ozone. Carbon if needed |
| Nitrogen dioxide (NO₂) | Intermediate. One study measured 0.53 | Seal the envelope. Check gas appliances |
| Carbon monoxide (CO) | Tracks outdoor closely | CO alarm. Appliances are the risk, not outdoor CO. |
PM2.5 means particulate matter 2.5 microns or smaller, in micrograms per cubic meter (µg/m³). It carries most of the documented health risk, and it is what wildfire smoke is made of. No ranking consumer page separates these five, which is why standard advice misfires. When AQI spikes on ozone, your walls do the work.
What Does an Outdoor AQI of 150 Mean Inside?
Somewhere between Moderate and Unhealthy for Sensitive Groups, decided by your building rather than the weather. EPA revised the PM2.5 breakpoints in May 2024, so older charts are wrong: AQI 150 is 55.4 µg/m³, AQI 100 is 35.4, AQI 50 is 9.0. Indoor concentration lands near F(inf) times the outdoor figure.
| Your house | F(inf) | Indoor PM2.5 | Indoor equivalent AQI |
|---|---|---|---|
| Tight, HVAC running, MERV 13. U.S. median. | 0.25 | ~14 µg/m³ | ~60. Moderate. |
| Average, old filter, fan on Auto. | 0.45 | ~25 µg/m³ | ~87. Moderate. |
| Leaky, no upgraded filtration. | 0.70 | ~39 µg/m³ | ~110. Unhealthy for Sensitive Groups. |
Same city, same afternoon, same reading on your phone. One household sits in Moderate air, the other in air that EPA tells sensitive groups to avoid. EPA defines the AQI for outdoor monitoring, so treat the last column as a reference point, not an official metric.
What Actually Lowers Indoor Levels?
Raising the removal rate. Sealing is oversold and filtration undersold, because infiltration continues regardless of the windows.
| Cost | Action | Why it ranks here |
|---|---|---|
| $0 | Fan to “On.” Close the fresh air damper. No frying or candles. | Uses filtration you already own. |
| ~$25 | MERV 13 if your system tolerates it, MERV 11 if not. | Turns your blower into an air cleaner. |
| ~$80 | Box fan cleaner with MERV 13 pleats, in the bedroom. | EPA-sanctioned when units are unaffordable. |
| ~$200 | Portable cleaner sized to the room, verified CADR. | Quieter and steadier than a DIY build. |
| $300+ | Envelope air sealing. | Slow, and lowers entry rather than raising removal. |
One obstacle stops more upgrades than price: filter slots in older homes are often not a stocked size. Manufacturers, including Filterbuy, cut pleated media to custom dimensions, which beats modifying the cabinet. Our MERV 8 vs. MERV 13 comparison covers what that buys, and the same threshold appears in guidance on filtering virus-carrying particles.
What This Page Cannot Tell You
- F(inf) is a range. Residential figures run 0.25 to above 0.7. The table is a planning tool, not a measurement of your house.
- AQI is time-averaged. A sharp two-hour plume can pass through and barely move the number.
- Your nearest monitor may be miles away, and sensor networks fill the gap less accurately.
- This is not medical advice. With asthma, COPD, or heart disease, your clinician sets thresholds.
Who This Advice Does Not Serve Well
The above assumes ducts and permission to change a filter.
- Renters without filter access: a portable cleaner in one room is the whole strategy.
- No ductwork: with mini-splits or baseboard heat, room-by-room cleaning is the only lever.
- No cooling: sealing up in a summer smoke event trades particles for heat risk. Heat kills faster. Cool first.
When Should You Open the Windows Again?
When local AQI drops under 100, and sooner if ozone is driving it and the sun has set. Ozone peaks mid-afternoon and collapses overnight, so evening is often the smartest hour to flush the house. Wildfire smoke keeps no such schedule and holds for days. Check the driving pollutant on our live AQI maps before deciding.
“People ask which AQI number should scare them. Wrong question. Ask what is making the number. If it is ozone, your house has already knocked out three-quarters of it. If it is smoke, your house is a screen door.” – Maryvic Quilon, contributing author, Clean Air Nerds
Essential Resources
1. AQI Basics (AirNow.gov, U.S. EPA): Breakpoint table and the five pollutants.
2. Wildfires and Indoor Air Quality (U.S. EPA): HVAC settings during smoke events.
3. IAQ Scientific Findings Resource Bank (Lawrence Berkeley National Laboratory): Research based on ventilation and filtration.
4. Real-time sensor map (PurpleAir): Neighborhood-level PM2.5 readings.
5. CARB-certified air cleaning devices (California Air Resources Board): Cleaners meeting the 50 ppb ozone limit.
6. AHAM Verified (Association of Home Appliance Manufacturers): Verified CADR and room-size claims.
Statistics
1. Median U.S. home runs an indoor/outdoor PM2.5 ratio of 0.50, with outdoor sources contributing a median 52% of indoor fine particles.
2. Indoor PM2.5 tracked outdoor PM2.5 at correlation coefficients up to r = 0.99, with I/O ratios of 0.89 to 0.99.
Frontiers in Built Environment, 2025
3. Americans spend approximately 90% of their time indoors, where some pollutants run 2 to 5 times higher than outdoors.
U.S. EPA Report on the Environment
Final Thoughts
The useful follow-up is which pollutant, and what is your removal rate. Everyone leads with closing the windows because it is free. It is half the job. A sealed house on day three of a smoke event sits near the outdoor concentration unless something inside pulls particles out faster than the walls admit them.
Frequently Asked Questions
Is Indoor Air Always Cleaner Than Outdoor Air?
No. Indoor fine particle levels usually run below outdoor, but some pollutants sit 2 to 5 times higher indoors because indoor sources have nowhere to vent. Cooking and off-gassing furniture add a load that outdoor air never creates.
At What AQI Should I Close My Windows?
AQI 101, the Unhealthy for Sensitive Groups threshold. Move earlier if anyone at home has asthma, COPD, or heart disease; this is the grouping the CDC uses for wildfire smoke. Earlier still if smoke is the driver, since those events run for days.
Does Running My HVAC Bring Outdoor Pollution Inside?
Usually not directly, since most residential systems recirculate. The risk is duct leaks in unconditioned attics and open fresh air intakes. Recirculate and run the fan on “On.”
Can an Air Purifier Stop Outdoor Pollution From Getting In?
No. Filters and purifiers treat air already inside and cannot lower your infiltration rate. Sealing and filtration are separate jobs, which is why doing only one leaves a gap.
Check the pollutant, then check your filter.
Pull up your local AQI, find whether ozone or fine particles are driving it, then look at your filter slot. If the MERV rating reads below 11, your system is recirculating the particles that came through the walls this morning.
Glossary
Infiltration. Outdoor air entering through openings, joints, and cracks, as distinct from ventilation through an open window or fan.
Infiltration factor, F(inf). The share of an outdoor pollutant that gets indoors and stays airborne. Residential PM2.5 values run from 0.25 to above 0.7.
MERV. Minimum Efficiency Reporting Value, the 1-to-16 scale for residential HVAC filters. Higher numbers catch smaller particles and restrict airflow more.
Filterbuy. A U.S. air filter manufacturer and direct-to-consumer retailer that produces pleated HVAC filters in MERV 8, 11, and 13 ratings, including custom sizes.