What Is the AQI? The Scale, How It's Calculated, and How EPA Changed It
EPA data through 2025 · Updated September 28, 2026 · Methods
The Air Quality Index (AQI) is EPA's answer to a simple question: how healthy is the air where you are today? It gives the answer as a single number from 0 to 500. The higher the number, the more pollution and the greater the health concern.
- It covers five pollutants: ozone, particle pollution (fine and coarse), carbon monoxide, sulfur dioxide and nitrogen dioxide. Each gets its own score, and the day's AQI is the highest one, not a total.
- Above 100 is unhealthy. 100 generally marks the pollutant's short-term health standard. 101 to 150 is Unhealthy for Sensitive Groups; higher scores are Unhealthy, Very Unhealthy and Hazardous.
- EPA has made the scale stricter three times since 2012. It lowered the pollution level that counts as 100 for fine particles (2012) and ozone (2015), and in 2024 lowered the level that counts as Good for fine particles. The same air can now score higher than it used to, so today's numbers and older ones are not directly comparable.
How U.S. air rated in 2025: shares of all 170,306 metro-days in EPA's published daily AQI for 498 metropolitan and micropolitan areas. A metro-day is one area on one day, rated by its highest-scoring monitor.
The AQI scale: six categories
Each band has a name, a color, and a plain-language description. The descriptions below are AirNow's, quoted as written. When an area forecasts the orange or red band, local agencies often announce it as a “Code Orange” or “Code Red” air quality action day.
| AQI | Category | What AirNow says | U.S. metro-days, 2025 |
|---|---|---|---|
| 0-50 | Good Green |
Air quality is satisfactory, and air pollution poses little or no risk. | 118,741 |
| 51-100 | Moderate Yellow |
Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution. | 49,380 |
| 101-150 | Unhealthy for Sensitive Groups Orange |
Members of sensitive groups may experience health effects. The general public is less likely to be affected. | 1,718 |
| 151-200 | Unhealthy Red |
Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects. | 385 |
| 201-300 | Very Unhealthy Purple |
Health alert: The risk of health effects is increased for everyone. | 36 |
| 301 and higher | Hazardous Maroon |
Health warning of emergency conditions: everyone is more likely to be affected. | 46 |
Categories and descriptions: AirNow, AQI Basics. Action-day practice: AirNow, Action Days, which notes that some places call action days at Code Orange and many at Code Red.
How the AQI is calculated
The day's AQI is the highest of the individual pollutant scores. Each score comes from measurements taken over the time period that pollutant's health standard uses. Four steps:
- Measure each pollutant over its own time window. The windows differ because each health
standard is written for a different length of exposure; for particles, EPA notes that “health research
for PM measures exposure over a 24-hour period.” The AQI uses the same window as the standard, so a
score of 100 lines up with it.
- Ozone: the day's highest 8-hour average
- Fine particles (PM2.5): the 24-hour average
- Coarse particles (PM10): the 24-hour average
- Carbon monoxide: the highest 8-hour average
- Sulfur dioxide: the highest 1-hour value (the 24-hour average at AQI 200 and above)
- Nitrogen dioxide: the highest 1-hour value
- Drop the extra digits; don't round. EPA truncates the concentration, the amount of pollutant in the air, to a fixed number of digits. Ozone keeps three decimal places in parts per million (ppm). Fine particles keep one decimal place in micrograms per cubic meter (µg/m³). Coarse particles keep a whole number. Carbon monoxide keeps one decimal place in ppm. Sulfur dioxide and nitrogen dioxide keep whole parts per billion (ppb). This matters right at a category line: an ozone average of 0.0709 ppm counts as 0.070, which scores 100, Moderate. Rounding would make it 0.071 and score 101, Unhealthy for Sensitive Groups.
- Convert the concentration to a score. EPA's breakpoint table (below) pairs concentration
ranges with AQI ranges. Find the range that holds the concentration. Inside it, the score follows a straight
line between the two ends: a concentration halfway through the range gets a score halfway through its AQI
range. The result is rounded to a whole number.
