DAQI to AQI: How the UK Index Maps to the US Scale (Conversion Table)
The UK DAQI (1–10) and the US AQI (0–500) are different scales for the same air. Here's how they map, a conversion table, and why the same day scores differently.
If you have ever seen the UK report "Low" air pollution on the same day a US-style app flags "Moderate," you have met the core confusion: the UK DAQI and the US AQI are two different indices measuring the same air. Neither is wrong. They were built by different agencies, on different scales, with different thresholds. This guide explains how to translate between them, with a conversion table, and why an exact one-to-one mapping is not really possible.
You can see any concentration land on both scales at once with our AQI calculator and converter; this article explains what is happening underneath.
The two indices are built differently
The US AQI is a 0–500 scale calculated from a pollutant concentration using linear interpolation between breakpoints (we walk through that math in how AQI is calculated). It reports the single worst pollutant.
The UK DAQI (Daily Air Quality Index) is a 1 to 10 scale grouped into four bands: Low (1–3), Moderate (4–6), High (7–9) and Very High (10). It is a public-health advice scale set by the UK's Committee on the Medical Effects of Air Pollutants (COMEAP), and its thresholds were chosen around health advice, not to line up with the US numbers.
Because one is a fine-grained 500-point interpolation and the other is a coarse 10-point band system, no formula converts one into the other precisely. What you can do is map the bands.
DAQI to AQI: band conversion table
The cleanest way to translate is by health-advice band. This table lines up the UK DAQI bands with the approximate US AQI ranges and categories they correspond to:
| UK DAQI | UK band | Approx. US AQI | US category |
|---|---|---|---|
| 1–3 | Low | 0 – ~50 | Good |
| 4–6 | Moderate | ~51 – 150 | Moderate → Unhealthy for Sensitive Groups |
| 7–9 | High | ~151 – 250 | Unhealthy → Very Unhealthy |
| 10 | Very High | 250+ | Very Unhealthy → Hazardous |
Treat these as approximate. The exact US AQI depends on which pollutant is driving it and its precise concentration, so a DAQI of 4 might land anywhere from the high 50s to around 100 on the US scale depending on the day.
Why the same air scores differently
The disagreement comes down to breakpoints. The UK and US drew their PM2.5 thresholds in different places, so a single concentration can sit in a "Low" UK band and a "Moderate" US category simultaneously. For example, a PM2.5 around 27 µg/m³ can read as DAQI 3 (Low) in the UK while scoring near the top of the US "Moderate" range at the same moment.
Add to that: the US AQI reports its worst pollutant on a 500-point scale, while the DAQI's 10 points can't express fine differences. So even on identical air, the two systems will often give you different-sounding messages. Both are internally consistent; they just weren't designed to agree.
What about AQHI and the EU EAQI?
Two other indices come up in the same searches:
- AQHI (Air Quality Health Index): used in Canada and some other regions, a 1–10+ scale built around health risk from a pollutant mix rather than a single worst pollutant. Like the DAQI, it maps to US AQI only by band, not by formula.
- EU EAQI (European Air Quality Index): a six-band scale (Good → Extremely poor) that classifies concentrations directly rather than interpolating. Our converter shows the EAQI band alongside the US and UK numbers for any reading.
The theme is the same across all of them: these are regional translation layers over the same underlying pollutant concentrations. Convert by band, not by exact arithmetic.
The practical takeaway
If you are trying to make a UK DAQI figure make sense on the US scale, use the band table above: Low ≈ Good, Moderate ≈ Moderate to USG, High ≈ Unhealthy to Very Unhealthy, Very High ≈ Hazardous. If you need real precision, convert from the underlying PM2.5 concentration instead of the index, because the concentration is the common ground both scales are built on.
That is exactly what the Partycle AQI converter does: enter one concentration and see the US AQI, EU EAQI and UK DAQI at the same time, so you never have to guess which "score" a given lungful of air deserves.
Sources
- UK Defra. "Daily Air Quality Index (DAQI)": bands, 1–10 scale, and health advice. uk-air.defra.gov.uk/air-pollution/daqi
- UK COMEAP. "Review of the UK Air Quality Index." Basis for the DAQI band thresholds. gov.uk
- US EPA AirNow. "AQI Basics": the 0–500 US scale and categories. airnow.gov/aqi/aqi-basics
- European Environment Agency. "European Air Quality Index": the EAQI six-band system. eea.europa.eu
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