The Vietnam scale, band by band
Vietnam has its own index, called the VN_AQI. The Vietnam Environment Administration set out how to work it out in Decision 1459/QD-TCMT, dated 12 November 2019. It replaced an older method from 2011.
| VN_AQI | Air quality | Colour |
|---|---|---|
| 0 to 50 | Good (Tốt) | Green |
| 51 to 100 | Medium (Trung bình) | Yellow |
| 101 to 150 | Poor (Kém) | Orange |
| 151 to 200 | Bad (Xấu) | Red |
| 201 to 300 | Very bad (Rất xấu) | Purple |
| 301 to 500 | Hazardous (Nguy hại) | Brown |
The index looks at six pollutants: sulphur dioxide, carbon monoxide, nitrogen dioxide, ozone, and fine dust known as PM10 and PM2.5. A station works out a score for each one. The number you see is the highest of them. So one bad pollutant is enough to push the whole reading up.
The US scale that most apps use
Many phone apps and websites show the US AQI instead. It also has six steps: Good (0 to 50, green), Moderate (51 to 100, yellow), Unhealthy for Sensitive Groups (101 to 150, orange), Unhealthy (151 to 200, red), Very Unhealthy (201 to 300, purple) and Hazardous (301 and higher, maroon). It covers five pollutants: ground-level ozone, particle pollution, carbon monoxide, sulphur dioxide and nitrogen dioxide.
The two scales share the same idea, but the names and the top colour differ. Vietnam calls 101 to 150 "poor" and shows the top band in brown. The US calls the same range "unhealthy for sensitive groups" and shows the top band in maroon. Check which scale an app uses before you compare numbers.
The US scale was also changed recently. On 7 February 2024 the US Environmental Protection Agency said it was revising its AQI to explain the risks from fine particles (PM2.5) more clearly.
Sources: AirNow, AQI Basics; US EPA, Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (PM), 7 February 2024.
What Hanoi looked like in January 2025
Winter shows why the bands matter. Hanoi's air got worse from October 2024. On 8 January 2025 at 9 a.m., IQAir recorded an AQI of 219 in Hanoi, which is "very unhealthy". PM2.5 reached 144 micrograms per cubic metre, about 29 times the World Health Organization's yearly guideline, according to VnExpress.
The day before, on 7 January, Hanoi ranked first in the world for the most polluted city, with an AQI of 272. Early in the month, peaks went above 400. In mid-January some single sites near West Lake read in the 300s and 400s, which is the top band. VietnamPlus puts the worst months at September to March.
For these levels, Vietnam's Ministry of Health advised people to limit outdoor activity at unhealthy levels, especially children, pregnant women, older adults and people with lung or heart conditions. At very unhealthy levels it advised staying indoors where possible, wearing a fine dust mask if you must go out, and keeping windows and doors closed. This is what the ministry published, not personal medical advice. Ask a doctor about your own case.
Sources: VnExpress International, Residents urged to stay indoors as Hanoi air quality hits 'very unhealthy' levels, 8 January 2025; VietnamPlus, Hanoi tops pollution rankings, seeks solutions to improve air quality, 25 January 2025.
Hanoi and Ho Chi Minh City over a full year
A single bad day tells you little about a year. The IQAir 2023 World Air Quality Report, covered by Vietnamese media in March 2024, gives yearly PM2.5 averages for both cities.
| Place | PM2.5, 2023 average | World rank among 7,812 cities |
|---|---|---|
| Hanoi | 43.7 micrograms per cubic metre | 233rd |
| Ho Chi Minh City | 21.8 micrograms per cubic metre | 1,048th |
Hanoi's average was nearly nine times the WHO yearly guideline and was the highest in Vietnam. Across the whole country the average was 29.6, which put Vietnam 22nd of 134 countries and regions in the report. Ho Chi Minh City sits well below Hanoi, but its average still passes the WHO guideline. These are yearly averages of one measure, so they do not match the daily AQI numbers above.
Sources: VnExpress International, Vietnam, Asian neighbors face alarming air pollution crisis: report, 21 March 2024; VietnamNet, Vietnam's air quality second worst in ASEAN, 22 March 2024.
Where to check, and how to read it
Start with the number, then the colour, then the hour. Air can change a lot between morning and afternoon, and between one district and the next.
- moitruongthudo.vn is the Hanoi capital environment monitoring portal. It publishes station readings and air quality reports for the city.
- IQAir publishes live readings and city rankings, and it reports in the US scale.
Pick one source, note which scale it uses, and check it at the same time each day. If two apps disagree, the cause is often a different scale or a different station. Our healthcare in Vietnam guide covers where to get care if you feel unwell, and the emergency number guide lists who to call in an urgent case.
Sources: VietnamPlus, Hanoi tops pollution rankings, seeks solutions to improve air quality, 25 January 2025; VnExpress International, Residents urged to stay indoors as Hanoi air quality hits 'very unhealthy' levels, 8 January 2025; Hanoi capital environment monitoring portal (moitruongthudo.vn).
Sources
- Vietnam Environment Administration, Decision 1459/QD-TCMT, 12 November 2019 (technical guide to the VN_AQI)
- AirNow, AQI Basics
- US EPA, Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (PM), 7 February 2024
- VnExpress International, Residents urged to stay indoors as Hanoi air quality hits 'very unhealthy' levels, 8 January 2025
- VietnamPlus, Hanoi tops pollution rankings, seeks solutions to improve air quality, 25 January 2025
- VnExpress International, Vietnam, Asian neighbors face alarming air pollution crisis: report, 21 March 2024
- VietnamNet, Vietnam's air quality second worst in ASEAN, 22 March 2024