The US AQI Is Not a Concentration: 100 Generally Corresponds to the Short-Term Standard, and the Daily Ozone AQI Runs on an 8-Hour Clock

A common mistake is to treat the AQI as a concentration or a percentage. It is neither. It is an index, and the reference point is 100. AirNow puts it this way: 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 generally corresponds to the level of the short-term standard, and AirNow describes values above 100 as unhealthy "at first for certain sensitive groups of people, then for everyone as AQI values get higher." A single AQI reading is a communication tool, not a legal finding that an area meets or violates a standard.^[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 different quantities and cannot be equated directly. Concentrations are converted to the index using each pollutant's own concentration breakpoints (EPA's Table 6, "Breakpoints for the AQI"), which is why the same AQI value can come from very different concentrations depending on the pollutant. This page does not reproduce the breakpoint table.^[1][4]

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 count six pollutants have merged the two counts.^[1][2]

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). The orange band warns sensitive groups: AirNow says "Members of sensitive groups may experience health effects. The general public is less likely to be affected."^[1]

For the daily AQI, 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." A daily ozone value therefore describes an eight-hour period within the day, not the whole day.^[2]

AQI 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." A next-day ozone forecast and a daily PM average are not the same kind of measurement.^[2]

The current air quality shown on AirNow is different again. It uses the NowCast, which AirNow says "shows you air quality for the most current hour available by using a calculation that involves multiple hours of past data." EPA calculates a NowCast for two pollutants, ozone and particle pollution. The 8-hour and 24-hour rule describes the daily AQI and forecasts, not the hourly NowCast figure.^[2]

When more than one pollutant is measured, EPA's technical guidance says "The AQI is the highest value calculated for each pollutant". Its worked example computes 126 for ozone, 102 for PM2.5 and 90 for carbon monoxide, and concludes "The AQI is 126, with O3 as the main pollutant." The main pollutant is the pollutant with the highest AQI value.^[4]

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 the latest health-based air quality standards. On availability, metro areas with a population of more than 350,000 are required to report the daily AQI, and many more 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 reproduced here: the numeric breakpoint table, the dates and content of the post-1999 updates, the current numeric value of any pollutant standard, and the NowCast formula. This page does not examine any specific fire or smoke event.^[1][2][4]