What the AQI number represents
The Air Quality Index is a standardized scale that converts measured concentrations of air pollutants into a single number from 0 to 500. The number is designed to be immediately interpretable without requiring knowledge of micrograms per cubic meter or parts per billion. Higher numbers mean worse air quality and greater health risk.
In the United States, the EPA calculates AQI for five criteria pollutants: ground-level ozone (O3), particulate matter (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2). The overall AQI for a location at any given time is the highest AQI value among all measured pollutants. The pollutant driving the highest value is called the 'responsible pollutant.'
The six color-coded categories
AQI values are grouped into six health concern categories, each with a color code that has become widely recognized. Green (0–50) means air quality is good and poses little or no risk. Yellow (51–100) is moderate — acceptable for most people, but sensitive individuals may experience mild effects. Orange (101–150) is unhealthy for sensitive groups including children, older adults, and people with respiratory or heart conditions.
Red (151–200) is unhealthy for everyone. Purple (201–300) is very unhealthy, meaning significant health effects are likely for all groups. Maroon (301–500) is hazardous — emergency conditions where the entire population is likely to be affected. Values above 500 are considered 'beyond the AQI' and indicate unprecedented pollution levels.
- Green (0–50): Good — air quality is satisfactory
- Yellow (51–100): Moderate — acceptable, some sensitive individuals may notice effects
- Orange (101–150): Unhealthy for Sensitive Groups — active children and adults, and people with respiratory disease should limit prolonged outdoor exertion
- Red (151–200): Unhealthy — everyone may begin to experience health effects
- Purple (201–300): Very Unhealthy — health alert, everyone may experience more serious effects
- Maroon (301–500): Hazardous — health emergency, entire population likely affected
PM2.5: the pollutant that matters most
Among the criteria pollutants, PM2.5 — particulate matter with a diameter smaller than 2.5 micrometers — is responsible for the most significant health effects at concentrations commonly encountered. These particles are small enough to penetrate deep into the lungs, enter the bloodstream, and affect the heart and brain. Long-term exposure increases the risk of heart disease, stroke, lung cancer, and premature death.
PM2.5 is the dominant pollutant during wildfire smoke events, agricultural burning, and many urban pollution episodes. When you see AQI values above 150 during a smoke event, PM2.5 is almost always the responsible pollutant. The AQI calculation for PM2.5 uses a 24-hour average concentration, which means the AQI can lag behind rapid changes in air quality — a limitation that the NowCast algorithm attempts to address.
Ground-level ozone: the summer pollutant
Ozone at ground level (distinct from the protective ozone layer in the stratosphere) is formed by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight. It is the primary component of photochemical smog and is most problematic during hot, sunny summer afternoons.
Ozone irritates the respiratory system, causing coughing, throat irritation, reduced lung function, and aggravation of asthma. Unlike PM2.5, which can come from both local and distant sources, elevated ozone is primarily a warm-season, daytime phenomenon. The AQI for ozone uses an 8-hour average concentration, reflecting the cumulative nature of its health effects.
How AQI is monitored
Official AQI values in the US come from the EPA's network of approximately 4,000 monitoring stations operated by state and local air quality agencies. These stations use regulatory-grade instruments that meet strict quality assurance requirements. Data is reported to the AirNow system and made available through public websites and APIs.
In recent years, lower-cost sensor networks — most notably PurpleAir — have dramatically expanded the spatial density of PM2.5 monitoring. These sensors are less accurate than regulatory monitors but provide neighborhood-level data in areas where EPA monitors may be miles apart. EPA has developed correction factors for popular sensor models that improve their agreement with regulatory instruments.
International AQI variations
Different countries use different AQI scales, thresholds, and pollutant lists, which can cause confusion when comparing air quality across borders. China's AQI includes PM10 prominently and uses different breakpoint concentrations. India's AQI adds ammonia and lead. The European Air Quality Index uses a different category structure entirely.
This lack of global standardization means that an 'AQI of 100' in the US and 'AQI of 100' in China or India represent different pollutant concentrations and different health risk levels. When viewing global air quality data on platforms like PlanetSentry, understanding which scale is being reported is important for accurate interpretation.
Practical decisions based on AQI
The AQI is designed to inform daily activity decisions. At Green levels, all outdoor activities are appropriate. As AQI rises through Yellow and Orange, sensitive individuals should consider reducing prolonged outdoor exertion. At Red levels, everyone should reduce extended outdoor physical activity. At Purple and Maroon levels, all outdoor activity should be minimized and indoor air filtration becomes important.
PlanetSentry displays air quality data alongside other environmental monitoring layers, helping users understand whether pollution from wildfires, industrial sources, or atmospheric conditions is affecting their region. Combining the AQI with wildfire smoke tracking data creates a more complete picture of air quality threats than either data source provides alone.