Technical Assistance Document for the Reporting of Daily Air Quality the Air Quality Index (AQI)


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Technical Assistance Document for the
Reporting of Daily Air Quality - the Air Quality
Index (AQI)

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EPA-454/B-24-002
May 2024

Technical Assistance Document for the Reporting of Daily Air Quality - the Air Quality Index (AQI)

U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Air Quality Assessment Division
Research Triangle Park, NC

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CONTENTS

I.	INTRODUCTION TO THE AQI	1

II.	REPORTING THE AQI	1
Who has to Report	1
When to Report	1
What to Report	2
Format for Reporting	3
Definition of the AQI Colors	3

III.	HEALTH INFORMATION FOR AQI POLLUTANTS	5
Health Effects	5
Sensitive Groups	7
Cautionary Statements	7

IV.	CALCULATING THE AQI	13
Calculating the AQI from pollutant concentration data	13
Using the table and the equation and concentration data to calculate the AQI	15
How to handle values from multiple pollutants	15
Using both 031-hour and 8-hour values	15
Calculating AQI values for S02	15
How to handle concentrations for pollutants that have blank places in the table for Breakpoints for the AQI 16
How the NowCast is calculated	16

V.	ACCESSING AQI DATA	18
AirNow Website	18

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AirNow Fire and Smoke Map	18

AirNow Mobile App (Android and iOS)	19

AirNow EnviroFlash	19

AirNow Widget Website	20

AirNow Department of State	20

AirData Website	21

AirCompare Website	21

VI.	FREQUENTLY ASKED QUESTIONS	22

VII.	ADDITIONAL RESOURCES	26

LIST OF TABLES

TABLE 1. NAMES AND COLORS FOR THE SIX AQI CATEGORIES	3

TABLE 2. STANDARD AQI COLOR FORMULAS	4

TABLE 3. COLORVISION ASSIST AQI COLOR FORMULAS	4

TABLE 4. POLLUTANT-SPECIFIC SENSITIVE GROUPS	7

TABLE 5. POLLUTANT-SPECIFIC SUB-INDICES AND CAUTIONARY STATEMENTS	8

TABLE 6. BREAKPOINTS FOR THE AQI	14

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This guidance is designed to aid local agencies in reporting air quality using the U.S. Air Quality Index as
required in 40 CFR Part 58.50 and according to 40 CFR Appendix G to Part 58.

I. INTRODUCTION TO THE AQI

The AQI is EPA's tool for communicating air quality. The AQI is divided into six categories that
correspond to different levels of health concern. For ease of use, the categories are color-coded and
provide statements about local air quality, which groups of people may be affected, and steps people
can take to reduce their exposure to air pollution. The AQI is also used as the basis for air quality
forecasts and current air quality reporting.

EPA establishes an AQI for five major air pollutants regulated by the Clean Air Act. Each of these
pollutants has a national ambient air quality standard (NAAQS) set by EPA to protect public health:

• ozone

• particle pollution (also called particulate matter)

• carbon monoxide

• nitrogen dioxide

• sulfur dioxide

The AQI for each pollutant draws on the scientific information that supports the health-based NAAQS for
that pollutant. The AQI value of 100 generally corresponds to an ambient air concentration that equals
the level of the short-term NAAQS for protection of public health. AQI values at or below 100 are
generally thought of as satisfactory. When AQI values are above 100, air quality is unhealthy: at first for
certain sensitive groups of people, then for everyone as AQI values get higher. The AQI does not apply to
hazardous air pollutants, extreme heat, and other aspects of outdoor air.

II. REPORTING THE AQI

Who has to Report

Metropolitan Statistical Areas (MSAs) with a population of more than 350,000 are required to report the
AQI daily to the public. The population of an MSA for purposes of index reporting is based on the latest
available U.S. census population.

When to Report

MSAs must report the AQI daily, which is defined as seven days each week on days for which you have
measured air quality data (e.g., from continuous PM2.5 monitors). This definition allows for equipment
failures and accommodates agencies using manual monitoring methods on a l-in-6-day schedule.

Low AQI values

If the AQI values for all of the pollutants remain below 50 for a year, then you may report the AQI at
your discretion. In subsequent years, if any pollutant level rises to where the AQI would be above 50,
then you must report the AQI.

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If a specific pollutant remains below 50 for an extended period of time (a season or a year), you may
exclude that pollutant from your AQI calculation.

What to Report

Required Reporting

It takes a full 24 hours to obtain an AQI value (that's 24 hourly values for PM or the max 1-hour or 8-
hour value in a 24-hour period for other pollutants), so you are in effect required to report yesterday's
AQI, including the:

•	reporting area

•	reporting period

•	main pollutant (the pollutant with the highest AQI value)

•	AQI value

•	category descriptor and color (if your report uses color) shown in Table 2

•	sensitive groups for all pollutants with an AQI over 100, as shown in Table 4

Voluntary Reporting

To make AQI reporting more useful to the public, most agencies also choose to report some or all of the
following:

•	Forecasts of the daily and/or sub-daily AQI values

•	Hourly concentrations used to compute the current AQI values based on the NowCast

•	Health effects and cautionary statements

•	Causes for unusual AQI values

•	The AQI for sub-areas of the reporting area

•	Pollutant concentrations

•	The name and AQI for other pollutants, particularly those with an AQI greater than 100

•	Statements that "blend" health effects and cautionary information for more than one
pollutant, if there is more than one pollutant with an AQI greater than 100

Forecasting the AQI

AQI Forecasts are a prediction of the day's air quality using AQI colors and scale. Forecasts usually are
issued by state, Tribal, and local air quality forecasters in the afternoon for the next day. An AQI forecast
can help plan outdoor activities for the day. Much like a weather forecast lets people know whether to
pack an umbrella, an air quality forecast lets people know when they may want to change their outdoor
activities to reduce the amount of air pollution they breathe in. Many forecasters also provide a
"forecast discussion," which lets people know when pollution is expected to be highest during the day -
and if there are times when air quality is expected to be better.

