Introduction and how to use the U.S. Air Quality Index calculator
The U.S. Air Quality Index, or AQI, converts measured pollutant concentrations into a shared health-communication scale. It is not a direct concentration measurement. Fine particles, ozone and combustion gases use different units and averaging periods, so their raw values cannot be compared directly. EPA breakpoint tables map each concentration to a pollutant sub-index, and the highest available sub-index becomes the reported AQI.
This calculator covers PM2.5, PM10, ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide. It uses the revised PM2.5 breakpoints that took effect on 6 May 2024. Under those breakpoints, the Good PM2.5 range ends at 9.0 µg/m³. A tool using the older table can therefore produce a lower result for the same air, especially at concentrations near the Good and Moderate boundary.
Enter one or more measured concentrations in the form below. Leave pollutants blank when no measurement is available. PM2.5 and PM10 inputs should represent 24-hour averages or appropriate reported NowCast values. Ozone inputs are daily maximum 8-hour or 1-hour averages. Carbon monoxide uses an 8-hour average, while nitrogen dioxide and the lower part of the sulfur dioxide scale use daily maximum 1-hour readings. The ozone selectors accept either ppm or ppb; the other units are printed beside their fields.
After calculation, the result identifies the headline AQI, health category and responsible pollutant. The breakdown also shows the concentration after EPA-required truncation, the applicable breakpoint segment and each sub-index. The permalink and download controls provide a reproducible record of the entered scenario.
The AQI formulas for interpolation, truncation and the overall maximum
The AQI formula performs linear interpolation inside the breakpoint segment containing the pollutant concentration:
Here is the concentration used in the calculation. The terms and are the concentration endpoints of the selected segment. The matching and values are the AQI endpoints. The segment slope is , so the rate at which AQI changes is not identical across all categories.
Concentrations are truncated before interpolation, not rounded. Ozone is truncated to three decimal places; PM2.5 and carbon monoxide use one decimal place; PM10, SO₂ and NO₂ use whole numbers. For a precision of decimal places, the operation is:
The interpolated sub-index is rounded to the nearest whole number only after the formula is applied:
Finally, the overall AQI is the maximum available pollutant sub-index:
The maximum rule is health-protective. Averaging the pollutant indices could allow a clean ozone reading to conceal hazardous smoke, or a low particle reading to conceal dangerous carbon monoxide. When two pollutants tie after rounding, the underlying unrounded values determine which one is identified as dominant by this calculator.
Current EPA breakpoints for the six AQI pollutants
The table summarizes the current U.S. EPA breakpoint ranges. Blank entries are intentionally undefined. A concentration in a blank part of a pollutant table is disregarded rather than assigned an invented index.
U.S. EPA AQI breakpoints. Particles are in µg/m³, ozone and CO are in ppm, and SO₂ and NO₂ are in ppb.
| AQI | Category | O₃ 8-hr | O₃ 1-hr | PM2.5 | PM10 | CO | SO₂ | NO₂ |
|---|
| 0–50 | Good | 0.000–0.054 | — | 0.0–9.0 | 0–54 | 0.0–4.4 | 0–35 | 0–53 |
|---|
| 51–100 | Moderate | 0.055–0.070 | — | 9.1–35.4 | 55–154 | 4.5–9.4 | 36–75 | 54–100 |
|---|
| 101–150 | Unhealthy for Sensitive Groups | 0.071–0.085 | 0.125–0.164 | 35.5–55.4 | 155–254 | 9.5–12.4 | 76–185 | 101–360 |
|---|
| 151–200 | Unhealthy | 0.086–0.105 | 0.165–0.204 | 55.5–125.4 | 255–354 | 12.5–15.4 | 186–304 | 361–649 |
|---|
| 201–300 | Very Unhealthy | 0.106–0.200 | 0.205–0.404 | 125.5–225.4 | 355–424 | 15.5–30.4 | 305–604 (24-hr) | 650–1249 |
|---|
| 301–500 | Hazardous | — | 0.405–0.604 | 225.5–325.4 | 425–604 | 30.5–50.4 | 605–1004 (24-hr) | 1250–2049 |
|---|
The top published concentration is anchored to an index of 500, but it is not a hard ceiling. EPA guidance continues the slope of the 301–500 segment when a concentration exceeds the last breakpoint. Such results remain in the Hazardous category and are shown as beyond the charted scale.
