Explainer · 2026
PM2.5 air quality index: how to read the numbers in Vietnam
PM2.5 air quality index scores turn a raw dust reading into one number from 0 to 500, so that anyone can read the air at a glance. A low number means cleaner air, and a high number means more fine dust. The trouble is that the same dust can get a different score in different apps. This guide explains why.
What PM2.5 is and what the index does
PM2.5 means particles in the air that are 2.5 micrometres across or smaller. That is much thinner than a human hair. Monitors count them in micrograms per cubic metre of air, written as µg/m3.
A raw figure such as 38 µg/m3 means little to most readers. An air quality index, or AQI, fixes this. It maps each pollutant reading onto a shared scale of 0 to 500 and gives each range a name and a colour. The scale is the same for dust, ozone and other gases, so the labels line up.
When several pollutants are measured at once, the agency reports the worst one. In Vietnam's method, the final hourly or daily score is the highest score among the pollutants checked.
Sources: US EPA, Final Updates to the Air Quality Index for Particulate Matter, fact sheet, February 2024; Vietnam Environment Administration, Decision 1459/QD-TCMT of 12 November 2019.
The colour bands in Vietnam
Vietnam calls its index the VN_AQI. The technical guide was issued by decision 1459/QD-TCMT on 12 November 2019. It sets six bands, each with a colour.
- 0 to 50: good, green.
- 51 to 100: average, yellow.
- 101 to 150: poor, orange.
- 151 to 200: bad, red.
- 201 to 300: very bad, purple.
- 301 to 500: hazardous, brown.
The guide turns a PM2.5 reading into a score with set breakpoints. A 24-hour average of 25 µg/m3 scores 50. A reading of 50 µg/m3 scores 100. A reading of 80 scores 150, 150 scores 200, 250 scores 300, 350 scores 400, and 500 scores 500. Scores between these points are worked out in a straight line.
Sources: Vietnam Environment Administration, Technical guidance on calculating and publishing VN_AQI, Decision 1459/QD-TCMT, 12 November 2019 (PDF on cem.gov.vn).
Hourly scores and daily scores
The Vietnamese guide describes two kinds of score. The daily score uses the 24-hour average of PM2.5. The hourly score uses a weighted average of the last 12 hourly readings, called a Nowcast value. The Nowcast gives more weight to recent hours when the air is changing fast, and it is used for both PM2.5 and PM10.
So a live number on a map and a daily number in a report are not the same kind of figure. The live one moves fast. The daily one is steadier. If two sources look far apart, check which kind each one shows.
Sources: Vietnam Environment Administration, Decision 1459/QD-TCMT, 12 November 2019.
The US scale is not the Vietnamese scale
Many phone apps and public air monitors show the US AQI instead of the Vietnamese one. The US Environmental Protection Agency announced a change to its PM2.5 breakpoints on 7 February 2024, and the new table took effect on 6 May 2024. Under it, the "good" band runs from 0 to 9.0 µg/m3. "Moderate" runs from 9.1 to 35.4. The next bands start at 35.5, 55.5, 125.5 and 225.5.
Now compare. In the US table, a PM2.5 reading of 35.4 µg/m3 is the top of the moderate band, which is an AQI of 100. In the Vietnamese table, a score of 100 sits at 50 µg/m3. Put another way, the same reading of 40 µg/m3 lands in the US "unhealthy for sensitive groups" band, which is orange. In Vietnam's table it lands under 100, which is yellow.
Neither table is wrong. They are two sets of rules that use different cut points. That is the first reason two apps can disagree.
Sources: US EPA, Final Updates to the Air Quality Index for Particulate Matter, fact sheet, February 2024.
Why two apps can show different numbers
There are four common causes.
- Different scales. One app may use the US table and another the Vietnamese table.
- Different averaging. One may show a live Nowcast and another a 24-hour average.
- Different sensors. A government station and a low-cost home sensor can sit streets apart and read different dust levels.
- Different update times. Some apps refresh every hour, and some lag behind.
Before you compare, write down three things for each source: the scale, the averaging time and the place of the sensor. If all three match and the numbers still differ, the sensors are measuring different air.
How the WHO guideline fits in
The World Health Organization is a separate yardstick. Its air quality guidelines, published on 22 September 2021, set recommended levels for PM2.5 of 5 µg/m3 as a yearly average and 15 µg/m3 as a 24-hour average. The 2005 version had the yearly level at 10. WHO says the guidelines are not legal limits. They are a global target that governments can work toward.
An AQI band and a WHO guideline answer different questions. The band describes today's air on a colour scale. The guideline is a long-term reference level. A day can be "good" on a national scale and still be above the WHO guideline, because the national bands are wider.
Sources: World Health Organization, What are the WHO Air quality guidelines? (22 September 2021); WHO, Air quality guidelines global update 2021.
A short checklist for reading any reading
- Find the unit. Is it a score from 0 to 500, or a figure in µg/m3?
- Find the scale. Is it the VN_AQI or the US AQI?
- Find the time frame. Is it live, hourly or a 24-hour average?
- Find the sensor. Is it an official station or a small private device, and how far away is it?
With those four answers, you can compare two sources fairly and know why they differ. This guide explains how the numbers are made. It does not give health advice, and anyone with a health question should ask a doctor.