AQI Scale Explained: What Is a Good Air Quality Index?

The Air Quality Index (AQI) is a 0 to 500 scale that turns air pollution into a single number and color. A good AQI is 0 to 50, shown in green: the air is satisfactory and poses little or no risk. Once the number passes 100 the air is no longer considered fine for everyone, and at 101 to 150 (orange) sensitive groups should start easing off heavy outdoor activity. The higher the number and the deeper the color, the more people it affects.

What does AQI mean?

AQI stands for Air Quality Index, the system the US EPA uses to report daily air pollution to the public. It runs from 0 (perfectly clean) to 500 (immediate danger), according to the American Lung Association, and is calculated from ground-level ozone, particle pollution (PM2.5 and PM10), and several other widespread pollutants.

One thing that trips people up: the same AQI number can come from very different air. Each pollutant gets its own AQI value, and the reading you see tracks whichever one is worst at the time. So an AQI of 120 might be almost all ozone on a hot afternoon, or almost all fine particles on a wildfire day, which is why the number alone does not tell you what is in the air, only how concerning it is.

What do the AQI colors mean? The scale band by band

The AQI is split into six categories, each with a number range and a color. These are the official bands and descriptions from the US EPA’s AirNow.

AQI	Category	Color	What it means
0–50	Good	Green	Air quality is satisfactory and poses little or no risk.
51–100	Moderate	Yellow	Acceptable, but unusually sensitive people may notice symptoms.
101–150	Unhealthy for Sensitive Groups	Orange	Sensitive groups may feel effects; the general public is less likely to be affected.
151–200	Unhealthy	Red	Some of the general public may feel effects; sensitive groups feel them more seriously.
201–300	Very Unhealthy	Purple	Health alert: the risk of effects is increased for everyone.
301–500	Hazardous	Maroon	Emergency conditions: everyone is more likely to be affected.

The colors are the fast way to read a forecast. Green and yellow mean go about your day. Orange is the first line where the air starts to matter for some people. Red and above mean it matters for everyone, and the effects get more serious as you climb.

What does “Unhealthy for Sensitive Groups” actually mean?

“Unhealthy for Sensitive Groups” is the orange band, an AQI of 101 to 150. It is the level people misread most often, because the name sounds alarming while the air is still fine for the majority. AirNow puts it plainly: members of sensitive groups may experience health effects, but the general public is less likely to be affected.

So who is a sensitive group? The American Lung Association lists:

• Children and teens
• Adults over 65
• People who are pregnant
• People with asthma or other respiratory disease
• People with heart disease or diabetes
• Anyone who spends a lot of time active outdoors

If you are in one of those groups, orange is your cue to take it easier: shorten the outdoor workout, keep a reliever inhaler handy, or move the effort to a cleaner part of the day. If you are not, orange is mostly a heads-up that the air is trending the wrong way.

This is also the kind of number worth handing to an app instead of checking by hand. Skyvo sends a morning summary that pairs the day’s air quality with the UV index, so an orange day reaches you as a heads-up before you head out, not halfway through a run.

So what actually counts as a “good” AQI?

Strictly, “Good” is the official name of the 0 to 50 band, so that green range is the only level the EPA labels good outright. In practice, anything up to 100 is fine for most healthy people, since Moderate air is still rated acceptable. The line worth remembering is 100: below it, air quality is a non-issue for the general public; above it, more and more groups need to pay attention.

This post covers what the scale is. What a specific reading like 150 or 200 means for your plans, and exactly who should do what, is a decision on top of the scale, and worth its own guide.

Is the AQI the same everywhere? US vs. European scales

No, and this trips a lot of people up. There are two different air quality scales, and which one you see depends on where you are and where your app pulls its data.

• The US scale (AQI): the 0 to 500 number described above, from the US EPA. Six color bands, and the number is set by whichever pollutant is worst.
• The European scale (EAQI): the European Air Quality Index, published by the European Environment Agency. Instead of a 0 to 500 number it reports six named bands, from Good to Extremely Poor, updated hourly and again set by the worst of its pollutants.

Because the two systems do the math differently, the same air can land in different categories depending on which scale you read. That is a common reason two apps disagree about the “same” air, and our US AQI vs. European index guide breaks down exactly why. A quick tell: if your app shows a 0 to 500 number, it is using the US AQI; if it shows a named band with no number, it is using the European index. Skyvo takes the guesswork out of that: in Settings you pick which scale to use, the US AQI or the European index, and every reading is colored by the same bands in the table above, so you can tell the situation at a glance.

How to use the AQI day to day

The practical habit is not memorizing every band, it is checking your local AQI before you commit to time outdoors, especially a run, a long walk, or anything with kids or older relatives. Match the plan to the number: green and yellow, go; orange, sensitive people ease off; red and above, move it indoors or reschedule.

That is the “check before you head out” habit Skyvo is built around. It tracks air quality and the UV index side by side, so one glance covers your whole day, not just half of it. If you want the sun side in depth, our UV index scale guide breaks that scale down the same way.