Air quality action days program

The use of air quality action days or community action programs, which are usually based on AQI
forecasts, is voluntary. However, action days and similar programs can help alert the public and
encourage them to take protective actions to reduce their exposure. Air quality action days may be
called by state or local air agencies when the AQI is expected to get into unhealthy ranges. Different
2

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agencies call them at different levels, such as Orange or Red or sometimes Yellow. To learn more about
air quality action days, go to https://www.airnow.gov/aqi/action-davs/.

Real-time AQI reporting: the NowCast

Submitting hourly data in real-time to the EPA's AirNow (or future analogous) system is recommended,
but not required, and assists the EPA in providing timely air quality information to the public for making
health-protective decisions. Submitting hourly data for appropriate monitors (referenced in section 3.2
of 40 CFR Part 58 Appendix G) satisfies the daily AQI reporting requirement because the AirNow system
makes daily and sub-daily AQI reports widely available through its website and other communication
tools. AirNow uses the hourly concentrations to report current air quality based on the NowCast AQI.
The NowCast uses an algorithm that relates hourly readings from air quality monitors to the AQI for
ozone and particle pollution. The NowCast gives you the latest information on air quality where you are.
Because air quality can change during the day, you can expect to see the NowCast AQI change. Even on
days when the daily AQI forecast isn't good, there may be times during the day when air quality is OK for
outdoor activity. Providing current conditions gives people the power to take action when necessary to
reduce outdoor activities and exposure and protect their health. The NowCast methods for ozone and
PM (PM2.5 and PM10 use the same method) are described in section IV Calculating the AQI. Because the
sub-indices for S02 and N02 are based on 1-hour averages, a NowCast is not needed for these
pollutants.

Format for Reporting

AQI reports can take many forms - from television and radio news to apps, web pages, and social media.
The purpose of the AQI is to inform people about their air quality so they can take steps to protect their
health. This is especially important whenever the AQI exceeds 100. To reach the most people, try to
deliver the AQI in as many ways as possible.

Definition of the AQI Colors

Table 1. Names and colors for the six AQI categories

For this AQI...

use this descriptor...

and this color

0 to 50

Good

Green

51 to 100

Moderate

Yellow

101 to 150

Unhealthy for Sensitive Groups

Orange

151 to 200

Unhealthy

Red

201 to 300

Very Unhealthy

Purple

301+

Hazardous

Maroon

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The standard AQI colors are defined by the formulas RGB (red, green, blue) and CMYK (cyan, magenta,
yellow, black) as shown in Table 2. RGB is traditionally used for screen colors, while CMYK is traditionally
used for printing. The color models are based on a 0 - 255 scale (e.g., 50% is 126).

Table 2. Standard AQI color formulas

Standard AQI Color

Green

228

100

Yellow

255

100

Orange

255

126

100

Red

255

100

Purple

143

151

Maroon

126

100

While the standard AQI colors listed in Table 2 are required, reporting entities may also use ColorVision
Assist colors that enhance contrast to assist individuals with color vision deficiencies. South Coast Air
Quality Management District's Air Quality Sensor Performance Evaluation Center and Air Quality
Assessment Group led the development of these modified colors to provide better accessibility for those
with color vision deficiencies. The changes were primarily made to the hues, keeping the essential colors
associated with each AQI category and health impacts (green, yellow, orange, red, purple, maroon). The
standard AQI color scale can be difficult to discern for those individuals who have challenges
distinguishing certain colors, especially red and green colors.

Table 3. ColorVision Assist AQI color formulas

ColorVision Assist Color

Green

158

255

145

Yellow

255

201

Orange

255

130

Red

240

100

Purple

137

151

Maroon

100

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III. Health Information for AQI Pollutants

Health Effects

The information below describes the health effects that may be associated with each of the AQI
pollutants. In general, the proportion of people affected and the likelihood of more severe effects
increase as pollutant concentrations increase. More information can be found at
https://www.epa.gov/criteria-air-pollutants.

Ozone

Depending on the level of exposure, ozone can:

• Cause coughing and sore or scratchy throat

• Make it more difficult to breathe deeply and vigorously and cause pain when taking a deep
breath

• Inflame and damage the airways

• Make the lungs more susceptible to infection

• Aggravate lung diseases such as asthma, emphysema, and chronic bronchitis

• Increase the frequency of asthma attacks

Some of these effects have been found even in healthy people, but effects can be more serious in
people with lung diseases such as asthma. They may lead to increased school absences, medication use,
visits to doctors and emergency rooms, and hospital admissions. Some studies in locations with elevated
concentrations also report associations of ozone with deaths from respiratory causes.

In addition to the health effects that can be experienced in response to short-term (i.e., 1-8 hours)
ozone exposure (which is the focus of the AQI), long-term exposure to ozone is linked to aggravation of
asthma and is likely to be one of many causes of asthma development.

