Reference
the aqi scale.
Every value from 0 to 500, with the pollutant concentration behind it, what official guidance actually says at that level, and the one question the number never answers on its own — which pollutant produced it.
look up a value
If you are checking a number you just saw, jump to it below. For the mechanism — how a plume finds you, what changes at each threshold, why two days can both read 88 — read the air quality index explained.
the six categories
The concentrations below are the post-2024 EPA breakpoints, effective 6 May 2024. The revision lowered the PM2.5 ceiling for “Good” from 12.0 to 9.0 µg/m³ and lowered the upper-category thresholds — so a reading that scored 48 under the old table can score above 50 under the current one, with no change in the air.
Good
0–50 · Green
Air quality is satisfactory and pollution poses little or no risk. At this level the atmosphere is doing what it should — mixing efficiently, with no accumulation of particulate or photochemical pollutant near the surface.
PM2.5 0.0–9.0 µg/m³ · ozone up to 54 ppb
Moderate
51–100 · Yellow
Air quality is acceptable. There is a measurable pollutant load — enough that a small number of unusually sensitive people may notice it — but for the general population this is an ordinary day.
PM2.5 9.1–35.4 µg/m³ · ozone up to 70 ppb
Unhealthy for Sensitive Groups
101–150 · Orange
Members of sensitive groups may experience health effects. The general public is less likely to be affected. This is the threshold at which the index stops being informational and starts being a recommendation.
PM2.5 35.5–55.4 µg/m³ · ozone up to 85 ppb
Unhealthy
151–200 · Red
Some members of the general public may experience health effects; members of sensitive groups may experience more serious effects. The distinction between sensitive and general populations begins to collapse in this band.
PM2.5 55.5–125.4 µg/m³ · ozone up to 105 ppb
Very Unhealthy
201–300 · Purple
A health alert. The risk of health effects is increased for everyone, not only for people with existing conditions. Readings in this band in the United States are overwhelmingly driven by wildfire smoke.
PM2.5 125.5–225.4 µg/m³ · ozone up to 200 ppb
Hazardous
301–500 · Maroon
An emergency-conditions health warning. Everyone is likely to be affected. Readings here are rare and almost always indicate a major wildfire smoke event, a dust storm, or an industrial release close to the monitor.
PM2.5 225.5–325.4 µg/m³
every value, 0 to 500
501 readings. Each one carries its category, the PM2.5, PM10 and ozone concentration it corresponds to, and what official guidance says at that level.
Good · 0–50
50s
Moderate · 51–100
100s
Unhealthy for Sensitive Groups · 101–150
150s
Unhealthy · 151–200
200s
Very Unhealthy · 201–300
300s
Hazardous · 301–500
500s
What does the AQI number on my forecast mean?
The Air Quality Index is a unit-less 0–500 scale that translates measured pollutant concentrations into a single comparable number, reported as whichever pollutant is currently worst. 0–50 is Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy, and 301 and above Hazardous. Because it reports only the worst pollutant, the number alone cannot tell you whether you are in an ozone day — which peaks in the afternoon — or a wildfire smoke day, which often peaks overnight. That distinction reverses most practical advice.
Frequently Asked Questions
What is a good AQI number?
Anything from 0 to 50 is the "Good" category, where no group is advised to change behaviour. 51 to 100 is "Moderate" — acceptable for the general population, with a small number of unusually sensitive people potentially noticing the top of that range. 101 is the first value at which official guidance recommends anyone actually change a plan.
What are the six AQI categories?
Good (0–50, green), Moderate (51–100, yellow), Unhealthy for Sensitive Groups (101–150, orange), Unhealthy (151–200, red), Very Unhealthy (201–300, purple) and Hazardous (301 and above, maroon). The scale is conventionally capped at 500.
What PM2.5 level is AQI 100?
Under the post-2024 EPA breakpoints, an AQI of 100 corresponds to 35.4 µg/m³ of PM2.5 as a 24-hour average. The 2024 revision lowered the "Good" ceiling from 12.0 to 9.0 µg/m³, so an AQI of 50 is now 9.0 µg/m³ rather than 12.0. A great many published conversion tables still show the older numbers.
Why does the AQI only report one pollutant?
By design. The index is computed separately for each monitored pollutant — particulate, ozone, carbon monoxide, sulphur dioxide and nitrogen dioxide — and the reported AQI is the highest of those sub-indices. It answers "how bad is the worst thing in this air", not "how good is this air overall", so two days reporting the same number can be chemically unrelated.
Is the air better in the morning or the afternoon?
It depends entirely on which pollutant is driving the number, and the two common drivers behave in opposite ways. Ozone is formed by sunlight acting on precursor pollutants, so it builds through the day and peaks mid-to-late afternoon — mornings are genuinely cleaner. Wildfire smoke is transported rather than formed, and settles toward the surface overnight when the atmosphere stops mixing, so smoke days frequently peak at dawn and improve as daytime heating breaks the inversion.
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