Air Stagnation Advisory or Air Quality Action Day: what to check before going outside

Four products · Four questions

Start with the product that matches the decision

Current AQI

What is the reported outdoor air quality now? AirNow presents the AQI as a color, category, and number; higher values mean greater pollution and health concern.

AQI forecast

What air-quality category is expected today or tomorrow? It is a prediction, so compare it with the latest current AQI before changing an outdoor plan.

Air Quality Action Day or Air Quality Alert

What health or pollution-reduction action is a local agency asking people to take? Names and trigger levels vary by community.

Air Stagnation Advisory

Are weak winds and a stagnant weather pattern expected to allow pollution, smoke, dust, or industrial gases to build near the ground?

The key distinction is simple: the AQI measures or forecasts pollution, while an Air Stagnation Advisory describes meteorology that can make buildup more likely. A stagnation advisory is not proof that every monitor is already reporting unhealthy air, and a green current AQI does not guarantee that a later forecast will remain green.

Outlook, forecast, advisory, watch, warning

Do not promote one label into another

Outlook

A broad planning signal that a pattern may support poor air quality later. It is not a local observation, an AQI value, or permission to ignore later updates.

Forecast

A prediction for a location and period. An AQI forecast can help schedule outdoor work, exercise, school activities, or errands before conditions change.

Advisory

An Air Stagnation Advisory is a weather message about the potential for pollutants to accumulate. Read its area, timing, and local instructions.

Action Day or Air Quality Alert

A local health-action message tied to expected or observed pollution. EPA notes that agencies issue these at different AQI levels, so the attached instructions matter more than the nickname.

Watch or warning

An AQI color, Action Day, and Air Stagnation Advisory are not automatically an NWS watch or warning. Smoke, dust, fire, or other hazards may have separate official products; follow the exact alert issued for your location.

Read the number and the pollutant

The AQI color is a shortcut, not the whole message

EPA's AQI has six categories: Good (0–50), Moderate (51–100), Unhealthy for Sensitive Groups (101–150), Unhealthy (151–200), Very Unhealthy (201–300), and Hazardous (301 and higher). Above 100, air quality becomes unhealthy first for sensitive groups and then for everyone as the value rises.

• Check the leading pollutant. The people most at risk and the useful precautions can differ for ozone and particle pollution.
• Check the time. A current reading and a forecast for later answer different questions. Recheck near the activity.
• Check the place. The nearest useful reading may not represent every neighborhood, valley, roadway, or smoke plume.
• Use EPA activity guidance. At unhealthy levels, reduce exposure in the way the official message recommends—often by shortening or reducing heavy outdoor exertion, especially for the named sensitive group.

“Hazardous” is the name of EPA's highest AQI category. It does not mean an NWS warning has automatically been issued. Treat the category's health guidance seriously and separately check emergency and weather alerts.

Why calm weather matters

Weak winds can keep pollution close to where people breathe

NWS defines an Air Stagnation Advisory as a product issued when major buildups of air pollution, smoke, dust, or industrial gases are expected near the ground for a period of time. It usually develops beneath stagnant high pressure, when weak winds cannot bring in fresher air.

That weather setup can last longer than a short-lived pollution spike, but local emissions, terrain, temperature, sunlight, and incoming smoke still influence the pollutant and AQI. The advisory therefore belongs beside—not in place of—the current AQI, AQI forecast, and local agency message.

A five-minute outdoor-plan check

Use observations, forecasts, and alerts in that order

1. Enter the actual location in AirNow. Read the current AQI, category, time, and leading pollutant.
2. Read today's and tomorrow's AQI forecast. Plan strenuous or longer activity for the cleaner period when official guidance supports that choice.
3. Open any Action Day or Air Quality Alert. Follow the instructions for the pollutant and the named sensitive groups.
4. Check the NWS forecast and active alerts. An Air Stagnation Advisory explains the weather setup; smoke, dust, heat, fire, or fog may require additional decisions.
5. Recheck before and during the activity. Stop relying on an earlier screenshot if the smell, visibility, wind, or official message changes.

People with heart or lung disease, older adults, children, and people who are active outdoors may be included in a sensitive group depending on the pollutant. Use the specific AirNow guidance for the reported pollutant and follow advice from your health-care professional for personal medical needs.

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