Particle Pollution

The size of particles is directly linked to their potential for causing health problems. Particles less than 10
micrometers can be inhaled into the body, with smaller particles less than 2.5 micrometers posing the
greatest problems because they can get deep into the lungs, and some may even get into the
bloodstream.

Exposure to such particles can affect both lungs and heart. Numerous scientific studies have linked
particle pollution exposure to a variety of health problems, such as:

• premature death in people with heart or lung disease

• heart attacks

• irregular heartbeat

• aggravated asthma

• decreased lung function

• increased respiratory symptoms, such as irritation of the airways, coughing or difficulty
breathing.

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In addition to the health effects that can be experienced in response to short-term (or daily) particle
pollution exposure (which is the focus of the AQI), long-term exposure (exposures experienced over
months and years) can lead to the development of cardiovascular and respiratory diseases, such as
atherosclerosis and asthma, nervous system effects (e.g., cognitive effects), lung cancer, and premature
death.

Carbon Monoxide (CO)

Breathing air with a high concentration of CO reduces the amount of oxygen that can be transported in
the blood stream to critical organs like the heart and brain. At very high levels, which are possible
indoors or in other enclosed environments, CO can cause dizziness, confusion, unconsciousness and
death.

Very high levels of CO are not likely to occur outdoors. However, when CO levels are elevated outdoors,
they can be of particular concern for people with some types of heart disease. These people already
have a reduced ability for getting oxygenated blood to their hearts in situations where the heart needs
more oxygen than usual. They are especially vulnerable to the effects of CO when exercising or under
increased stress. In these situations, short-term exposure to elevated CO may result in reduced oxygen
to the heart accompanied by chest pain also known as angina.

Nitrogen Dioxide (N02)

Breathing air with a high concentration of N02 can irritate airways in the human respiratory system.
Such exposures over short periods (i.e., 1-5 hours) can aggravate respiratory diseases, particularly
asthma, leading to respiratory symptoms (such as coughing, wheezing or difficulty breathing), hospital
admissions and visits to emergency rooms.

In addition to the health effects that can be experienced in response to short-term N02 exposures
(which is the focus of the AQI), long-term (exposures experienced over months and years) exposure to
elevated concentrations of N02 may contribute to the development of asthma and potentially increase
susceptibility to respiratory infections.

Sulfur Dioxide (S02)

Short-term exposures (i.e., 1-6 hours) to S02 are linked with respiratory effects including difficulty
breathing and increased asthma symptoms. These effects are particularly problematic for people with
asthma while breathing deeply such as when exercising or playing. Short-term exposures to S02 have
also been connected to increased emergency department visits and hospital admissions for respiratory
illnesses, particularly for at-risk populations including children, older adults, and those with asthma.

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Sensitive Groups

The sensitive groups for each pollutant are listed in Table 4.

Table 4. Pollutant-specific Sensitive Groups

When this pollutant has
an AQI above 100.,.1

Report these Sensitive Groups

Ozone

People with lung disease, such as asthma; children, including teenagers;
people who are active outdoors; and older adults.

Fine particle pollution2
(PM2.5) and particle
pollution (PM10)

People with heart or lung disease; older adults; and children, including
teenagers. In addition, some minority groups, people with lower incomes,
and outdoor workers may experience higher exposure that can worsen
underlying health conditions.

Carbon monoxide (CO)

People with heart disease.

Nitrogen dioxide (N02)

People with lung disease, such as asthma; children, including teenagers;
and older adults.

Sulfur dioxide (S02)

People with asthma; children, including teenagers; and older adults.

Cautionary statements may be combined, for all AQI pollutants above 100, so that each sensitive group
is mentioned only once.

2 During a smoke event, EPA recommends including pregnant people as an at-risk group as noted in the
At-Risk Groups of People Wildfire Smoke Factsheet found here

https://www.airnow.gov/publications/wildfire-guide-factsheets/at-risk-groups-of-people-fact-sheet/.

Cautionary Statements

Table 5 (see next page) lists cautionary statements for each pollutant by AQI category. Use these
statements when showing current air quality or for the AQI forecast. Please note Table 5 spans several
pages.

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Table 5. Pollutant-Specific Sub-indices and Cautionary Statements

AQI Categories

Ozone

Particulate Matter

Carbon Monoxide

Nitrogen

Sulfur

(Index Values)

(8-hr)

(1-hr)

PM2.5

PM10

(8-hr)

Dioxide

Dioxide







(24-hr)

(24-hr)



(1-hr)

(1-hr)

Good

It's a great day to be



It's a great day to be

It's a great day to

It's a great

It's a great

(0-50)

active outside.



active outside.

be active outside.

day to be

day to be













active

active













outside.

outside.

Moderate

Unusually sensitive



Unusually sensitive

It's a great day to

Unusually

It's a great

(51-100)

people: Consider



people: Consider

be active outside.

sensitive

day to be



making outdoor



making outdoor



people:

active



activities shorter and



activities shorter and



Consider

outside.



less intense. Watch for



less intense. Go



limiting





symptoms such as



inside if you have



prolonged





coughing or shortness



symptoms such as



exertion





of breath. These are



coughing or



especially





signs to take it easier.



shortness of breath.



near busy















roads.