Worked example: ozone and smoke on a summer afternoon
Consider an 8-hour ozone average of 0.07853333 ppm, PM2.5 of 35.9 µg/m³ and an 8-hour carbon monoxide average of 8.4 ppm. The calculator truncates ozone to 0.078 ppm. That falls in the 0.071–0.085 segment, which maps to AQI 101–150:
The PM2.5 reading already has the required one-decimal precision. It falls in the 35.5–55.4 segment:
Carbon monoxide falls in the 4.5–9.4 ppm segment:
The three sub-indices are 126, 102 and 90. Their maximum is 126, so the headline category is Unhealthy for Sensitive Groups and ozone is responsible. This example illustrates why raw pollutant values should not be compared: 0.078 ppm and 35.9 µg/m³ look unrelated, but their health-scaled indices are close.
Interpreting AQI categories, ozone and sulfur dioxide rules
An AQI from 0 to 50 is Good, while 51 to 100 is Moderate. At 101 to 150, sensitive groups such as children, older adults and people with heart or lung conditions should reduce prolonged outdoor exertion. Values from 151 to 200 are Unhealthy for everyone, 201 to 300 are Very Unhealthy, and anything above 300 is Hazardous. Follow local public-health guidance during elevated conditions, especially when symptoms, wildfire smoke or emergency alerts are present.
Protective measures should match the responsible pollutant. HEPA filtration can reduce indoor particle concentrations but does not remove ozone or carbon monoxide. A well-fitted N95 or P100 respirator filters particles, not gases. During ozone episodes, reducing strenuous afternoon activity may be more useful. Carbon monoxide requires locating and stopping the combustion source; suspected indoor exposure is an emergency rather than a reason to rely on an outdoor AQI calculation.
Ozone has two tables. The 1-hour ozone table is blank below 0.125 ppm, while the 8-hour table stops at 0.200 ppm. When both periods produce valid indices, EPA reports the higher one. Sulfur dioxide also changes procedure: below a 1-hour value of 305 ppb, the 1-hour table applies. At or above 305 ppb, a 24-hour average is needed for an index above 200. If the 1-hour maximum reaches 305 ppb but the supplied 24-hour average is below 305 ppb, the prescribed result is fixed at 200. In formula terms, both and are set to 200.
Related planning tools include the Air Purifier Smoke Clearance Time Calculator, the Indoor Air Quality Improvement Cost Calculator and the Seasonal Allergy Risk Estimator.
Limitations and assumptions of this AQI estimate
This calculator applies the published transformation exactly, but the result can only be as reliable as the measurements. Inputs should be quality-assured concentrations collected over the stated averaging periods. A short PM2.5 spot reading is not interchangeable with a 24-hour average, and an NO₂ concentration in ppm must not be entered into a field expecting ppb.
- NowCast is not calculated. AirNow’s hourly particle reporting uses a weighted 12-hour NowCast that responds to rapid changes. Enter an existing NowCast value when trying to reproduce an AirNow result.
- Sensor quality matters. Low-cost optical particle sensors can be affected by humidity, particle composition, placement and ageing. Corrected official measurements are preferable for health or regulatory decisions.
- Partial data gives a lower bound. A PM2.5-only result is that pollutant’s sub-index. Another unmeasured pollutant could produce a higher official AQI.
- AQI is not personal dose. The index does not account for time spent outdoors, breathing rate, individual susceptibility or cumulative exposure.
- Air toxics are excluded. Benzene, formaldehyde and many other hazardous pollutants are outside the six-pollutant AQI.
- This is the U.S. scale. Other countries use different breakpoints and averaging periods, so similarly numbered international indices may not be equivalent.
Frequently asked questions about U.S. AQI reporting
Which PM2.5 breakpoints are used?
The calculator uses the revised breakpoints effective 6 May 2024. The Good range ends at 9.0 µg/m³, and 325.4 µg/m³ corresponds to AQI 500.
Must every pollutant be entered?
No. At least one value is required, and blank fields are omitted. A partial result may be lower than the AQI produced by a complete monitoring network.
Why is AQI not an average of the sub-indices?
The highest sub-index is reported so that a low concentration for one pollutant cannot conceal a health concern caused by another pollutant.
Can AQI exceed 500?
Yes. The calculator extrapolates using the slope of the final published segment. Every result above 300 remains Hazardous.
Sources for the AQI formulas and breakpoints. The breakpoint ranges, truncation rules, interpolation equation, ozone procedure, sulfur dioxide averaging rule and above-500 guidance are based on the U.S. Environmental Protection Agency’s Technical Assistance Document for the Reporting of Daily Air Quality — the Air Quality Index (AQI), Table 6 and section IV, and 40 CFR part 58 appendix G. See the EPA technical document and AirNow AQI Basics. PM2.5 values reflect the revision effective 6 May 2024.
Enter pollutant levels to compute the AQI.