To calculate a city’s overall Air Quality Index (AQI) value, AirVisual’s real-time AI-based (Artificial Intelligence) system takes the median average of all the city’s stations that have reported data in the past hour. 

The median average means that any stations that are reporting significantly higher values than other stations will NOT be taken into account. For example, if in the last hour we have stations reporting the following values: 125, 130, 131, 135, 140, 145, 250, the AirVisual system will select 135 (the median) as the value for the city average. The value 250 will NOT have any impact on the city average.

Stations that update late will not go into the real-time city average AQI calculation.

Hanoi is one of around 90 global cities that is included on the AirVisual Major Cities Ranking. This ranks cities according to their AQI, from highest to lowest. It is a live ranking and updates several times per hour. When Hanoi tops the list, this is not to say that Hanoi has the worst air quality in the world measured ever, but rather the worst air quality in that hour of the 90 ranked cities. Discover more information about the Major Cities Ranking.

For alternative insights into how cities compare based on longer-term data, AirVisual recently compiled a detailed ranking which compares over 3,000 global cities during 2018 based on their total measurements during that year. View the full 2018 World Air Quality report page to explore this longer-term data.