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AQI Categories

Ozone

Particulate Matter

Carbon Monoxide

Nitrogen

Sulfur

(Index Values)

(8-hr)

(1-hr)

PM2.s

PMio

(8-hr)

Dioxide

Dioxide







(24-hr)

(24-hr)



(1-hr)

(1-hr)

Unhealthy for Sensitive Groups

Sensitive groups: Make outdoor

Sensitive groups:

Sensitive group:

Sensitive

Sensitive

(101-150)

activities shorter and less



Make outdoor

Limit heavy

groups: Limit

groups:



intense. Take more breaks.

activities shorter and

exertion outdoors

prolonged

Consider



Watch for symptoms such as

less intense. It's OK

and avoid sources

exertion

limiting



coughing or shortness of breath.

to be active

of CO, such as

outdoors,

outdoor



Plan outdoor activities in the

outdoors but take

heavy traffic.

especially

exertion.



morning when ozone is lower.

more breaks. Watch



near busy

People with



People with asthma: Follow

for symptoms such



roads.

asthma:



your asthma action plan and

as coughing or



People with

Follow your



keep quick-relief medicine

shortness of breath.



asthma:

asthma



handy.



People with asthma:



Follow your

action plan







Follow your asthma



asthma

and keep







action plan and keep



action plan

quick relief







quick relief medicine



and keep

medicine







handy.





quick relief

handy.







People with heart



medicine









disease: Symptoms



handy.









such as palpitations,













shortness of breath,













or unusual fatigue













may indicate a













serious problem. If













you have any of













these, contact your













health care provider.







9

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AQI Categories

Ozone

Particulate Matter

Carbon Monoxide

Nitrogen

Sulfur

(Index Values)

(8-hr)

(1-hr)

PM2.s

PMio

(8-hr)

Dioxide

Dioxide







(24-hr)

(24-hr)



(1-hr)

(1-hr)

Unhealthy

Sensitive groups: Do not do

Sensitive groups:

Sensitive group:

Sensitive

Sensitive

(151-200)

long or intense outdoor



Consider



Limit moderate

groups: Avoid

groups: Limit



activities. Schedule outdoor

rescheduling or

outdoor exertion

prolonged

outdoor



activities in the morning when

moving all

activities

and avoid sources

outdoor

exertion.



ozone is lower. Consider moving

inside. Go inside if

of CO, such as

exertion near

People with



activities indoors.



you have symptoms.

heavy traffic.

roadways.

asthma:



People with asthma: Follow

People with asthma:



People with

Follow your



your asthma action plan and

Follow your asthma



asthma:

asthma



keep quick-relief medicine

action plan and keep



Follow your

action plan



handy.



quick-relief medicine



asthma

and keep



Everyone else: Reduce long or

handy.





action plan

quick relief



intense outdoor activity. Take

People with heart



and keep

medicine



more breaks, do less intense

disease: Symptoms



quick relief

handy.



activities. Schedule outdoor

such as palpitations,



medicine





activities in the morning when

shortness of breath,



handy.





ozone is lower.



or unusual fatigue



Everyone









may indicate a



else: Limit









serious problem. If



prolonged









you have any of



outdoor









these, contact your



exertion









health care provider.



especially









Everyone else: Keep



near busy









outdoor activities



roads.









shorter and less













intense. Go inside if













you have symptoms.







10

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AQI Categories

Ozone

Particulate Matter

Carbon Monoxide

Nitrogen

Sulfur

(Index Values)

(8-hr)

(1-hr)

PM2.s

PMio

(8-hr)

Dioxide

Dioxide







(24-hr)

(24-hr)



(1-hr)

(1-hr)

Very Unhealthy

Sensitive groups: Avoid a

1

Sensitive groups:

Sensitive group:

Sensitive

Sensitive

(201-300)

physical activity outdoors. Move

Avoid all physical

Avoid outdoor

groups: Avoid

groups:



activities indoors* or reschedule

activity outdoors.

exertion and

all outdoor

Avoid



to when air quality will be

Reschedule to a time

sources of CO,

exertion.

outdoor



better.



when air quality is

such as heavy

Everyone

exertion.



People with asthma: Follow

better or move

traffic.

else: Avoid

Everyone



your asthma action plan and

activities indoors.*



prolonged

else: Reduce



keep quick-relief medicine

People with asthma:



outdoor

outdoor



handy.



Follow your asthma



exertion

exertion.



Everyone else: Avoid long or

action plan and keep



especially

People with



intense outdoor exertion.



quick-relief medicine



near busy

asthma:



Schedule outdoor activities in

handy.





roads.

Follow your



the morning when ozone is

People with heart



People with

asthma



lower. Consider moving



disease: Symptoms



asthma:

action plan



activities indoors.*



such as palpitations,



Follow your

and keep







shortness of breath,



asthma

quick relief







or unusual fatigue



action plan

medicine







may indicate a



and keep

handy.







serious problem. If



quick relief









you have any of



medicine









these, contact your



handy.









health care provider.













Everyone else: Limit













outdoor physical













activity. Go indoors*













if you have













symptoms.







11

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AQI Categories

Ozone

Particulate Matter

Carbon Monoxide

Nitrogen

Sulfur

(Index Values)

(8-hr)

(1-hr)

PM2.s

PMio

(8-hr)

Dioxide

(24-hr)

(1-hr)

Hazardous

Everyone: Avoid all physical

Sensitive groups:

Sensitive group:

Sensitive

(>301)

activity outdoors.*

Stay indoors and

Avoid outdoor

groups:

People with asthma: Follow

keep activity levels

exertion and

Remain

your asthma action plan and

light. Follow tips for

sources of CO,

indoors.*

keep quick-relief medicine

keeping particle

such as heavy

Everyone

handy.

levels low indoors.*

traffic.

else: Avoid all

else: Avoid

People with asthma:

Everyone else:

outdoor

Follow your asthma

Limit heavy

exertion.

action plan and keep

outdoor exertion.

