US AQI vs. EU Air Quality Index: What's the Difference?

The difference is simple on the surface and meaningful underneath. The US Air Quality Index (AQI) reports air pollution as a single number from 0 to 500, while the European Air Quality Index (EAQI) reports it as a named band, from Good to Extremely poor. The US scale covers six pollutants and includes carbon monoxide; the European one covers five and leaves carbon monoxide out. Because the two use different cutoffs, and Europe now pegs its bands to the stricter WHO 2021 guidelines, the very same air can read “Good” on one scale and “Fair” on the other. That mismatch, not a broken sensor, is usually why two apps disagree.

What is the difference between the US AQI and the European index?

Both scales answer the same question, “how clean is the air right now,” and both work the same basic way: each pollutant gets its own score, and the worst pollutant sets the overall result. From there they diverge. The US EPA runs the AQI through its AirNow service and expresses it as a 0 to 500 number across six categories. The European Environment Agency runs the EAQI and expresses it as one of six named bands, with no number attached. Here is the side by side.

	US AQI	European AQI (EAQI)
What you see	A number from 0 to 500	A named band
The six categories	Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, Hazardous	Good, Fair, Moderate, Poor, Very poor, Extremely poor
Run by	US EPA (AirNow)	European Environment Agency
Pollutants	Six, including PM2.5, PM10, ozone, NO2, SO2, and carbon monoxide	Five: PM2.5, PM10, ozone, NO2, SO2 (no carbon monoxide)
Overall value	The single highest pollutant score	The worst of the five pollutant bands
Time basis	Daily average, with NowCast for the current hour	Hourly, for every pollutant
Health anchor	US national air standards	WHO 2021 guidelines

Notice that both scales top out at a band most people will thankfully never see. The day-to-day question is what the second and third bands mean for you, and that is where the two systems disagree the most.

The European Air Quality Index, explained

If you have only ever seen the US number, the European scale can look vague, but it is precise in its own way. The EAQI sorts air into six bands, Good, Fair, Moderate, Poor, Very poor, and Extremely poor, based on five pollutants, and the European Environment Agency sets the overall band from whichever pollutant is worst that hour.

Two things make it distinct. First, since its 2024 revision the two cleanest bands, Good and Fair, are anchored directly to the World Health Organization’s 2021 air quality guidelines, so “Good” means fine-particle levels are within the WHO annual guideline of about 5 micrograms per cubic meter. Second, the index is now calculated hourly for every pollutant, including fine particles, which it previously averaged over 24 hours. That makes it quick to react when pollution spikes or clears.

Why does the same air read differently on each scale?

Three design choices pull the two scales apart, so identical air can carry different labels.

• Different cutoffs. The thresholds are not aligned. After the US 2024 update, air is “Good” up to 9 micrograms per cubic meter of PM2.5, but Europe’s “Good” band stops around the WHO guideline of 5. So air at 7 micrograms is “Good” in the US and already “Fair” in Europe.
• Different pollutants. The US AQI includes carbon monoxide and Europe’s does not, so a CO spike from heavy traffic can lift the US number while the European band does not move.
• Different timing. The US AQI is built around daily averages and estimates the current hour with NowCast, while Europe recalculates every hour. During a fast-moving event like drifting wildfire smoke, the hourly European index can shift a band before the US number catches up.

There is also a labeling trap. Both scales have a band called “Moderate,” but it is the second band in the US (still acceptable, colored yellow) and the third band in Europe. The same word means two different levels, so never compare the words alone. An app that shows both scales side by side sidesteps the confusion by letting you read the same air on either one.

Why do two apps show different AQI numbers?

Usually for one of two reasons, and the scale is the bigger one. If one app shows “AQI 62” and another shows “Fair,” they are simply reporting on different rulers, the US number versus the European band, for the very same air. The fix is not to trust one blindly but to check which scale each app is using.

The second reason is the data source. Two apps may read different monitoring stations, one nearby and one across town, or mix in low-cost sensors. When two readings disagree, prefer the one that names an official monitoring station near you over an unnamed or crowd-sourced feed.

This is exactly the confusion Skyvo is built to remove. It shows both the US AQI and the European index and lets you choose which one to see in Settings, so you are always comparing like with like instead of guessing why two apps tell you different things.

Which scale should you use?

Use the one that matches where you are. In the United States, the AQI number is what local agencies, forecasts, and health alerts all speak, so it is the most useful reference. In Europe, the EAQI band is the official language and is tuned to the WHO health thresholds. If you travel between the two, or just want to make sense of a reading from the other side of the Atlantic, seeing both at once is the clearest way to keep your bearings.

That is the everyday case for Skyvo: it puts your local air quality on whichever scale you prefer, right next to the UV index, so one check tells you what the air is doing and what to do about it, wherever you happen to be.