AQI Explained: What the Air Quality Index Means for You

What Is the AQI and Why It Matters

The AQI, or air quality index, is a standardized scale used by the U.S. Environmental Protection Agency (EPA) to communicate how clean or polluted the air is. It runs from 0 to 500: the higher the number, the greater the health concern. The index translates pollutant concentrations—ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide—into a single, easy-to-read value. On a summer day in Phoenix, for example, ozone can push the AQI into the 120s, while a wildfire smoke event in Seattle can spike PM2.5 readings above 200.

The scale is color-coded, and you will see it on weather apps, local news, and government monitoring sites. Understanding it helps you plan outdoor exercise, protect children and seniors, and decide when to close windows or run an air purifier.

AQI Levels: The Six Color Categories

The EPA defines six categories. Each has a specific color and health message.

• Good (0–50, green): Air quality is satisfactory; little or no risk.
• Moderate (51–100, yellow): Acceptable, but unusually sensitive people may feel effects.
• Unhealthy for Sensitive Groups (101–150, orange): Children, older adults, and people with heart or lung disease should limit prolonged outdoor exertion.
• Unhealthy (151–200, red): Everyone may begin to experience health effects; sensitive groups should avoid outdoor exertion.
• Very Unhealthy (201–300, purple): Health alert; everyone should avoid outdoor exertion.
• Hazardous (301–500, maroon): Emergency conditions; everyone should stay indoors.

These AQI levels are not linear in risk. A jump from 50 to 100 is less concerning than a jump from 150 to 200, because the health effects accelerate at higher concentrations. During the 2023 Canadian wildfire smoke episodes, New York City reached an AQI above 200, prompting health advisories and event cancellations.

How to Check AQI Today

To find AQI today, you can use official and independent sources. The EPA's AirNow website provides current readings from thousands of monitors across the United States. Many state environmental agencies also publish hourly data. For a global view, the World Air Quality Index project aggregates data from government and crowd-sourced sensors.

When you check, look at the dominant pollutant and the time stamp. Ozone peaks in the afternoon on hot, sunny days, while PM2.5 from wildfire smoke can be highest overnight or in the early morning. A single reading is a snapshot; trends over several hours are more useful for planning.

Reading an AQI Map

An AQI map shows color-coded readings across a region, often with dots or shaded areas. This helps you see whether pollution is local or regional. For example, a plume of smoke from a fire in Northern California may appear as a band of orange and red moving eastward. On a map, you can also spot urban heat islands or industrial corridors with consistently higher values.

Maps update at different intervals. Some government monitors report hourly, while satellite-derived estimates may update less frequently. Always check the legend and the timestamp to avoid misinterpreting a stale reading.

Tools to Monitor Air Quality and Weather

Several platforms help you track air quality alongside weather and alerts. Here are some reliable options:

• AirNow (EPA): Official U.S. government site with current AQI, forecasts, and interactive maps.
• IQAir: Global platform with real-time data from government and private sensors, plus historical trends.
• PurpleAir: Community-driven network of low-cost sensors; useful for hyperlocal readings, though data quality varies.
• Contingencias: A weather and alerts app that includes real-time air quality, a live weather map with radar and satellite imagery (NOAA GOES-19), severe storm detection with possible hail, and global earthquake alerts. You can explore Contingencias' live weather and air quality features on iOS and Android.

Each tool has strengths. Government monitors are calibrated and reliable but may be sparse in rural areas. Low-cost sensors fill gaps but can drift over time. Using more than one source gives a more complete picture.

Practical Tips for High AQI Days

When the index climbs, simple actions reduce exposure:

• Check the AQI before outdoor exercise; shift workouts indoors or to early morning when ozone is lower.
• Keep windows closed during smoke events and use a high-efficiency particulate air (HEPA) filter if available.
• Follow local health advisories, especially if you have asthma, heart disease, or are pregnant.
• Use a mask (N95 or equivalent) if you must be outside in unhealthy conditions.

Remember that indoor air can also be affected. Cooking, candles, and poor ventilation contribute to indoor pollution. A portable air cleaner in the bedroom can make a difference during multi-day episodes.

Frequently Asked Questions

What does AQI stand for?

AQI stands for Air Quality Index, a scale from 0 to 500 that indicates how polluted the air is and what health effects may be of concern.

What are the main AQI levels?

The six categories are Good (0–50), Moderate (51–100), Unhealthy for Sensitive Groups (101–150), Unhealthy (151–200), Very Unhealthy (201–300), and Hazardous (301–500).

Where can I see an AQI map?

You can view AQI maps on the EPA's AirNow website, IQAir, and some weather apps that include air quality layers, such as Contingencias.

How can I check AQI today?

Check AirNow.gov or a trusted weather app for current readings. Look at the dominant pollutant and the timestamp, and compare with nearby monitors for context.