The US AQI Is Not a Concentration: 100 Marks the Standard, and Ozone Runs on an 8-Hour Clock

The most common error with the AQI is reading it as a concentration or a percentage. It is neither. **It is an index, and the anchor point is 100.** AirNow puts it plainly: for each pollutant, **an AQI value of 100 "generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for protection of public health."** ★**So 100 is the standard, and values above it mean the air is over that standard** — first for sensitive groups, then for everyone.^[1]

This is why an AQI number cannot be compared directly with a microgram figure. **A PM2.5 reading of 35 µg/m³ and an AQI of 35 are unrelated quantities.** The conversion runs through each pollutant's own standard, which is why **the same AQI value can come from very different concentrations depending on which pollutant is driving it.** ★**This page does not reproduce the breakpoint tables that perform that conversion.**^[1]

There is a US AQI for **five major pollutants regulated by the Clean Air Act: ozone, particle pollution (particulate matter), carbon monoxide, nitrogen dioxide and sulfur dioxide.** Separately, **the index is divided into six categories**, each with its own colour. ★**Five pollutants, six categories** — answers that say "six pollutants" have merged the two counts.^[2][3]

The categories as AirNow lists them are **Good 0 to 50 (green), Moderate 51 to 100 (yellow), Unhealthy for Sensitive Groups 101 to 150 (orange), Unhealthy 151 to 200 (red), Very Unhealthy 201 to 300 (purple), and Hazardous 301 and higher (maroon).** Note that **the orange band is the one where, in AirNow's wording, "the general public is less likely to be affected"** — it is not a general warning.^[1]

A single daily AQI figure hides the fact that **the averaging period is not the same for every pollutant.** AirNow states it directly: **"It varies by pollutant. The ozone AQI is an 8-hour index; for particle pollution, it's 24 hours."** ★**An ozone value therefore describes an eight-hour window inside the day, not the whole day.**^[2]

Forecasts inherit that difference. **For ozone, a forecast "focuses on the period during the day when average 8-hour ozone concentrations are expected to be the highest," while for PM it "predicts the average 24-hour concentration for the next day."** ★**Comparing a morning ozone forecast with a daily PM average as though they were the same measurement is a category error.**^[2]

**EPA has issued a national air quality index since 1976**, and **the AQI in its current form was issued in 1999** and has been updated several times since to reflect newer health-based standards. On availability, **metro areas with a population of more than 350,000 are required to report the daily AQI**, and many smaller areas report it as a public service. ★**So an absent local AQI does not necessarily mean the air was not measured** — it may mean the area is under the reporting threshold.^[2]

★**Not established by this page:** which value is reported when several pollutants are measured at once. **The pages read here do not contain a sentence stating that rule, so this page does not assert one.** ★**Also not read:** the numeric breakpoint tables, the dates and content of the post-1999 updates, the current numeric value of any pollutant standard, and the AQI calculator's formula. **As of 2026-09-23** the AirNow site also carried a wildfire-smoke notice; **this page did not examine any specific fire event.**^[1][2]