AQI = (Ihi − Ilo) ÷ (Chi − Clo) × (C − Clo) + IloC is the concentration; Clo and Chi are the ends of its range in the table; Ilo and Ihi are the AQI values at those ends.
- Take the highest score. The pollutant with the highest score is the “main pollutant,” and its score is the day's AQI. Scores are never added together. Fine and coarse particles are scored separately, so a monitor can produce up to six scores.
Most monitors measure only some of the six. In 2025, 1,209 monitoring sites reported an ozone AQI, 1,043 fine particles, 624 coarse particles, 436 nitrogen dioxide, 372 sulfur dioxide and 219 carbon monoxide. Only 63 reported all six. The daily AQI EPA publishes for a metro area is the highest AQI among its monitors.
The two breakpoint tables that matter most
Ozone and fine particles set the AQI on nearly every day: together they were the main pollutant on 96% of U.S. metro-days in 2025.
| AQI band | Ozone, 8-hour (ppm) | Fine particles (PM2.5), 24-hour (µg/m³) |
|---|---|---|
| 0–50 | 0.000–0.054 | 0.0–9.0 |
| 51–100 | 0.055–0.070 | 9.1–35.4 |
| 101–150 | 0.071–0.085 | 35.5–55.4 |
| 151–200 | 0.086–0.105 | 55.5–125.4 |
| 201–300 | 0.106–0.200 | 125.5–225.4 |
| 301+ | 1–hour value used | 225.5+ |
Breakpoints for the other four pollutants
| AQI band | Coarse particles (PM10), 24-hour (µg/m³) | Carbon monoxide, 8-hour (ppm) | Sulfur dioxide, 1-hour (ppb) | Nitrogen dioxide, 1-hour (ppb) |
|---|---|---|---|---|
| 0–50 | 0–54 | 0.0–4.4 | 0–35 | 0–53 |
| 51–100 | 55–154 | 4.5–9.4 | 36–75 | 54–100 |
| 101–150 | 155–254 | 9.5–12.4 | 76–185 | 101–360 |
| 151–200 | 255–354 | 12.5–15.4 | 186–304 | 361–649 |
| 201–300 | 355–424 | 15.5–30.4 | 305–604 (24–hour) | 650–1249 |
| 301+ | 425+ | 30.5+ | 605+ (24–hour) | 1250+ |
Breakpoints: EPA, Technical Assistance Document for the Reporting of Daily Air Quality (May 2026), Table 6. Above AQI 300, ozone is scored from its 1-hour value; from AQI 200 up, sulfur dioxide is scored from its 24-hour average.
Two days, the same AQI, different air
An AQI of 119 can describe a day led by ozone or a day led by fine particles. Two days at one monitor show it. The monitor is Rubidoux, in Riverside County, California, one of only 63 sites that reported all six scores in 2025 (the five pollutants, with particles at both sizes), so every score can be compared at the same place. EPA's daily metro data show its metro area, Riverside-San Bernardino-Ontario, with 136 days above AQI 100 in 2025, more than any other U.S. metro.
Each table shows all six scores for the day. The box below it works through the arithmetic for the pollutant that set the AQI.
May 22, 2025: AQI 119, set by ozone
| Pollutant | Measured as | Reading | Score |
|---|---|---|---|
| Ozone: main pollutant | highest 8-hour average | 0.076 ppm | 119 |
| Fine particles (PM2.5) | 24-hour average | 13.9 µg/m³ | 60 |
| Coarse particles (PM10) | 24-hour average | 42 µg/m³ | 39 |
| Nitrogen dioxide | highest 1-hour value | 29.1 ppb | 27 |
| Carbon monoxide | highest 8-hour average | 0.4 ppm | 5 |
| Sulfur dioxide | highest 1-hour value | 0.5 ppb | 0 |
Ozone: 0.076 ppm falls in the 101–150 range (0.071–0.085 ppm).
(150 − 101) ÷ (0.085 − 0.071) × (0.076 − 0.071) + 101 = 118.5,
which rounds to 119: Unhealthy for Sensitive Groups.
