ISPU and US AQI: why there are two numbers
Published 19 October 2026
Indonesian city pages on Kabut show two indices for the same air. Here is why they differ, and which one the advice follows.
Two scales for the same air
An air quality index turns a pollutant concentration into a single number and a class, so that nobody has to know what 40 µg/m³ of PM2.5 means. Most countries publish their own. The two that matter on Kabut are:
- The US AQI, from the US Environmental Protection Agency. Kabut uses the breakpoints of its 2024 revision. It has six classes, from Good to Hazardous.
- ISPU, the Indeks Standar Pencemar Udara, Indonesia's official index. It is set by regulation of the Ministry of Environment (Permen LHK P.14/2020), and it is the index BMKG and the ministry use in their bulletins. It has five classes: Baik, Sedang, Tidak Sehat, Sangat Tidak Sehat and Berbahaya.
Both start at 0, both put "Good" at 0 to 50, and both have classes called unhealthy and hazardous. That makes them look interchangeable. They are not. The two scales cut the same concentrations at different points, so the same air can give two different numbers and sometimes two different classes.
Where the lines are drawn
Most of the time, and always in a haze event, the pollutant that decides both indices is PM2.5. Here is where each scale draws its lines for it, in µg/m³:
| PM2.5 µg/m³ | US AQI class | ISPU class |
|---|---|---|
| up to 9.0 | Good | Baik |
| 9.1 to 15.5 | Moderate | Baik |
| 15.6 to 35.4 | Moderate | Sedang |
| 35.5 to 55.4 | Sensitive groups | Sedang |
| 55.5 to 125.4 | Unhealthy | Tidak Sehat |
| 125.5 to 150.4 | Very unhealthy | Tidak Sehat |
| 150.5 to 225.4 | Very unhealthy | Sangat Tidak Sehat |
| 225.5 to 250.4 | Hazardous | Sangat Tidak Sehat |
| above 250.4 | Hazardous | Berbahaya |
The US scale is stricter at the bottom. ISPU calls air baik up to 15.5 µg/m³; the US AQI calls anything above 9.0 Moderate. And ISPU has no class between sedang and tidak sehat for sensitive people, so 35 to 55 µg/m³, which the US scale flags for children, older adults and people with asthma or heart conditions, is still sedang.
The same air, two readings
Some worked examples, using the formula each scale publishes:
- 12 µg/m³: US AQI 56, Moderate. ISPU 39, Baik.
- 20 µg/m³: US AQI 71, Moderate. ISPU 57, Sedang.
- 55 µg/m³: US AQI 149, Sensitive groups. ISPU 100, Sedang.
- 100 µg/m³: US AQI 182, Unhealthy. ISPU 147, Tidak Sehat.
So a reader who sees "149" on Kabut and "100, Sedang" in a bulletin is not looking at two measurements that disagree. It can be one measurement on two rulers. The numbers are not on the same scale, so don't compare them directly: compare like with like, Kabut's ISPU against the bulletin's ISPU.
Two other things can make the numbers differ even on the same scale. The reading may come from a different station, or from a model rather than a station; each city page says which source it used. And it may be from a different hour.
Which pollutant drives the index
Both indices are calculated for each pollutant separately, and the highest one becomes the index. ISPU defines its table for PM2.5, PM10, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide. Kabut computes ISPU from whatever concentrations it holds for the reading and shows the pollutant that drove it, for example "ISPU 101 · Tidak Sehat, from PM2.5".
Above 500, the top of the published ISPU table, Kabut shows "500+" rather than extending the scale past where the regulation stops.
Why Kabut leads with the US AQI
The number in the orb, the ranking, the colours and the health advice all follow the six US AQI classes. ISPU appears beside them on the page of every Indonesian city, with its own class. There are three reasons for leading with the US scale:
- One set of advice. Two sets of advice about one lungful of air could only contradict each other. At 45 µg/m³ the US scale tells sensitive people to ease up and ISPU says sedang. Kabut gives one answer, and it is the more cautious one.
- A finer scale where it matters. The US scale separates "Sensitive groups" from "Unhealthy", which is exactly the band where children and people with asthma need to act and everyone else does not yet.
- One scale for every city. Kabut is built to cover places outside Indonesia too, and a ranking only works if every city is on the same scale.
ISPU is there so you can check our number against the official one in the format you already know. Both are calculated once on our server and stored with the reading; the site and the app never recalculate either.
Where to read more
The methodology page has both class tables, generated from the same files the app colours its readings from, and the full advice for each US AQI class. For official air quality statements, follow BMKG and the Ministry of Environment. Kabut is not an official warning service.