People with

quick-relief medicine

asthma:

handy.

Follow your

People with heart

asthma

disease: Symptoms

action plan

such as palpitations,

and keep

shortness of breath,

quick relief

or unusual fatigue

medicine

may indicate a

handy.

serious problem. If

you have any of

these, contact your

health care provider.

Everyone: Avoid all

physical activity

outdoors.*

*Note: If you don't have an air conditioner, staying indoors with the windows closed may be dangerous in extremely hot weather. If you are hot, go
someplace with air conditioning or check with your local government to find out if cooling centers are available in your community.

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IV. CALCULATING THE AQI

Calculating the AQI from pollutant concentration data

The AQI is the highest value calculated for each pollutant as follows:

1.	Identify the highest concentration among all of the monitors within each reporting area and truncate
as follows:

Ozone (ppm) - truncate to 3 decimal places
PM2.5 (ng/m3) - truncate to 1 decimal place
PM10 (ng/m3) - truncate to integer
CO (ppm) - truncate to 1 decimal place
S02 (ppb) - truncate to integer
N02 (ppb) - truncate to integer

2.	Using Table 6 (next page), find the two breakpoints that contain the concentration.

3.	Using Equation 1, calculate the index.

4.	Round the index to the nearest integer.

Where lp = the index for pollutant p

Cp = the truncated concentration of pollutant p

BPHi = the concentration breakpoint that is greater than or equal to Cp
BPLo = the concentration breakpoint that is less than or equal to Cp
lHi = the AQI value corresponding to BPH,
lLo = the AQI value corresponding to BPLo

Equation 1:

13

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Table 6. Breakpoints for the AQI

These Breakpoints...

...equal
this AQI

...and this
category

o3

(ppm)
8-hour

o3

(ppm)
1-hour1

PM2.5

(Hg/m3)
24-hour

PMio

(Hg/m3)
24-hour

CO

(ppm)
8-hour

S02

(ppb)

1-hour

no2

(ppb)
1-hour

AQI



0.000 -
0.054



0.0-9.0

0-54

o
d

0-35

0-53

0-50

Good

0.055 -
0.070



9.1-35.4

55 -154

4.5-9.4

36-75

54 -100

51-100

Moderate

0.071 -
0.085

0.125 -
0.164

35.5-
55.4

155 - 254

9.5 -12.4

76 -185

101-360

101 -150

Unhealthy for
Sensitive Groups

0.086 -
0.105

0.165 -
0.204

(55.5 -
125.4)3

255 -354

12.5 -
15.4

3 186 - 304

361 - 649

151-200

Unhealthy

0.106 -
0.200

0.205 -
0.404

(125.5 -
(225.4)3

355 -424

15.5 -
30.4

3 305 -
604)

650-
1249

201-300

Very unhealthy

0.201-(2)

0.405+

225.5+

425+

30.5+

3 605+

1250+

301+

Hazardous 4

1	Areas are generally required to report the AQI based on 8-hour 03 values. However, there are a small
number of areas where an AQI based on 1-hour 03 values would be more precautionary. In these cases,
in addition to calculating the 8-hour 03 index value, the 1-hour 03 value may be calculated, and the
maximum of the two values reported.

2	8-hour 03 values do not define higher AQI values (> 301). AQI values of 301 or higher are calculated
with 1-hour 03 concentrations.

3	1-hr S02 concentrations do not define higher AQI values (>200). AQI values of 200 or greater are
calculated with 24-hour S02 concentration.

4	AQI values between breakpoints are calculated using equation 1 to this appendix. For AQI values in the
hazardous category, AQI values greater than 500 should be calculated using equation 1 and the
concentration specified for the AQI value of 500. The AQI value of 500 are as follows: 03 1-hour—0.604
ppm; PM2.5 24-hour—325.4 ng/m3; PM10 24-hour—604 ng/m3; CO ppm—50.4 ppm; S021-hour—1004
ppb; and I\I021-hour—2049 ppb.

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Using the table and the equation and concentration data to calculate the AQI

Suppose you have an 8-hour 03 value of 0.07853333. First, truncate the value to 0.078. Then refer to the
8-hour 03 in table 5 for the values that fall above and below your value (0.071-0.085). In this case, the
0.078 value falls within the index values of 101 to 150. Now you have all the numbers needed to use
equation 1.

150-101	49

(0.78 - 0.071) + 101 = ——(0.007) + 101 = 125.5 = 126

0.085 — 0.071	'	0.14

An 8-hour 03 value of 0.07853333 corresponds to an index value of 126.

How to handle values from multiple pollutants

Suppose you have an 8-hour 03 value of 0.078 ppm, a PM25 value of 35.9 ng/m3, and a CO value of 8.4
ppm. You apply the equation 3-times:

150 - 101

	r-r^T(0.78- 0.071) + 101 = 126

3 0.085 - 0.071 v	y

150 - 101

pm2.s ¦ 55 4_ 35 5 (35.9 - 35.5) + 101 = 102

100 - 51

CO: —	— (8.4 - 4.5) + 51 = 90

9.4 - 4.5

The AQI is 126, with 03 as the main pollutant.

Using both 031-hour and 8-hour values

You must calculate the 8-hour values, and you may also calculate the 1-hour values. If you calculate
both, you must report the higher AQI value.

