AQI turns several pollutant readings into a single health signal. It is useful only when you know which index, time period and data source you are looking at, because the same air can receive different values under different systems.
What the AQI means
The US Air Quality Index is the Environmental Protection Agency’s scale for outdoor air quality and health risk. The overall AQI is the highest valid pollutant sub-index, not an average of every pollutant. A reading of 120 therefore means that at least one pollutant has reached that level; it does not mean that all of them have.
The six colour-coded categories are:
- 0–50, Good: pollution poses little or no risk.
- 51–100, Moderate: acceptable for most people, although unusually sensitive people may be affected.
- 101–150, Unhealthy for Sensitive Groups: sensitive groups may experience health effects.
- 151–200, Unhealthy: some of the general public may be affected, with more serious effects possible for sensitive groups.
- 201–300, Very Unhealthy: the risk of health effects increases for everyone.
- 301–500, Hazardous: emergency conditions in which everyone is more likely to be affected.
EPA defines an AQI for ground-level ozone, particle pollution, carbon monoxide, nitrogen dioxide and sulphur dioxide. Particle pollution is split into PM2.5 and PM10, so a service may show six pollutant values even though EPA refers to five major pollutants.
EPA averaging periods and current breakpoints
An AQI value is not obtained by placing any instant sensor reading on the scale. EPA first applies the specified averaging period and concentration rules, then converts the result into an index value. The official daily AQI uses daily averages or daily maximums. AirNow uses separate NowCast methods to relate recent monitor readings to the AQI for near-real-time ozone and particle reporting.
The following ranges run from Good through Hazardous. They are grouped by pollutant because a wide table is difficult to read on a phone and can separate a number from its heading.
- PM2.5, 24-hour average in µg/m³: 0–9.0; 9.1–35.4; 35.5–55.4; 55.5–125.4; 125.5–225.4; 225.5–325.4.
- PM10, 24-hour average in µg/m³: 0–54; 55–154; 155–254; 255–354; 355–424; 425–604.
- Nitrogen dioxide, 1-hour average in ppb: 0–53; 54–100; 101–360; 361–649; 650–1249; 1250–2049.
- Carbon monoxide, 8-hour average in ppm: 0–4.4; 4.5–9.4; 9.5–12.4; 12.5–15.4; 15.5–30.4; 30.5–50.4.
These are the current EPA AQS breakpoints. EPA changed the PM2.5 bands in May 2024. The Good category now ends at 9.0 µg/m³ rather than 12.0, and several upper breakpoints also moved lower. The 24-hour PM2.5 air-quality standard itself remained at 35 µg/m³; the revised AQI reflects both the stronger annual standard and newer health evidence.
Ozone and sulphur dioxide use more than one period
Ozone cannot be represented accurately by one row labelled with a single averaging time. Its 8-hour ranges are 0–54, 55–70, 71–85, 86–105 and 106–200 ppb through AQI 300. The 1-hour ranges start at AQI 101: 125–164, 165–204, 205–404 and 405–604 ppb. EPA does not define Good or Moderate from the 1-hour value, and the 1-hour series supplies the Hazardous band.
Sulphur dioxide also changes period. The 1-hour ranges are 0–35, 36–75, 76–185 and 186–304 ppb through AQI 200. Above 200, EPA uses a 24-hour average: 305–604 ppb for Very Unhealthy and 605–1004 ppb for Hazardous. Describing all six SO2 ranges as 1-hour values, as a single broad table often does, is incorrect.
How the European index differs
The European Air Quality Index is defined hourly. It uses hourly concentrations of PM2.5, PM10, ozone, nitrogen dioxide and sulphur dioxide, then takes the poorest available category as the overall level. It does not include carbon monoxide. The EEA map has both station and modelled layers, and station gaps may be filled with modelled data.
Its categories are Good, Fair, Moderate, Poor, Very poor and Extremely poor. The concentration bands, in µg/m³ and in that order, are:
- PM2.5: 0–5; 6–15; 16–50; 51–90; 91–140; above 140.
- PM10: 0–15; 16–45; 46–120; 121–195; 196–270; above 270.
- Ozone: 0–60; 61–100; 101–120; 121–160; 161–180; above 180.
- Nitrogen dioxide: 0–10; 11–25; 26–60; 61–100; 101–150; above 150.
- Sulphur dioxide: 0–20; 21–40; 41–125; 126–190; 191–275; above 275.
The European index describes short-term conditions. It is not a measure of annual exposure and cannot establish whether legal air-quality limits have been met. A European category and a US AQI number are therefore not interchangeable.
Who should take extra care?
The sensitive group depends on the pollutant. EPA names people with heart or lung disease, children and older adults for particle pollution; people with lung disease, children, older adults and active outdoor workers for ozone; people with heart disease for carbon monoxide; and people with asthma or other lung disease, children and older adults for nitrogen dioxide or sulphur dioxide. During smoke events, pregnancy can also increase risk.
The World Health Organization also identifies pregnant people, children, older people, people with cardiorespiratory conditions and outdoor workers as groups that may need to reduce short-term exposure. Local advice matters because it accounts for the pollutant and the people most at risk from it.
What to do when pollution rises
- Check the category, dominant pollutant, source and local health advice before strenuous outdoor activity. Change the time, route, duration or intensity when the advice applies to you.
- Pay attention to coughing, wheezing, chest tightness, sore eyes or shortness of breath. Follow your treatment plan and seek medical help for severe or persistent symptoms.
- During smoke or particle events, closed windows can help only if indoor air remains cleaner and the building stays at a safe temperature. Avoid adding particles indoors through smoking, candles or frying.
- A portable air cleaner can reduce particles when it is correctly sized for the room. Choose a mechanical filter that produces little or no ozone; ozone-generating devices can irritate the lungs.
Staying indoors is not a complete answer. Outdoor pollution can enter buildings, indoor sources still matter, and a sealed room can overheat. Official alerts from local health or environmental authorities should take priority over a general-purpose app.
How Airpult shows AQI
Airpult shows a model-derived AQI on the familiar US 0–500 scale where data is available. It comes from modelled hourly pollutant concentrations, not an official monitoring station, EPA daily AQI or AirNow NowCast. Differences between those readings are expected because their inputs and averaging methods are different.
Airpult currently uses the earlier EPA PM2.5 breakpoint implementation. In that implementation, Good extends to 12.0 µg/m³ rather than the current official limit of 9.0, and the upper PM2.5 bands also differ. The displayed Airpult value can therefore be lower than a value calculated with the current EPA PM2.5 rules. Read more about PM2.5 and PM10, and use local authorities for regulatory reporting and health instructions.
Open Airpult’s air-quality overview to compare current city readings, then use Explore to find a forecast location.
