What is the AQI (Air Quality Index)?
In one line: the AQI turns several pollutant measurements into one easy number and colour, from "good" (low) to "hazardous" (high). It's usually an outdoor measure, but it's a useful cue for when to shut windows and run an air purifier indoors.
What it measures
The Air Quality Index combines key pollutants — chiefly PM2.5 and PM10 particulates, plus gases like ozone, nitrogen dioxide and sulphur dioxide — into a single score. Whichever pollutant is worst at that moment drives the headline number, so a high AQI usually means one specific pollutant has spiked.
How the bands work
Different countries use different scales (the US EPA runs 0–500; the UK uses a 1–10 Daily Air Quality Index), but they share the same idea: low numbers/green are good, high numbers/red are unhealthy, with advisory levels for sensitive groups in between.
| General band | Meaning |
|---|---|
| Low / Good (green) | Air quality poses little risk. |
| Moderate (yellow) | Acceptable; very sensitive people may notice effects. |
| High / Unhealthy (orange–red) | Sensitive groups, then everyone, may be affected — reduce exposure. |
| Very high / Hazardous (purple–maroon) | Serious risk — stay indoors, seal up, filter the air. |
AQI and your indoor air
The AQI you see on a weather app is almost always outdoor air. But outdoor pollution seeps inside, so a high outdoor AQI is your signal to close windows and run a purifier — and indoor sources (cooking, smoke, candles) can push your indoor levels well above outdoor even on a "green" day. A cheap indoor PM2.5 monitor shows what you're actually breathing at home.
Next step
Want to keep indoor levels low whatever the AQI? See units chosen on clean-air delivery at Best Air Purifier UK, and read what PM2.5 is.
FAQ
Is the AQI indoor or outdoor? Usually outdoor. Indoor air quality can be better or worse depending on your own sources and ventilation.
What AQI should make me run a purifier? Any time outdoor air is "moderate" or worse, or whenever your indoor monitor shows a spike — and continuously if anyone in the home is sensitive.