Suppose you had a 1-hour value of 0.162 ppm and an 8-hour value of 0.078 ppm. Then you apply the
equation twice:

150 - 101

1 - hour: ——		——(0.162 - 0.125) + 101 = 148

0.164- 0.125 v	y

150 - 101

8 - hour: 			(0.078 - 0.071) + 101 = 126

0.085 - 0.071 v	y

In this case, the index is 148 (the maximum of 148 and 126) and the main pollutant is 03.

Calculating AQI values for S02

EPA strengthened the primary standard for S02 in 2010. Because there was not enough health
information to inform changing the upper end of the AQI for S02, the upper end continues to use the 24-
hour average S02 concentration. The lower end of the AQI uses the daily max 1-hour S02 concentration.

If you have a daily max 1-hour S02 concentration below 305 ppb, then use the breakpoints in Table 6 to
calculate the AQI value.

15

-------
If you have a 24-hour average S02 concentration greater than or equal to 305 ppb, then use the
breakpoints in Table 6 to calculate the AQI value. If you have a 24-hour value in this range, it will always
result in a higher AQI value than a 1-hour value would.

On rare occasions, you could have a day where the daily max 1-hour concentration is at or above 305
ppb but when you try to use the 24-hour average to calculate the AQI value, you find that the 24-hour
concentration is not above 305 ppb. If this happens, use 200 for the lower and upper AQI breakpoints
(lLo and lHi) in Equation 1 to calculate the AQI value based on the daily max 1-hour value. This effectively
fixes the AQI value at 200 exactly, which ensures that you get the highest possible AQI value associated
with your 1-hour concentration on such days.

How to handle concentrations for pollutants that have blank places in the table
for Breakpoints for the AQI

Disregard numbers that have blank places in the table 6: Breakpoints for the AQI. Suppose you had a 1-
hour 03 value of 0.104 ppm and an 8-hour 03 value of 0.078 ppm. First you disregard the 1-hour 03
value because it is less than 0.125ppm. Then you calculate the index for the 8-hour 03 value as before:

150 - 101

¦(0.078 - 0.071) + 101 = 126

0.085 - 0.071

How the NowCast is calculated

EPA updated the NowCast method for PM2.s on August 1, 2013, and for PMi0 on December 9, 2014. The
same method is used for both PM25 and PMi0. It was designed to be more responsive than the previous
method in rapidly changing air quality conditions, such as those we see during fire events. The PM
NowCast is an average of the previous 12 hours. When air quality is stable, the hours are weighted more
evenly (approaching a 12-hour average). When air quality is variable, the most recent hours are
weighted more (approaching a 3-hour average). To calculate the PM NowCast, use the past 12 hours of
PM measurements in micrograms per cubic meter (ng/m3):

1. Select the minimum and maximum PM measurements.

2. Subtract the minimum measurement from the maximum measurement to get the range.

3. Divide the range by the maximum measurement in the 12-hour period to get the scaled rate of
change.

4. Subtract the scaled rate of change from 1 to get the weight factor. The weight factor must be
between 0.5 and 1. The minimum limit approximates a 3-hour average.

5. If the weight factor is less than 0.5, then set it equal to 0.5.

6. Multiply each hourly measurement by the weight factor raised to the power of the number of
hours ago the value was measured (for the current hour, the factor is raised to the zero power).

7. Compute the NowCast by summing the products from Step 6 and dividing by the sum of the
weight factor raised to the power of the number of hours ago each value was measured.

-------
8. Convert this value to an AQI. A "concentration to AQI" converter is available
at https://airnow.gov/aqi/aqi-calculator-concentration.

Missing data: 2 of the last 3 hours of data must be valid for a NowCast calculation.

EPA implemented the latest NowCast method for ozone on August 1, 2019. The ozone NowCast method
is based on the relationship of 1-hour and 8-hour data over the most recent two-week rolling window. It
is designed to rise and fall with the 1-hour concentration and still be representative of an 8-hour
average. A paper describing the method and accompanying R code is available at
https://github.com/USEPA/03-NowCast/tree/master.

17

-------
V. Accessing AQI Data

AirNow Website

AirNow.gov is the one-stop source for air
quality data. The website highlights air
quality at the local level first, while also
providing air quality information at the
state, national, and world level. The AirNow
Dial allows users to quickly see what their
current air quality profile is where they
live—or anywhere they want to check. It is
available immediately on the AirNow home
page. There are also menu items to help find
more information and resources about air
quality, health, and much more.

https://www.airnow.gov/

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AirNow Fire and Smoke Map

The Fire and Smoke Map, a joint project
between the U.S. Forest Service and the
EPA, is AirNow's mapping tool specifically
for fires and the smoke they bring. The Fire
and Smoke Map displays information on
ground level air quality monitors recording
fine particulates (PM2.5) from smoke and
other sources using both federal monitors
and thousands of sensors, giving most
users the nearest readings possible and
recommendations on how to protect their

health. It also includes information on fire locations, smoke plumes, and special announcements related
to fires from the LISFS.

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https://fire.airnow.gov/

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-------
AirNow Mobile App (Android and iOS)

EPA's AirNow mobile app provides a simple interface for quickly checking
current and forecast air quality information for planning daily activities
and learning how to protect health. The app automatically displays the
current AQI for the local area and allows the ability to store multiple areas
for quick reference. The app also includes the AirNow interactive map and
the AirNow Fire and Smoke map.

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AirNow EnviroFlash

AirNow's EnviroFlash sends air quality information for
the selected location to any email or mobile phone.

These forecasts and alerts allow people to take air quality
into consideration when making outdoor plans.

