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What is measured—and what is the AQI?
Monitoring equipment measures concentrations of specific pollutants. The AQI is a communication scale that maps those concentrations to an index number and health category; it is not a concentration unit or a direct reading from a single sensor.
The U.S. AQI covers five pollutant groups: ozone, particle pollution (including PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. AirNow says its maps and AQI readings currently show ozone, PM10, and PM2.5, so a particular AirNow display should not be assumed to show all five groups. See AirNow’s AQI Basics and About the Data.
AirNow uses six categories: Good (0–50), Moderate (51–100), Unhealthy for Sensitive Groups (101–150), Unhealthy (151–200), Very Unhealthy (201–300), and Hazardous (301 and higher). These ranges describe the AQI scale, not pollutant concentrations.
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How monitoring turns outdoor air into concentration data
Agencies collect pollutant-specific samples
State, local, and tribal monitoring agencies collect measurements using EPA-approved federal reference or equivalent methods. Sites and instruments are selected for the pollutants being monitored; there is not one universal sensor technique that measures every pollutant. AirNow describes the public data it shares as preliminary and quality-checked, rather than fully validated regulatory records.
Particle monitors use different methods from gas monitors
For particulate matter, one reference approach draws air through a size-selective inlet and collects particles on a filter, which can later be weighed. A continuous beta attenuation monitor collects particles on filter tape and estimates particle mass concentration from changes in beta-ray transmission. These are examples of particle-monitoring methods, not descriptions of how ozone or carbon monoxide is measured. The EPA’s forecasting guidance discusses these particle measurement approaches.
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Instrument output is processed into concentration
Applicable methods and quality procedures convert instrument output into pollutant concentration values in the relevant units. The AQI calculation uses those concentrations along with pollutant-specific averaging periods and breakpoint tables. A consumer monitor may display useful information, particularly about particles, but the cited sources do not establish that any particular consumer model is an EPA-approved reference or equivalent monitor for official AQI reporting.
How the daily AQI is calculated
For each pollutant, the calculation produces a sub-index. The reported AQI is the highest pollutant-specific sub-index for the reporting area, so the pollutant driving the number can change by place and day.
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- Select the concentration. The technical guidance describes using the highest relevant concentration among monitors in the reporting area for each pollutant.
- Apply the prescribed precision. The concentration is truncated to the precision specified for that pollutant.
- Find the matching breakpoints. The truncated concentration is placed between the two concentration breakpoints that surround it, along with their corresponding AQI values.
- Interpolate and round. The index is calculated by linear interpolation between those breakpoint pairs and rounded to the nearest integer.
- Report the highest sub-index. The largest pollutant-specific index becomes the AQI reported for the area.
The calculation sequence is described in AirNow’s technical assistance document. AirNow’s publications page reports that the daily AQI technical assistance document was updated in May 2026. Because breakpoint values and implementation details can change, use the current document for exact values; this guide does not reproduce an older table or calculate a numerical example.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Daily AQI, current NowCast, and forecast: what is the difference?
| Information | Purpose and time frame | What it represents |
|---|---|---|
| Daily AQI | Summarizes daily air quality; AirNow describes an 8-hour ozone index and a 24-hour particle pollution index. | Calculated from monitor concentrations using pollutant-specific breakpoints. |
| NowCast or current AQI | Describes the latest available conditions using recent hourly readings and multiple previous hours. | A current public reading for ozone and particle pollution; its averaging window can be longer when conditions are stable and shorter when concentrations change quickly. |
| AQI forecast | Predicts expected conditions, often for the next day. | A forecast informed by observations and other inputs, not a measurement of future air. |
| Regulatory AQS data | Supports regulatory analysis and decisions. | Records held in EPA’s Air Quality System and subject to the full validation and certification process described by AirNow. |
AirNow explains the distinctions among daily reporting, NowCast, and forecasts in Using the Air Quality Index. A NowCast can differ from a daily AQI because it emphasizes more recent hourly readings, while a forecast estimates what conditions may be.
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How forecasts are made
Forecasters combine monitoring observations with weather forecasts, satellite information, transport models, and local expertise to estimate expected conditions. Those inputs help predict how pollutants may change; they do not turn a forecast into a measurement. AirNow’s AQI guidance describes forecast inputs and time frames.
Why an AirNow reading may differ from a regulatory record
AirNow provides preliminary, checked public AQI data for timely reporting. EPA’s Air Quality System (AQS) is the regulatory database, where data go through the full validation and certification process described by AirNow. The two serve different purposes and should not be treated as interchangeable records. AirNow’s data explanation also identifies which pollutants its maps and readings currently display.
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