AirNow API - Air Quality 101

Air Quality Index (AQI)

The EPA developed the AQI, which reports levels of ozone, particle pollution, and other common air pollutants on the same scale. An AQI reading of 101 corresponds to a level above the national air quality standard - the higher the AQI rating, the greater the health impact.

AQI colors

The AQI is divided into color-coded categories and each category is identified by a simple informative descriptor. The descriptors are intended to convey information to the public about how air quality relates to public health. The table below defines the AQI categories.

AQI Numbers AQI Category (Descriptor) AQI Color   Hexadecimal Color Value Category Number
0 - 50 Good Green   (00e400) 1
51 - 100 Moderate Yellow   (ffff00) 2
101 - 150 Unhealthy for Sensitive Groups Orange   (ff7e00) 3
151 - 200 Unhealthy Red   (ff0000) 4
201 - 300 Very Unhealthy Purple   (99004c) 5
301 - 500 Hazardous Maroon   (7e0023) 6

Air quality observations

Hourly or daily observations are measured and reported to AirNow by federal, state, local, and tribal air quality agencies. Historical (daily) AQI values are calculated using an averaging method, and real-time AQI values are based on a surrogate calculation.

Data averaging

The AQI reported for ground-level ozone and fine particles (PM2.5) is based on an average of hourly data. For ozone, the AQI is based on the maximum observed 8-hour average from midnight to midnight. For PM2.5, the AQI is simply the 24-hour average. For AQI values reported in real-time, before a full day's data are available, the AQI is based on a surrogate calculation.

Surrogate values

Real-time observations are reported using a surrogate AQI value or NowCast. More information about the surrogate calculations can be found at for PM2.5 and for ozone.

Air quality forecasts

Many agencies issue air quality forecasts for reporting areas. These forecasts are predictions of the observed daily AQI based on air quality and weather information. These predictions can vary by reporting area, depending on the agency issuing the forecast. Additional information can be found on the FAQ page under "How can forecasts differ by reporting area?"

Action days

Action days are usually called when the AQI reaches unhealthy or higher. Different air pollution control agencies call Action Days at different levels. In some places, action days are called when the AQI is forecast to be Unhealthy for Sensitive Groups, or Code Orange. In this case, the groups that are sensitive to the pollutant should reduce exposure by eliminating prolonged or heavy exertion outdoors. For ozone this includes children and adults who are active outdoors and people with lung disease, such as asthma. For particle pollution this includes: people with heart or lung disease, older adults and children. Occasionally, an action day is declared when the AQI is Moderate, or Code Yellow, if the levels are expected to approach Code Orange levels.