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know when it's a good time to be active outside.

To start getting air quality messages today, or to update
your EnviroFlash account, enter your email here:

Enter your email address here

EnviroFlash is a partnership between the US EPA and
your state or local air quality agency.

19

-------
AirNow Widget Website

AirNow widgets allow organizations to show air quality
information to the people they want to reach on their own
website in various formats.

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-------
AirData Website

AirData provides air quality data
collected at outdoor monitors
across the United States, Puerto
Rico, and the U.S. Virgin Islands.
Users can download, output, view
or visualize the data.

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AirCompare Website

AirCompare maps provide
information for counties in the
United States that monitored
outdoor air quality in recent
years, and tailor that
information for groups more
likely to be affected by different
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21

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VI. Frequently Asked Questions

Q. Can AirNow help me meet the reporting requirements for the AQI?

A. Yes, AirNow is one way you can submit, store, and display your AQI values. This includes the required
elements of AQI reporting and the voluntary elements. The information you submit to airnow.gov is
reported in national and state/local pages on the website. It is available to anyone through the AirNow
app, AirNow widgets, and Enviroflash emails. It is also available for distribution to media and weather
service provider companies.

Q. Why doesn't my area report an Air Quality Index value?

A. Towns and cities with 350,000 or fewer inhabitants are not required to report the AQI. Some may not
have monitors. In addition, AirNow is a voluntary program and some state or local air quality monitoring
agencies may not submit data to AirNow.

Q. The other day, the air quality in my area was reported as green, or good air
quality. However, it was pretty hazy outside. Why didn't the AQI report this
accurately?

A. If this was an AQI forecast, there are still a few areas of the United States that only forecast for ozone
and not particle pollution. It is possible that the ozone AQI forecast was "GOOD" while the hazy
conditions experienced were due to particle pollution. In this instance, the reported AQI forecast may
have only represented ozone.

If this was a real-time air quality report, it may be because there are only ozone monitors nearby, which
were reading that ozone was in the "GOOD" category while the hazy conditions experienced were due
to particle pollution.

There are also occasions where hazy conditions may be due primarily to high humidity and not pollution.
On these days, it is still good to check the AQI maps and forecasts to make sure that pollution is not the
primary cause of the haze.

Q. It looks smoky outside, but the AirNow dial is reporting green or yellow air
quality. How can that be right?

A. During smoky condition, particle pollution levels can vary considerably, even over small distances and
short periods of time. This is particularly true when smoke plumes move in and out of an area. As a
result, monitors in the same area may show different NowCast AQI categories. The main AirNow
website currently does not include sensor information and cannot always account for these rapidly
changing conditions. The AirNow Fire and Smoke Map, which EPA operates in partnership with the U.S.
Forest Service, does include sensor data, which updates more frequently. The Fire and Smoke Map also
shows smoke plumes from National Oceanic and Atmospheric Administration satellites to give the public
information on how far smoke has traveled. Note that smoke plume images on the map cannot tell you
whether the smoke is high in the atmosphere or at ground level.

-------
If it looks smoky where you are, check your air quality on the AirNow Fire and Smoke Map. You can click
on a colored icon to get additional information, including steps you can take to reduce your pollution
exposure. The map is available at https://fire.airnow.gov, or by clicking the "Smoke "icon on the lower
right of the free AirNow smartphone app.

Q. What does it mean when the AQI values are above 500?

A. All AQI values above 300 are part of the Hazardous category. This includes values above 500 and the
hazardous category information should be used. The 500 breakpoint of the AQI is used to calculate the
AQI values in the hazardous category (AQI values above 300), with AQI values above 500 based on the
same linear slope as the AQI values between 301 and 500.

Q. What is the U.S. AQI and how is it different than some AQIs I see reported?

A. The U.S. AQI is the official Air Quality Index for the United States. The U.S. AQI was developed to
provide a uniform scale for communicating air quality - so no matter where you are in the country, you
know what the AQI categories mean. The U.S. AQI is what you see on AirNow.gov, the AirNow app, and
on the AirNow Fire and Smoke Map, on most state and local air quality websites, and in many private
applications.

EPA reassesses the U.S. AQI for specific pollutants as new science becomes available, always doing so
through a transparent process that gives the public the opportunity to comment. The Agency does not
assess private air quality indexes; please contact the provider if you have questions about what those
indexes are based on.

Q. If the AQI information reported in the local media is incorrect, what should I
do?

A. Common problems with AQI information reporting in the local media include reporting inaccurate
cautionary statements and health information, reporting data values that are wrong or reporting
pollutant concentrations instead of the AQI. Another frequent mistake is to report inconsistent AQI
colors or terminology, as well as incorrect pollutant names. To minimize potential problems establish a
good working relationship with your local media and educate them about how using the correct
information can be helpful to their audiences, and how misleading or erroneous AQI information can be
harmful. If you spot a significant error in a weathercast or news story, don't wait until the next day to
request a correction. Reach out quickly.

Q. Why doesn't the U.S. AQI cover toxic air pollutants or air toxics?

A. While the AQI is an excellent indicator of the air quality resulting from ozone, particulate matter,
carbon monoxide, nitrogen dioxide and sulfur dioxide, it does not directly include health implications
from all air pollutants such as air toxics. Levels of air toxics that would pose adverse health effects
generally do not occur episodically on a regional scale, like ozone and particulate matter; therefore, air
toxics are monitored and evaluated differently. For information on air toxics and risk, visit EPA's Air
Toxics Screening Assessment. For information on air toxics monitoring, visit:
https://www.epa.gov/amtic/air-toxics-ambient-monitoring

-------
Q. Why does AirNow provide AQI forecasts only for ozone and particle pollution?