December 13, 2025: AQI 119, set by fine particles (PM2.5)
| Pollutant | Measured as | Reading | Score |
|---|---|---|---|
| Fine particles (PM2.5): main pollutant | 24-hour average | 42.7 µg/m³ | 119 |
| Coarse particles (PM10) | 24-hour average | 67 µg/m³ | 57 |
| Nitrogen dioxide | highest 1-hour value | 40.0 ppb | 38 |
| Ozone | highest 8-hour average | 0.035 ppm | 32 |
| Carbon monoxide | highest 8-hour average | 1.2 ppm | 14 |
| Sulfur dioxide | highest 1-hour value | 1.1 ppb | 1 |
Fine particles (PM2.5): 42.7 µg/m³ falls in the 101–150 range (35.5–55.4 µg/m³).
(150 − 101) ÷ (55.4 − 35.5) × (42.7 − 35.5) + 101 = 118.7,
which rounds to 119: Unhealthy for Sensitive Groups.
All twelve scores match the AQI values EPA published for this monitor on these days (EPA AirData daily files, site 06-065-8001).
What this shows. The same AQI can describe very different air. On May 22, ozone set the score and fine particles were at 60. On December 13, fine particles set it and ozone was at 32. Because the AQI reports only the highest score, the number alone doesn't say which pollutant it is about or where the others stood. Two smaller points from the tables: a score of 0 does not mean none of the pollutant was present (sulfur dioxide measured 0.5 ppb on May 22, which truncates to 0 and scores 0), and the readings record what was in the air, not what caused it.
One number for five pollutants
The two Rubidoux days follow the national pattern. In 2025, fine particles set the AQI on 51% of U.S. metro-days and ozone on 45%, but on the days above 100 the order flipped:
| Main pollutant | All metro-days | Share | Days above 100 | Share |
|---|---|---|---|---|
| Ozone | 77,405 | 45.5% | 1,124 | 51% |
| Fine particles (PM2.5) | 86,521 | 50.8% | 797 | 36% |
| Coarse particles (PM10) | 5,576 | 3.3% | 255 | 12% |
| Nitrogen dioxide | 584 | 0.3% | 9 | <1% |
| Carbon monoxide | 220 | 0.1% | 0 | 0% |
The consequence for anyone reading AQI statistics: a count of unhealthy days cannot tell you which pollutant changed. A year with fewer days led by fine particles and more led by ozone can post the same total.
EPA has changed the scale
All three changes made the scale stricter: each lowered the amount of pollution that earns a given score, so the same air now scores the same or higher. When EPA tightens a health standard, it usually redraws the AQI scale to match. It moved AQI 100 for fine particles in 2013 and for ozone in 2015. The 2024 change lowered the top of Good for fine particles but left 100 where it was:
| Change | Before | After |
|---|---|---|
| Fine particles, rule of December 2012, in effect 2013 | AQI 100 at 40.4 µg/m³; Good up to 15.4 | AQI 100 at 35.4 µg/m³; Good up to 12.0 |
| Ozone, rule of October 2015 | AQI 100 at 0.075 ppm | AQI 100 at 0.070 ppm |
| Fine particles, in effect May 6, 2024 | Good up to 12.0 µg/m³ | Good up to 9.0 µg/m³; upper bands redrawn; AQI 100 unchanged |
The air did not change when the tables did. A day with 0.072 ppm of ozone was Moderate in 2014 and is Unhealthy for Sensitive Groups today. Scoring the same 2025 monitor readings both ways shows how large the effect is:
Each change was a deliberate policy choice, made when EPA revised the health standard. Where it causes trouble is comparison over time: a chart of unhealthy days that runs across 2013 or 2015 mixes two measurements. Is air quality getting better or worse? scores the air in 35 major metros both ways for every year from 2000 to 2025.
What the AQI is good for, and what it is not
- Good for today's decisions. EPA calls it its “color-coded tool for communicating air quality to the public”: one number and one color for today's conditions.
