"Check an AQI app" appears on four pages of this site. Three of them add that northern cities can pass 150 for days at a stretch in winter. Neither the instruction nor the number is wrong, and neither is any use unless you know what the scale is.
There are two things to know. The first is what the number means. The second is that a hot number and a formal warning are different systems run by different authorities, and only one of them closes anything.
The six bands#
China's air quality index runs from 0 upwards and is divided into six categories, each with a colour and a health message:
| AQI | Category | What it means in practice |
|---|---|---|
| 0–50 | Excellent | No meaningful pollution; normal activity for everyone |
| 51–100 | Good | Acceptable; very sensitive people may notice it |
| 101–150 | Lightly polluted | Sensitive groups should cut long or strenuous outdoor exertion |
| 151–200 | Moderately polluted | Healthy people begin to feel it; vulnerable groups avoid long outdoor activity |
| 201–300 | Heavily polluted | Healthy people show symptoms; vulnerable groups stay indoors |
| Above 300 | Severely polluted | The top band, and the one that triggers emergency responses |
So the 150 this site keeps quoting is the boundary between lightly and moderately polluted. It is the point at which a healthy visitor starts to notice the air rather than the point at which a day is written off.
The standard behind it changed in March 2026#
This is the part that dates almost everything written about Chinese air quality, including the figures on this site until now.
Two national instruments were revised together and both took effect on 1 March 2026.
GB 3095-2026, the ambient air quality standard, tightened the concentration limits — for fine particulates most of all — and does it in two stages. From 1 March 2026 to 31 December 2030 a transitional set of limits applies; from 1 January 2031 the full revised limits take over. The Ministry of Ecology and Environment explained the revision as bringing the standard closer to the World Health Organization's 2021 guidance and protecting public health.
HJ 633-2026, the technical regulation defining the index itself, was reissued at the same time and repealed HJ 633-2012 on the day it took effect. Among its changes it adjusted the particulate concentrations that map onto each band, revised how the particulate sub-index is calculated for real-time reporting, and added provisions on sensitive groups.
A high number is not a warning#
Here is the distinction the site has never drawn, and it is the practically important one.
The typhoon and heat colours this site sets out elsewhere are meteorological signals, issued by the weather service under its own instrument. Wind runs on four colours and heat on three. Air pollution is not part of that system. It has its own, run from the environmental side, and it is graded on the forecast index rather than on today's reading.
The grading was unified nationally by the Ministry of Ecology and Environment in 环大气〔2024〕6号 (huán dàqì, atmospheric environment circular no. 6 of 2024), issued on 22 January 2024:
黄色预警以预测日AQI＞200或日AQI＞150持续48小时及以上、橙色预警以预测日AQI>200持续48小时或日AQI＞150持续72小时及以上、红色预警以预测日AQI>200持续72小时且日AQI>300持续24小时及以上为分级标准
Yellow where a daily index above 200 is forecast, or above 150 for 48 hours or more. Orange where above 200 is forecast for 48 hours, or above 150 for 72. Red where above 200 is forecast for 72 hours and above 300 for 24.
Note the shape. Like heat, and unlike wind, pollution has three colours rather than four — there is no blue. And every threshold is about duration, not a peak: one bad afternoon is not a warning, three bad days is.
The measures are local, not national#
This is where published advice most often overreaches, so it is worth being exact.
The same circular unifies the indicator — everyone grades on forecast AQI — and leaves the response to be set locally, requiring only that emergency reduction measures be drawn up in accordance with the relevant national technical guidance.
So there is no national rule that schools close at orange or that cars run on odd and even plates at red. Those measures exist, they are real, and they sit in each city's own heavy-pollution emergency plan. What travels across China is the colour and what it is based on; what does not travel is the list of things that stop.
What a visitor should actually do#
- Read the band, not the number. Under 100 is a normal day. 101–150 is a day to shorten the walking. Above 200 is a day to rearrange.
- A warning colour means a spell, not an afternoon. If one is in force, the forecast is for days rather than hours, so move the outdoor legs rather than waiting it out.
- Check the city you are going to as well as the one you are in. Air-quality episodes on the northern plain are regional and a train can take you into or out of one.
- The seasonal picture has not changed — the northern plain from November to March, and the clean places — only the way the number is calculated.
- An N95 is the item that helps, and it belongs in the bag if you have a respiratory condition, as the health and safety guide already says.
- Hong Kong and Macau run their own indices on their own standards, so a mainland figure and a Hong Kong one are not the same measurement.
Frequently asked questions#
What does the AQI number mean in China?#
It is a six-band scale: 0–50 excellent, 51–100 good, 101–150 lightly polluted, 151–200 moderately polluted, 201–300 heavily polluted, and above 300 severely polluted. The 150 mark commonly quoted for Chinese cities is the boundary between the third and fourth bands.
Did China change its air quality standard?#
Yes. The ambient air quality standard GB 3095-2026 and the AQI technical regulation HJ 633-2026 both took effect on 1 March 2026, the latter repealing the 2012 version. The revision tightened particulate limits in two stages, with fuller limits from 1 January 2031, and adjusted the concentrations that map onto each index band.
Is an AQI figure from a few years ago still comparable?#
Not directly. The bands are unchanged but the concentrations behind them were adjusted when the standard was reissued in March 2026, so an older Chinese figure and a current one are not measuring in quite the same way. An app using another country's standard is a different index again.
What are China's pollution warning colours?#
Yellow, orange and red — three levels, with no blue. National guidance from January 2024 grades them on forecast daily AQI over sustained periods, so a warning describes a spell of several days rather than a single bad afternoon.
Do schools close and cars get restricted during Chinese smog alerts?#
Those measures exist but are set locally. National guidance unifies how warnings are graded and requires emergency reduction measures to follow national technical guidance, while each city writes its own heavy-pollution emergency plan. There is no single national rule attaching specific closures to a colour.
How bad is air pollution for travellers in China?#
Much less than its reputation, and highly seasonal and regional. The northern plain from November to March is the problem period; Kunming, the southeast coast, Hainan and the plateau are clean nearly year round. Bad days are usually cleared by wind within a day or so.