AQI Hub United States of America (AQI)

Background

The United States of America (US) uses an Air Quality Index that accounts for PM_2.5, PM₁₀, CO, O₃, NO₂, and SO₂. The US reports a daily AQI as well as a NowCast and Forecast AQI.

The US requires that Metropolitan Statistical Areas (MSAs) with a population of over 350,000 are required to report a daily AQI to the public, while forecasting and NowCast AQIs are voluntary ¹. AQI values may be reported at the MSA’s discretion if AQI values for all pollutants remain below 50 for a year. However the AQI must be reported if a pollutant level rises to be above 50 during subsequent years. If the AQI for a specific pollutant remains below 50 for an extended period of time, such as a season or a year, that pollutant may be excluded from the AQI calculation ¹.

The US AQI methodology was most recently updated in May 2024 to implement stricter annual standards for PM_2.5 by altering breakpoint concentrations. The EPA outlines an in-depth history of the changes in PM National Ambient Air Quality Standards ².

Color scale

The US EPA uses a six color scale:

0-50

51-100

101-150

151-200

201-300

301+

Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous

Good0-50

Moderate51-100

Unhealthy for Sensitive Groups101-150

Unhealthy151-200

Very Unhealthy201-300

Hazardous301+

As of May 2024, the US EPA also offers a “ColorVision Assist” version of the color palette, designed to be more accessible for those with color vision deficiencies. The South Coast Air Quality Management District’s Air Quality Sensor Performance Evaluation Center and Air Quality Assessment Group led the development of this palette.

0-50

51-100

101-150

151-200

201-300

301+

Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous

Good0-50

Moderate51-100

Unhealthy for Sensitive Groups101-150

Unhealthy151-200

Very Unhealthy201-300

Hazardous301+

Note: “Technical Assistance Document for the Reporting of Daily Air Quality the Air Quality Index (AQI)” (May 2024), https://document.airnow.gov/technical-assistance-document-for-the-reporting-of-daily-air-quailty.pdf. Accessed 24 November 2024.

Previous to the most recent update of the AQI Technical Assistance Document for Reporting of Air Quality Data issued in May 2024, the ‘Hazardous’ category had an upper limit of 500. AQI values greater than 500 were considered “Beyond the AQI,” and the same warnings and cautions as the ‘Hazardous’ category were to be reported.

Methods

Pollutant	Averaging period	Good 0-50	Moderate 51-100	Unhealthy for Sensitive Groups 101-150	Unhealthy 151-200	Very Unhealthy 201-300	Hazardous 301+
PM_2.5 μg/m³	24 hr.	0-9	9.1-35.4	35.5-55.4	55.5-125.4	125.5-225.4	225.5+
PM₁₀ μg/m³	24 hr.	0-54	55-154	155-254	255-354	355-424	425+
CO ppm	8 hr.	0-4.4	4.5-9.4	9.5-12.4	12.5-15.4	15.5-30.4	30.5+
O₃ ppm	1 hr.	-	-	0.125-0.164	0.165-0.204	0.205-0.404	0.405+
O₃ ppm	8 hr.	0-0.054	0.055-0.07	0.071-0.085	0.086-0.105	0.106-0.2	0.201+
SO₂ ppb	1 hr.	0-35	36-75	76-185	186-304	305-604	605+
NO₂ ppb	1 hr.	0-53	54-100	101-360	361-649	650-1249	1250+

Note: “Technical Assistance Document for the Reporting of Daily Air Quality the Air Quality Index (AQI)” (May 2004), https://www.airnow.gov/sites/default/files/2020-05/aqi-technical-assistance-document-sept2018.pdf. Accessed 25 July 2024.

Forecast AQI

Forecast AQI predicts the next day’s AQI, usually issued in the afternoon for the next day by state and local air quality forecasters ³. These utilize numerous sources such as weather forecast models, satellite images, air monitoring data, and computer models that estimate the travel of pollution through the air ³. Forecast AQIs are mainly issued for O₃ and PM, however some areas also issue NO₂ and CO forecasts.

Daily AQI

The daily AQI accounts for PM_2.5, PM₁₀, CO, O₃, NO₂, and SO₂ concentrations.

