The Vietnamese government websites frequently report air quality data with a delay of three or more hours. If the data aren’t timely enough, the stations don’t display on the map. The AirVisual map only displays real-time or near real-time data.
Below is a screenshot taken from the Hanoi Environment Monitoring Portal at 9:10 am on Oct. 10 Hanoi time. It shows that they haven’t updated the data for Hoan Kiem station since 6 a.m. At 6 a.m., the PM2.5 concentration value was 85.4 micrograms per cubic meter (μg/m3).

Here is a screenshot of the AirVisual app’s Hoan Kiem page at the same time. We clearly state that the time of the data is 6 a.m. and the PM2.5 concentration: 85.4 μg/m3.

Are there other reasons why stations may suddenly disappear from AirVisual’s platform?
Yes. Stations may go online and offline for short or long periods of time because of power outages, internet outages or maintenance.
If the AirVisual A.I. system flags data as potentially inaccurate, it automatically runs checks on it before deciding whether to publish it or not.