The UK Daily Air Quality Index (DAQI) Explained

Last updated May 29, 2026 · 6 min read

If you check air quality in the UK, you won't see a number like 87 or 152 — you'll see an index from 1 to 10, grouped into four colour-coded bands. That's the Daily Air Quality Index (DAQI), defined by Defra on the advice of COMEAP. It looks nothing like the US 0–500 AQI, but it does the same job: turn pollutant concentrations into a single number with health advice attached. This guide takes it apart.

What the DAQI is

The DAQI is a 1-to-10 scale that summarises levels of five pollutants: nitrogen dioxide (NO₂), sulphur dioxide (SO₂), ozone (O₃), fine particulate matter (PM2.5), and coarse particulate matter (PM10). Each pollutant gets its own index value from its own breakpoint table, and the overall DAQI for a location is the highest of those five — the same "worst pollutant wins" logic the US AQI uses. Unlike the US system, the DAQI deliberately compresses the scale: ten coarse steps instead of 500 fine ones, on the view that the public mainly needs to know which of four action bands they're in.

The four bands

The ten index values roll up into four bands, each with its own colour and health message.

IndexBandColourIn plain English
1–3Low GreenEnjoy your usual outdoor activities. No measurable risk for the general population.
4–6Moderate YellowAdults and children with lung or heart problems who experience symptoms should consider reducing strenuous outdoor activity.
7–9High RedAnyone with symptoms should reduce physical exertion outdoors; people with asthma may need their reliever inhaler more often.
10Very High MaroonReduce physical exertion outdoors, especially if you feel symptoms. The general population may notice effects too.
 Bands, index ranges, and colours are defined by UK Defra (Daily Air Quality Index). The "in plain English" column paraphrases Defra/COMEAP health advice.

The per-pollutant breakpoints

Each pollutant is assigned its index value from a fixed concentration table. Note the different averaging periods — ozone is an 8-hour running mean, NO₂ a 1-hour mean, SO₂ a 15-minute mean, and both particulates a 24-hour running mean.

IndexO₃ (8-hr, µg/m³)NO₂ (1-hr, µg/m³)SO₂ (15-min, µg/m³)PM2.5 (24-hr, µg/m³)PM10 (24-hr, µg/m³)
10–330–670–880–110–16
234–6668–13489–17712–2317–33
367–100135–200178–26624–3534–50
4101–120201–267267–35436–4151–58
5121–140268–334355–44342–4759–66
6141–160335–400444–53248–5367–75
7161–187401–467533–71054–5876–83
8188–213468–534711–88759–6484–91
9214–240535–600888–106465–7092–100
10241+601+1065+71+101+
 Breakpoints from UK Defra / COMEAP (Daily Air Quality Index). The overall DAQI is the maximum index across the five pollutants.

How DAQI compares to the US AQI

The two scales aren't directly convertible — they use different averaging windows and different category boundaries — but the bands map roughly like this:

One structural difference: the DAQI PM2.5 breakpoints climb steeply. A 24-hour PM2.5 of 60 µg/m³ — a serious smoke day — lands at DAQI 8, while the same concentration sits at the boundary of the US "Unhealthy" range. The categorical interpretation translates better than the numbers do.

Putting it to work

  1. Know your band, not just the number. The action message is attached to the band (Low / Moderate / High / Very High), not to each of the ten steps.
  2. If you have a lung or heart condition, "Moderate" (4–6) is your watch level. That's the point where Defra suggests sensitive people consider easing off strenuous outdoor activity.
  3. For cross-border comparison, look at raw PM2.5 in µg/m³. The WHO 24-hour guideline of 15 µg/m³ is the same benchmark everywhere on Earth.

Air quality on your home screen

Smog Report shows real-time air quality with widgets and Live Activities, and lets you switch between regional scales including the UK DAQI. Free on iOS.

Download for iOS

Primary sources: UK Defra — Daily Air Quality Index · COMEAP — Review of the UK Air Quality Index