First, the highest pollutant concentrations over their allotted averaging periods are identified and truncated. O₃ is truncated to 3 decimal places, while PM_2.5 and CO are truncated to 1 decimal place. PM₁₀, NO₂, and SO₂ are truncated to an integer value.

The following piecewise linear function is used to calculate the sub-index AQI values using each pollutant’s breakpoint concentration:

$AQI_P = \frac{I_{HI} - I_{LO}}{BP_{HI} - BP_{LO}} \times (C_P - BP_{LO}) + I_{LO}$

$I_P$ – AQI for pollutant P
$I_{HI}$ – AQI value corresponding to $BP_{HI}$
$I_{LO}$ – AQI value corresponding to $BP_{LO}$
$BP_{HI}$ – Breakpoint ≥ $C_P$
$BP_{LO}$ – Breakpoint ≤ $C_P$
$C_P$ – Concentration of pollutant P

The resulting value is rounded to the nearest integer. The maximum AQI for all the pollutants is reported as the daily AQI.

For O₃, the AQI corresponding to the 8-hour averaging period is required to be calculated. The AQI corresponding to the 1-hour averaging period may be calculated; if so, the higher AQI value must be reported ¹. 8-hour O₃ values do not define AQi values greater or equal to 301, therefore 1-hour concentrations are used to calculate these higher AQI values ¹.

The EPA strengthened the primary annual standard for SO₂ in 2010. Due to a lack of health information regarding upper breakpoint concentrations for SO₂, the upper breakpoint continues to use a 24-hour averaging period for determining concentrations. The lower breakpoint concentrations were updated to use the daily maximum 1-hour SO₂ concentration ¹. 1-hour SO₂ concentrations do not define AQI values greater or equal to 200, therefore 24-hour concentrations are used to calculate these higher AQI values ¹.

If the daily maximum 1-hour concentration for SO₂ is less than 305 ppb or if the 24-hour average is at least 205 ppb, the breakpoint concentrations can be used in the AQI calculation as normal. On the rare occasion where the daily maximum 1-hour concentration is greater or equal to 305, the value of the lower and upper breakpoints is set to 200 to ensure the resulting AQI value to be the highest possible associated with the 1-hour concentration ¹.

For AQI values within the Hazardous category greater than 500, the calculation is performed using the standard piecewise linear function and the concentration specified is that of the AQI value of 500. The pollutant concentration values at an AQI of 500 are as follows¹:

PM_2.5 24 hr/	PM₁₀ 24 hr.	CO 8 hr.	O₃ 1 hr.	SO₂ 1 hr.	NO₂ 1 hr.
325.4 µg/m³	604 µg/m³	50.4 ppm	0.604 ppm	1004 ppb	2049 ppb

NowCast AQI

Given that the AQI represents a full day, which is a fairly long timeframe, the US EPA developed the NowCast algorithm to better represent short-term air quality changes. The NowCast calculation methods use longer averaging periods when there is stable air quality and shorter averaging periods when the air quality is rapidly changing, such as during a fire or extreme winds ⁴.

% above referenced [^4] after "must be valid"

Particulate Matter

For calculating the NowCast for PM, the past 12 hours of PM measurements are used in units of µg/m3. The range of concentration is calculated by subtracting the minimum value of the series ( ${M_{min}}$ ) from the maximum value of the series ( ${M_{max}}$ ). Then the the resulting range is divided by the maximum value of the series ( ${M_{max}}$ ). This yields what is called the scaled rate of change ( ${SRoC}$ ).

{SRoC} = \frac{M_{max} - M_{min}}{M_{max}}

Next, the weight factor (WF) is determined from the scaled rate of change (SRoC) using the following equation:

{WF} = {Max}(1 - SRoC, 0.5)

If the WF is less than 0.5, it is set equal to 0.5 as it approximates a 3-hour average ⁴.

The NowCast is calculated using the following equation using the appropriate WF:

\frac{\sum_{i=1}^{n} ({C}_i \times {WF}^i )}{\sum_{i=1}^{n} {WF}^i}

The final concentration value is then truncated to the nearest integer for PM₁₀ and 1 decimal place for PM_2.5. It is then converted to an AQI and reported using the aforementioned piecewise linear function.

