At what AQI should I close my windows?

Close windows when AQI reaches 101 (Unhealthy for Sensitive Groups) or higher. Between AQI 51-100, sensitive groups should limit open-window time.

When should I run an air purifier?

Run a HEPA air purifier when AQI exceeds 100. At AQI 151 or higher, run it continuously on the highest setting and seal obvious drafts.

At what AQI should I stay indoors?

Stay indoors as much as possible when AQI reaches 151 or higher. If you must go outside, wear an N95 respirator.

What To Do Indoors When AQI Is High

When AQI rises above 100, close your windows, run a HEPA air purifier in your main living area and bedroom, and limit outdoor exposure. At AQI 151 or higher, stay indoors as much as possible, seal drafts, and wear an N95 if you must go outside. This guide breaks down exactly what to do at each AQI level.

AQI action steps by level

These recommendations follow EPA AQI categories and apply to any U.S. city. Use your local city air quality page or ZIP code page to check current conditions.

AQI 0-50 (Good)

Windows: Open freely for fresh air.
Air purifier: Not needed for most homes.
Indoor activity: Normal activity is fine.

AQI 51-100 (Moderate)

Windows: Sensitive groups (asthma, elderly, children) should limit open-window time.
Air purifier: Use fans or filtration if dust or pollen is bothering you.
Indoor activity: Most people can stay active indoors.

AQI 101-150 (Unhealthy for Sensitive Groups)

Windows: Keep windows closed.
Air purifier: Run a HEPA purifier in the main room and bedroom.
Indoor activity: Sensitive groups should reduce outdoor exposure.

AQI 151+ (Unhealthy)

Windows: Keep windows and doors closed. Seal obvious drafts with weatherstripping or damp towels.
Air purifier: Run HEPA continuously on the highest setting.
Indoor activity: Stay indoors as much as possible. Use an N95 mask if you must go outside.

How to check your local AQI

You don't need to guess. Clean Air Data provides three ways to check:

City page: Each city report shows annual PM2.5, unhealthy days per year, and links to live AQI modules. For example, see Yuma, AZ or Tucson, AZ.
ZIP code page: If you want neighborhood-level detail, check your ZIP code page for real-time AQI and the nearest EPA monitor distance.
Live forecast: Every city and ZIP page includes a 3-day AQI forecast from the World Air Quality Index project.

How long to keep purifiers running

Once AQI drops back below 100, you can reduce purifier use. But don't turn it off immediately — indoor PM2.5 can lag outdoor readings by several hours, especially if your home has poor sealing. A good rule: run the purifier for at least 2-3 hours after outdoor AQI returns to the Moderate range.

Why this matters for long-term health

Short-term AQI spikes are one thing, but repeated exposure to elevated PM2.5 is linked to cardiovascular and respiratory problems. The Clean Air Score methodology tracks this through two components: annual PM2.5 average (40% weight) and unhealthy days per year (25% weight). If your city regularly hits AQI 101+, that shows up in both components.

For cities with chronic high-AQI days, indoor air quality becomes a daily concern, not just an occasional one. See our city-specific indoor air guides for guidance tailored to your area's pollution patterns — wildfire smoke in the West, traffic pollution in dense metros, or seasonal dust in the Southwest.