How Does The U.S EPA Report on Air Quality?

The United States Environmental Protection Agency (EPA) has developed two air quality Indexes for communicating air quality to the public, the U.S AQI and the Nowcast AQI. (N.B The Nowcast is used for all the AQI values reported on the airnow website).

Understanding the US AQI Scale

In both the US AQI and the Nowcast, the higher the value, the greater the level of air pollution, and the greater the health concern. Both indexes use color-coded categories, and each category is identified by a simple informative descriptor.

They both also use a 500-point scale – a rating between 0 and 50 is considered good, and a rating between 301 to 500 range is deemed hazardous:

 Air Quality Index | US EPA

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What’s the Difference Between the US AQI & the Nowcast?

The information provided by both U.S AQI and the Nowcast AQI is reliable and accurate, but the reporting of each index differs in a number of ways. To make things easy, we’ve highlighted some of the key differences below:


U.S AQINowcast AQI
What is the Purpose of the AQI?Inform regulatory planning & inform on air-quality health effectReport shorter-term AQ data to caution people in time to reduce their exposure
What Pollutants Does it Cover?Ground-level ozone (03)

Particle pollution (PM2.5 and PM10)

Carbon monoxide (CO)

Sulfur dioxide (SO2)

Nitrogen dioxide (NOx)
Ground-level ozone (03)

Particle pollution (PM2.5 and PM10)

Carbon monoxide (CO)

Sulfur dioxide (SO2)

Nitrogen dioxide (NOx)
What Time Frame Does it Cover?It varies by pollutant.

E.g. The ozone AQI has an 8-hour exposure index & a 1-hour index, while the calculation of PM2.5  AQI is provided as a 24-hour average of exposure.
The NowCast is designed to be responsive to rapidly changing air quality conditions, such as during a wildfire.

The NowCast calculation uses longer averages during periods of stable air quality & shorter averages when air quality changes rapidly. It uses the past 12 hours of PM measurements in micrograms per cubic meter.

Why Color is Key for Clear Communication

Both the US AQI & Nowcast are divided into six categories. Each category also has a specific color. These colors make it easy for people to quickly determine whether air quality is reaching unhealthy levels, or if the air they’re breathing is safe. The breakpoint of the AQI scale is 0-500 unless mentioned otherwise.

How Does BreezoMeter Calculate the U.S AQI?

BreezoMeter uses a hybrid index that is based on AirNow’s NowCast and U.S. AQI. For PM 2.5 and PM 10, we adopt the AirNow NowCast averaging method – giving more weight to recent hours and to erratic changes in order to better reflect sudden changes in air pollution levels due to smoke from fires or dust storm events.

This approach enables us to align more closely with what people are actually seeing or experiencing. However, unlike the Nowcast method, we also display all the pollutants that are part of the AQI (US) calculation and not only PM 2.5, PM 10, and O3.

We take the average of each pollutant type and convert average pollutant concentrations to an AQI for each pollutant. The final AQI is determined by taking the worst AQI among all pollutants. This unique approach to air quality reporting and our use of a hybrid index allows us to provide accurate and actionable air quality data.

We believe that by providing accurate, simple-to-understand information about air quality, people will find it easier to manage their exposure and take actions to improve their health.

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Dr Gabriela Adler
Dr Gabriela Adler Katz

Chief Scientist @BreezoMeter. I hold an MSc & PhD in atmospheric science and formerly worked as a research scientist at NOAA. I believe it is my greatest duty as a scientist to bring science to the people!