Ozone

The EPA implemented the latest calculation method of the O₃ NowCast to take into account the variability of the 1-hour concentrations while still remaining representative of the 8 hour average concentration ¹. The original method was established in 2012. The method uses a Partial Least Squares (PLS) method based on the relationship of the 1-hour and 8-hour concentration data over the most recent 2-week rolling period ¹.

There are two different methods used to calculate the O₃ NowCast value depending on whether the criteria for PLS calculation is met ³.

If the criterion is met and there is at least 1 hour of valid O₃ data within the past 3 hours, the PLS method is used ³. The previous 2 weeks of hourly O₃ data, consisting of 336 data points, are considered in this calculation ³. This method is used to better represent the 8-hour average value of O₃ concentrations and output an AQI value that more closely aligns with the daily AQI.

If the criterion is not met and there is at least 1 hour of valid O₃ data within the past 3 hours, the Surrogate method is used as a secondary method to produce an O₃ NowCast value ³. This method uses the following simple linear equation:

y = 0.85x + 4.5 ppb

-{x} = Hourly O_3 Value -{y} = Resulting O_3 NowCast Value

The Surrogate method is used as a secondary method used to produce an O₃ NowCast value. It has been derived from linear regression between concurrent 8-hour average and 1-hour O₃ concentrations using historical data from approximately 40 monitoring sites across major continental US cities³.

If there is no hourly O₃ data within the previous 3 hours, the NowCast is shown as N/A³.

References

“Air Quality Index (AQI) Basics.” AirNow.gov., [www.airnow.gov/aqi/aqi-basics/].(https://www.airnow.gov/aqi/aqi-basics/).

“Final Updates to the Air Quality Index (AQI) for Particulate Matter. Fact Sheet and Common Questions.” United States Environmental Protection Agency, www.epa.gov/system/files/documents/2024-02/pm-naaqs-air-quality-index-fact-sheet.pdf.

Hylton, Marcus. “The AQI Equation (2024 - Valid Beginning May 6th, 2024).” AirNow.gov., 18 Mar. 2024, forum.airnowtech.org/t/the-aqi-equation-2024-valid-beginning-may-6th-2024/453.

Hylton, Marcus. “The Nowcast for Ozone - 2019 Update (Partial Least Squares Method).” AirNow.gov., 26 Jul. 2019, forum.airnowtech.org/t/the-nowcast-for-ozone-2019-update-partial-least-squares-method/356.

Hylton, Marcus. “The NowCast for PM2.5 and PM10.” AirNow.gov., 15 Jul. 2016, forum.airnowtech.org/t/the-nowcast-for-pm2-5-and-pm10/172.

“How is the NowCast algorithm used to report current air quality?” AirNow.gov., usepa.servicenowservices.com/airnow/en/how-is-the-nowcast-algorithm-used-to-report-current-air-quality?id=kb_article_view&sys_id=bb8b65ef1b06bc10028420eae54bcb98&spa=1.

“NowCast Calculator.” AirNow.gov., www3.epa.gov/airnow/aqicalctest/nowcast.htm.

“PM NowCast.” AirNow.gov., Dec. 2014, www.epa.gov/sites/default/files/2018-01/documents/nowcastfactsheet.pdf.

“Technical Assistance Document for the Reporting of Daily Air Quality the Air Quality Index (AQI).” United States Environmental Protection Agency, May 2004, https://www.airnow.gov/sites/default/files/2020-05/aqi-technical-assistance-document-sept2018.pdf.

“Timeline of Particulate Matter (PM) National Ambient Air Quality Standards (NAAQS).” United States Environmental Protection Agency, 7 Feb. 2024, www.epa.gov/pm-pollution/timeline-particulate-matter-pm-national-ambient-air-quality-standards-naaqs.

“Using the Air Quality Index.” AirNow.gov., www.airnow.gov/aqi/aqi-basics/using-air-quality-index.

Air Quality Index Hub

United States of America

Air Quality Index (AQI)