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What is a good AQI? The 0–500 scale, explained

Weather24 editorial team · · 4 min read

In short: an AQI of 0–50 is good, 51–100 is moderate and acceptable for most people. Above 100 the air is unhealthy — first for sensitive groups, from 151 for everyone. The number is set by whichever pollutant is worst at the moment, most often fine particles (PM2.5) or ozone.

Air quality for your exact location, hour by hour, with the main pollutant — in the Weather24 app.

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01

The AQI scale, level by level

  1. 0–50 Good Air quality is satisfactory; little or no risk.
  2. 51–100 Moderate Acceptable; unusually sensitive people may want to take it easier outdoors.
  3. 101–150 Unhealthy for sensitive groups People with heart or lung conditions, older adults and children may be affected.
  4. 151–200 Unhealthy Everyone may begin to feel effects; sensitive groups more seriously.
  5. 201–300 Very unhealthy Health alert: the risk is higher for everyone.
  6. 301–500 Hazardous Emergency conditions: everyone is more likely to be affected.

The levels follow the US EPA Air Quality Index. This is general information, not medical advice — if you have asthma, heart or lung disease, follow your doctor’s guidance for poor-air days.

02

What drives the number

Most common PM2.5

Fine particles from smoke, traffic and industry — they go deepest into the lungs. Wildfire smoke days are PM2.5 days.

Summer afternoons Ozone

Forms in sunlight from traffic and industrial emissions, so it peaks on hot, sunny afternoons.

Dust PM10

Coarser particles — dust, pollen, construction; dry, windy days.

Near roads NO₂, SO₂, CO

Gases from engines, power plants and heating; usually local and close to the source.

03

Air quality in the Weather24 app

  1. Air quality card on the Today screen — the US AQI for your exact location, on the 0–500 scale with the level in plain words.
  2. Main pollutant — which of PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide or carbon monoxide sets today’s number.
  3. Hour by hour — see when the air is cleaner, to plan a run, a walk or opening the windows.
  4. Next to the weather — forecast, rain radar and lightning on the same screen.

Air quality in the Weather24 app

The Air quality card shows the US AQI for your location on the 0–500 scale, the level in plain words and the main pollutant — PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide or carbon monoxide — next to your forecast and radar.

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Frequently asked questions

What is a good AQI?
0 to 50 is “Good” — the air is clean and poses little or no risk. 51–100 is “Moderate”: acceptable for most people. Above 100 the air becomes unhealthy, first for sensitive groups.
What AQI is considered unhealthy?
101–150 is “Unhealthy for sensitive groups”, 151–200 “Unhealthy” for everyone, 201–300 “Very unhealthy” and 301–500 “Hazardous”. If you have a heart or lung condition, follow your doctor’s advice for these levels.
How is AQI calculated?
Each main pollutant — fine particles (PM2.5), coarse particles (PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide — gets its own 0–500 sub-index from its concentration. The overall AQI is the highest of them, and that pollutant is called the main (dominant) pollutant.
Which is more harmful, PM2.5 or PM10?
PM2.5 particles are smaller and reach deeper into the lungs, which is why they drive the AQI on smoky or polluted days more often than PM10. The Weather24 app shows which pollutant sets today’s number.

See also: Rain radar · Lightning map · Weather by city