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.
Looking for a unit that does this well? The Luggable XL is #1 of 247 on the US ENERGY STAR Most Efficient list, lab-tested by Intertek, and uses standard 3M MERV-13 filters.
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.