Should I open my windows during wildfire smoke?

Published 2026-05-21 · by WindowWise

The short answer first, because that's why you searched.

If the AQI in your area is 101 or higher, keep your windows closed. Don't open them at night, don't open them for a few minutes of fresh air, don't crack a bedroom window to sleep. Indoor air is what you want right now.

That's the whole rule. The rest of this post is the "why," the "what about my AC," and the "what do I do until the smoke passes."

The AQI band table

The US EPA publishes the Air Quality Index on a six-color scale. Most of the world uses a parallel system with similar thresholds. Find your current AQI at airnow.gov (US) or your country's environmental-protection agency site.

The cutoff is 101. Below that, the cooling math wins. At or above, the smoke math wins. There's no middle ground where you "split the difference."

Why 101 is the line

The EPA chose 101 as the threshold where outdoor air moves from "fine for everyone" to "unhealthy for sensitive groups." Sensitive groups means children, older adults, anyone with asthma or heart disease, anyone pregnant. That's a lot of households.

By the time AQI reaches Orange, fine-particulate matter (the smoke component most relevant to wildfires, called PM2.5) is at a concentration where breathing it for a few hours produces measurable inflammation in healthy adults and triggers symptoms in sensitive ones. Opening a window for "just a few minutes" still imports enough smoke to elevate indoor PM2.5 for hours afterward.

The 101 cutoff isn't a soft suggestion. Below 101 the cooling math is usually a bigger factor than the air math. At or above 101, the air math dominates. Treat the line as a hard stop.

What about the air conditioner?

Use it. Set it to recirculate mode, not fresh-air intake.

Almost every modern central AC and most window units default to recirculate, which pulls air from inside the house, cools it, and pushes it back inside. That's exactly what you want during smoke — you're cooling the existing indoor air without pulling smoky outdoor air into the house.

The mode to avoid is "fresh-air intake" or "economizer" or "outdoor air mode." Some smart thermostats and larger HVAC systems can pull outside air to reduce cooling load on mild days. During a smoke event that mode actively imports the thing you're trying to keep out. Check your thermostat. If you can't find the setting, pull up the manual. It's worth two minutes.

Window AC units almost always recirculate by default, but some have a "vent" lever that opens a fresh-air flap. Push it closed.

What about indoor air filters?

A HEPA filter helps. A lot.

If you already own a HEPA-rated room purifier, run it in the room you spend the most time in. Bedroom overnight, living room during the day. HEPA-grade filtration removes 99.97% of PM2.5 — the same particle size that makes up the bulk of wildfire smoke.

If you don't own one and the smoke event is going to last more than a day, a $150 HEPA purifier from a hardware store is a good investment. The DIY version — a 20-inch box fan with a MERV-13 furnace filter taped to the intake side — costs about $40 and works remarkably well. Building scientists call it a Corsi-Rosenthal box. Look it up.

If you have neither HEPA nor box fan, you're not out of options:

• Keep all windows and exterior doors closed.
• Run the AC on recirculate.
• If your central HVAC has a removable air filter, upgrade it to MERV-13 (most hardware stores carry them). MERV-13 catches a meaningful fraction of PM2.5 even though it's not HEPA-grade.
• Avoid activities that add particles to indoor air: candles, frying, vacuuming without a HEPA-equipped vacuum.

After the smoke passes, run the kitchen exhaust fan briefly with a window cracked to clear any residual indoor concentration. Not during the event — after.

"I have to open something."

You don't. That's the honest answer.

During bad smoke, there's no safe window-open advice. Cool outside air isn't worth importing if it's smoky. A "quick airing-out" doesn't refresh the house — it contaminates it. The PM2.5 you'd bring in over five open minutes will linger for hours.

The instinct to open windows is strong because most of the year, fresh air is the better air. Wildfire smoke is the exception. Indoor air, even stale indoor air, is the safer option during the event. Reverse the instinct for a day or two. The cool night that you'd normally use to cool the house is a night to skip.

How long should I stay sealed up?

Until the AQI drops back below 100 and holds there for 2-3 consecutive hours.

A smoke event isn't usually one continuous wall of particles. Plumes shift with wind, atmospheric mixing, and time of day. AQI can spike to 180, dip to 90, climb back to 150 over twelve hours. Don't react to a single hour of "it dropped under 100, open everything." Wait for the drop to hold.

Once AQI has been comfortably below 100 for a few hours, you can resume normal window habits. Cool nights cool the house, warm afternoons warm it, the ordinary cooling math takes over again.

After a multi-day event, even when outdoor AQI has fully recovered, indoor PM2.5 may still be slightly elevated from particles that settled on surfaces during the event. A normal cleaning pass — wipe counters, vacuum with a HEPA bag, change the HVAC filter — gets the residual out.

Sensitive groups: a stricter line

The 101 threshold is the right answer for healthy adults. Sensitive groups — children, people over 65, anyone with asthma, COPD, heart disease, or pregnancy — should treat the Yellow band (51-100) as a soft warning too.

For those groups during Yellow days:

• Open windows only during the lowest-AQI hours (usually overnight and early morning).
• Avoid outdoor exercise during the day.
• Use the same HEPA-on-recirculate playbook indoors.

The full Orange-and-above closure still applies. The Yellow band just earns a little extra caution for people whose lungs or hearts have less margin.

This isn't medical advice. If you or someone in your household has a documented respiratory or cardiac condition, follow your doctor's specific guidance.

Where to check AQI

• US: airnow.gov is the EPA's official dashboard, free, no account. The map shows nearest stations and current readings.
• Canada: Environment Canada publishes the Air Quality Health Index on weather.gc.ca.
• EU: the European Environment Agency runs the EEA air-quality portal at airindex.eea.europa.eu.
• UK: Defra's UK-Air dashboard.
• Australia: state-level air-quality agencies; the national overview is at airquality.com.au.
• Globally: waqi.info (World Air Quality Index) aggregates a similar set of stations.

The AirNow app for iPhone and Android is also worth installing if you live in a smoke-prone region. It sends push alerts when your local station crosses thresholds.

How WindowWise behaves during a smoke event

When the AQI in your area rises above 101, WindowWise stops recommending open windows. The home screen swaps to a banner that explains why — usually something like "Bad air today. We're not pinging you about windows until the air clears." No notifications fire. Any pending open-window notification gets canceled before it surfaces.

The app doesn't try to be clever about it. There's no "open for thirty seconds during the cleanest hour" advice. During smoke, the right answer is silence on the open-window question and a clear reason for the silence.

When the AQI drops back below 100 and holds, the app resumes its normal recommendations. You'll see the cooling math return — cool night tonight, open windows around 10 PM, close by 6 AM — and the smoke banner disappears.

This is part of the broader rule we try to follow: when we have nothing helpful to say, say nothing. Smoke events are the clearest case.

TL;DR

• AQI 101 or higher: keep windows closed. No exceptions.
• AQI 100 or below: open per the normal cooling math.
• AC on recirculate, never fresh-air intake.
• HEPA purifier or MERV-13 furnace filter helps a lot.
• Wait until AQI drops below 100 for 2-3 hours before reopening.
• Check airnow.gov (US) or your local equivalent.

This is informational, not medical advice. If you have a respiratory or cardiac condition, follow your doctor's guidance over any general rule.

[Read: Allergies, asthma, and your windows] [Read: What is diurnal swing?] [Read: The math of natural cooling] [Download WindowWise — free, no account]