- Not good for trends across years. The scale has moved, so a longer chart of AQI days mixes rulebooks unless it is rescored on one.
- Weak for comparing places. A metro's AQI is its highest-scoring monitor that day, so areas with more monitors, or monitors that run more days, have more chances to log a bad day.
- Not good for seeing what changed. One number hides which of the five pollutants moved. For that, look at each pollutant's concentration, as in why ozone stopped improving.
Why AirNow and sensor maps can show a different AQI
AirNow, EPA's air quality website and app, shows an hourly estimate that can differ from the official daily AQI. Called the NowCast, it describes current air quality on the AQI scale using the most recent hour available. EPA uses separate NowCast calculations for ozone and for particle pollution. AirNow also shows forecasts, which are different again: predictions of the daily AQI, usually issued in the afternoon for the next day.
The official daily AQI for particles uses a 24-hour average, so it can't be calculated until the day is over. The particle NowCast uses the past 12 hourly measurements and gives the most recent hours more weight when levels change quickly. A reading at 3 p.m. and the day's final AQI can differ for that reason alone.
The source of a reading matters too. AirNow's Fire and Smoke Map adds readings from PurpleAir sensors, most of them owned and operated by members of the public, which EPA and the Forest Service check and correct before showing. The daily AQI comes from regulatory monitors run by air quality agencies. The two don't always agree: EPA notes that sensors typically underestimate dust, so during a dust event sensors can read lower than monitors.
Sources: AirNow, Using the AQI; EPA NowCast fact sheet; AirNow Fire and Smoke Map questions and answers.
Frequently asked questions
What is a good AQI?
0 to 50. AirNow calls that range Good: "Air quality is satisfactory, and air pollution poses little or no risk." 51 to 100 is Moderate.
What AQI is unhealthy?
Above 100. 101 to 150 is Unhealthy for Sensitive Groups, 151 to 200 is Unhealthy, 201 to 300 Very Unhealthy, and 301 and above Hazardous.
Is an AQI of 100 bad?
100 is the top of the Moderate range. For each pollutant, an AQI of 100 generally corresponds to the level of its short-term national health standard, so 100 means the air is at about that level, not above it.
What does Code Orange mean?
An AQI forecast of 101 to 150, Unhealthy for Sensitive Groups. Some areas call an air quality action day at Code Orange; AirNow's advice is for people sensitive to the pollutant to reduce prolonged or heavy exertion outdoors.
What does Code Red mean?
An AQI forecast of 151 to 200, Unhealthy. AirNow says many places call action days at this level and that everyone should reduce exposure, especially sensitive groups.
What pollutants are in the AQI?
Five: ground-level ozone, particle pollution (fine PM2.5 and coarse PM10, scored separately), carbon monoxide, sulfur dioxide and nitrogen dioxide. The day's AQI is the highest of their scores, not a sum.
Did EPA change the AQI?
Yes. It moved the line for 100 for fine particles (effective 2013) and for ozone (2015), and in May 2024 lowered the top of the Good range for fine particles from 12.0 to 9.0 µg/m³. The same air can score higher today than it would have before. How much, metro by metro.
Why does AirNow show a different AQI from the daily number?
AirNow, EPA's air quality website and app, shows the NowCast, an estimate for the most recent hour, alongside forecasts for the next day. The official daily AQI for particles uses a 24-hour average, which can't be calculated until the day is over. The particle NowCast is a weighted average of the past 12 hourly readings, so a mid-afternoon reading and the day's final AQI can differ.
Sources
- EPA AirData, daily AQI by CBSA and daily monitor files, 2025 (files last updated July 13, 2026).
- AirNow, AQI Basics and Action Days.
- EPA, Final Updates to the AQI for Particulate Matter (2024); breakpoint history in our methods, including 40 CFR Part 58, Appendix G.
- EPA, PM NowCast fact sheet; AirNow, Using the AQI; AirNow, Fire and Smoke Map questions and answers.
- EPA, Technical Assistance Document for the Reporting of Daily Air Quality, May 2026 (breakpoints, truncation rules, equation).