A. AQI reporting is required for all criteria pollutants when they have an index value of 50 or above, but
forecasting the AQI is not required. Most cities forecast for ozone and particle pollution as these
pollutants are the major sources of unhealthy air quality around 99% of the time. However, several cities
forecast for all five pollutants: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide,
and nitrogen dioxide.

Q. What is the NowCast and what does it have to do with the AQI?

A. The NowCast is EPA's method for relating short-term (less than 24-hour) data to the AQI for the
purposes of real-time reporting. The AQI is based on daily air quality summaries, specifically daily
maximums or daily averages. It is not valid to use shorter-term (e.g., hourly) data to calculate an AQI
value. However, the real-time reporting of the NowCast requires shorter-term data to caution people in
time for them to reduce their 24-hour exposure. Learn more about the NowCast.

Q. When calculating the AQI for ozone, which 8-hour periods should be used?

A. When the ozone standard was revised in 2015, the data handling for the daily max 8-hour average
was modified to use only the 8-hour averages starting at 7am. This was done to avoid double counting
an exceedance from a single, short-term episode that spans the nighttime hours of the first day into the
early hours of the second day. The daily maximum 8-hour average used for computing the AQI value is
the same daily maximum 8-hour average described in the data handling for the revised ozone standard
(i.e., it is based on the 17 consecutive moving 8-hour periods in each day, beginning with the 8-hour
period from 7am to 3pm, and ending with the 8-hour period from 11pm to 7am).

Previous 8-hour ozone standards

Day
1

12 1 2

6 7 f

I 9 10 11 12 1 2
pm

7 i

I 9 10 11

Day

• Previous ozone standards considered 24 overlapping 8-hour averages

• The first 8-hour average began at 12am and ended at 7:59 am and the last began at 11pm and ended at 6:59 am of the next day.

• Because the 8-hour averages for each day started at 12am, the hours from 12am to 6:59am were "double counted."

• This double-counting could result in daily exceedances on different days influenced by the same hours.

New (2015) 8-hour ozone standards

Day
0

12 1 2 3
am

Day
1

7 S

J 9 10 11 12 1 2
pm

7 I

J 9 10 11 12 1
am

Day
2

7 f

} 9 10

• The new ozone standards eliminate double counting by starting the daily averages at 7am every day.

• The first 8-hour average begins at 7am and ends at 2:59 pm and the last begins at 11pm and ends at 6:59 am of the next day.

-------
Q. Should I use particulate matter or particle pollution when speaking with the
public?

A. Based on focus group testing by EPA, people better understand and prefer the term "particle
pollution" than "particulate matter."

Q. Why are some people using low-cost, compact sensors to measure air quality?

A. Many citizens are interested in learning more about local air quality where they live, work, and play.
Low-cost, compact, sometimes-portable sensors are becoming more popular for collecting real-time air
quality data. EPA scientists created the Air Sensor Toolbox for Citizen Scientists to provide information
about sensor performance and how to interpret the data from sensors.

Learn more at http://www.epa.gov/air-research/air-sensor-toolbox-citizen-scientists.

In addition, EPA and the U.S. Forest Service use data from sensors on the AirNow Fire and Smoke Map.
Before sensor data appears on the map, an EPA scientific correction equation is applied so sensor data
can be compared to data from permanent monitors.

Q. Can I apply the AQI colors to realtime data from low-cost air quality sensors?

A. As a general matter, EPA does not advise applying AQI values to realtime sensor data, especially 1-
minute readings. Health studies do not tell us what a single minute of exposure to a pollutant may
mean. The AQI is based on EPA's national air quality standards, which come from health studies that
show the effects of longer exposures.

Q. What is the ozone monitoring season for each state?

A. EPA requires ozone monitoring during the time of year when weather conditions are most favorable
for ozone formation. This season varies by state. In some states with warmer climates, monitoring is
required year-round. In states where the climate is colder, ozone monitoring is required for as little as
five months during the summertime. You can find a list of ozone monitoring seasons by state in 40 CFR
Part 58 Appendix D. Table D-3.

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VII. ADDITIONAL RESOURCES

AQI Final Rule 64 FR 42530, August 4 1999: https://www.airnow.gov/publications/air-quality-index/air-
quality-index-reporting-final-rule/

List of monitoring season by state in 40 CFR Part 58 Appendix D, Table D-3:

https://www.ecfr.gov/current/title-40/chapter-l/subchapter-C/part-58/appendix-

Appendix%20D%20to%20Part%2058

AirNow's Wildfires Page: https://www.airnow.gov/wildfires/

Air quality outreach materials in Spanish: https://www.airnow.gov/spanish-resources/

Basic information on the AQI in Spanish: https://www.airnow.gov/aqi/aqi-basics-in-spanish/

EPA's Air Quality Flag Program: https://www.airnow.gov/air-quality-flag-program/

Air Sensor Toolbox for Citizen Scientists: http://www.epa.gov/air-research/air-sensor-toolbox-citizen-
scientists

EPA's Air Toxics Screening Assessment: https://www.epa.gov/AirToxScreen

26

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United States
Environmental Protection
Agency

Office of Air Quality Planning and	EPA-454/B-24-002

Standards	May 2024

Air Quality Assessment Division
Research Triangle Park, NC

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