Key points
- Compare the pollutant concentration, not the index number: the same PM2.5 reading gives a different AQI in the US, China and Vietnam.
- Vietnam's AQI-VN follows Decision 1459/QD-TCMT (2019) and the QCVN 05:2023 national standard.
- Only 14% of cities met the WHO annual PM2.5 guideline of 5 µg/m³ in 2025, down from 17% the year before (IQAir).
- The final AQI is the highest of the pollutant sub-indices — pollutants are not added together.
Direct answer: An air quality index (AQI) number only means something on its own scale. The US, China and Vietnam all report air quality on a 0–500 scale with the same band numbers — but the pollutant concentrations behind those numbers differ. The rule for reading a foreign AQI is therefore simple: compare the underlying concentration, not the index number; check which scale produced it; and check the averaging period.
The one rule: compare concentrations, not index numbers
Every AQI translates a measured pollutant concentration into a band. Because each country sets its own breakpoints — the concentrations at which the index steps up a band — the same concentration can be “Good” in one country and “Moderate” in another. A number without its scale is not a measurement; it is a label. To read a foreign AQI, find the pollutant concentration behind it (usually PM2.5 in micrograms per cubic metre, µg/m³), identify the scale, and note the averaging period: a 1-hour reading, a 24-hour average and the US EPA’s NowCast (a weighted 12-hour average for PM) are not the same number.
The US scale
The US EPA scale runs 0–500 in six bands: 0–50 Good (green), 51–100 Moderate (yellow), 101–150 Unhealthy for Sensitive Groups (orange), 151–200 Unhealthy (red), 201–300 Very Unhealthy (purple), 301+ Hazardous (maroon). The US EPA revised its scale in May 2024, moving the top of “Good” from 12 to 9 µg/m³ of PM2.5. The AQI for a day is the highest sub-index across the pollutants measured — ozone, PM2.5, PM10, carbon monoxide, sulfur dioxide and nitrogen dioxide — and the pollutant behind it is named as the main pollutant.
The China scale
China uses its own scale, set by the technical regulation HJ 633-2012, with the same 0–500 range and the same band numbers as the US scale. The calculation is the same piecewise-linear method: each pollutant gets a sub-index from its concentration, and the final AQI is the maximum of the six individual sub-indices. What differs is the thresholds, not the maths. China’s scale reaches AQI 50 — the top of “Excellent” — at a 24-hour PM2.5 concentration of 35 µg/m³, against the US EPA’s 9 µg/m³. The 35 µg/m³ figure was China’s national annual PM2.5 limit under its ambient air quality standard GB 3095-2012; China revised the standard in February 2026, tightening the annual PM2.5 limit to 25 µg/m³, with a transitional limit of 30 µg/m³ from 1 March 2026 to 31 December 2030 and full effect from 2031. The same air is therefore “Excellent” in China and well up the scale in the United States.
The Vietnam scale
Vietnam’s index, AQI-VN, follows the Technical Guidelines for calculation and publication of the Vietnam Air Quality Index, issued with Decision No. 1459/QD-TCMT by the Vietnam Environment Administration on 12 November 2019. It measures six pollutants — PM2.5, PM10, NO2, SO2, CO and O3 — and follows the national standard QCVN 05:2023/BTNMT, which sets the maximum limit values for air pollutants including a PM2.5 annual average of 25 µg/m³. Hourly AQI values are calculated for each pollutant, and the daily VN_AQI is the maximum of the hourly values.
| AQI | Level | Colour |
|---|---|---|
| 0–50 | Good | Green |
| 51–100 | Moderate | Yellow |
| 101–150 | Unhealthy for sensitive groups | Orange |
| 151–200 | Unhealthy | Red |
| 201–300 | Very unhealthy | Purple |
| 301–500 | Hazardous | Maroon |
The band numbers match the US scale exactly; the health advice attached to each band differs, and the concentrations behind them are Vietnam’s own.
Worked example: PM2.5 of 35 µg/m³
Take a 24-hour PM2.5 concentration of 35 µg/m³ — a single, ordinary reading — and run it through the three scales:
- China: AQI 50, the top of “Excellent”.
- United States: around AQI 100, the top of “Moderate” — the 101–150 band starts at 35.5 µg/m³.
- WHO guideline: the WHO Global Air Quality Guidelines (2021) set the 24-hour PM2.5 guideline at 15 µg/m³ and the annual guideline at 5 µg/m³. The reading is more than twice the 24-hour guideline.
One concentration, three answers. None of them is wrong; each is a different scale’s verdict on the same air.
One number, several pollutants
Is the AQI a composite or a single-pollutant index? The labels “composite” and “non-composite” are not official WHO or EPA terms, and this guide does not present them as such. The mechanism is what matters: in the US, Chinese and Vietnamese systems, the final AQI is the maximum of the individual pollutant sub-indices. A single-pollutant reading reports one pollutant only. Pollutants are never added together — the worst one sets the number.
Where to find official readings for Vietnam
VN Air is the application developed by the Northern Center for Environmental Monitoring under the General Department of Environment, Ministry of Natural Resources and Environment. Its data comes from continuous automatic monitoring stations managed by the Vietnam Environment Administration and local Departments of Natural Resources and Environment. It publishes the AQI of automatic monitoring stations nationwide, with charts of changes over previous hours and days and health recommendations for each value. The Ministry of Agriculture and Environment’s monitoring system and the vietnam.vn government portal publish readings as well.
How many cities meet the WHO guideline
Almost none. IQAir’s 2025 World Air Quality Report (24 March 2026) found that only 14% of cities met the WHO annual PM2.5 guideline of 5 µg/m³, down from 17% the previous year. Only thirteen countries or territories met the guideline, and 130 out of 143 countries and territories (91%) exceeded it. In the 2024 report, the figures were 17% of cities and seven countries meeting the guideline (Australia, Bahamas, Barbados, Estonia, Grenada, Iceland and New Zealand). The direction, in both reports, is the wrong one.
Frequently asked questions
Why does the same PM2.5 number give a different AQI in the US and China?
Because the two scales use different breakpoints. The US EPA revised its scale in May 2024, moving the top of 'Good' from 12 to 9 µg/m³ of PM2.5; China's scale reaches AQI 50 at 35 µg/m³. The calculation method is the same — the thresholds differ.
Is an AQI from another country comparable to one from home?
Only through the underlying concentration. Compare PM2.5 in µg/m³, check which scale produced the number, and check the averaging period — 1-hour, 24-hour or NowCast readings are not the same number.
Where can I find official AQI readings for Vietnam?
VN Air, the application developed by the Northern Center for Environmental Monitoring under Vietnam's Ministry of Natural Resources and Environment, using data from continuous automatic monitoring stations.