How to Read Air Quality Data
Air quality displays usually contain several pieces of information: overall AQI, PM2.5 and PM10 concentrations, the latest measurement time, trend, and station location.
Looking only at one color or number can lead to the wrong conclusion. A useful assessment requires the full context.
Start with the overall AQI
Overall AQI provides the quickest summary.
On AirQuality.city, it is the higher value between:
- PM2.5 AQI
- PM10 AQI
The overall AQI therefore always represents the less favorable of the two PM components.
Check which pollutant determines the overall AQI
If PM2.5 AQI is higher, PM2.5 is dominant. If PM10 AQI is higher, PM10 determines the overall result.
This matters because the same overall AQI may be caused by different particle types and different sources.
Example:
| Indicator | Value |
|---|---|
| PM2.5 AQI | 78 |
| PM10 AQI | 116 |
| Overall AQI | 116 |
In this example, PM10 determines the overall AQI.
Look at concentration, not only AQI
PM2.5 and PM10 are displayed in micrograms per cubic metre:
µg/m³
Concentration is the actual measured amount of particulate matter in the air. AQI is an index derived from concentration according to the selected methodology.
Both values serve a purpose:
- concentration shows what was measured
- AQI shows which category the value belongs to
Check the latest measurement time
A value is not useful if it is outdated.
Always check:
- the date and time of the latest measurement
- how long it has been since the last update
- whether the station updates regularly
If the latest value is several hours old, it may no longer describe current conditions.
The trend matters more than one spike
A single high reading may result from a brief local event, such as:
- a passing vehicle
- smoke from a nearby chimney
- resuspended dust
- construction activity
- a temporary sensor disturbance
It is more useful to check:
- whether the value is rising
- whether it is falling
- how long the increase lasts
- whether nearby stations show the same pattern
A sustained rise across several readings is usually more important than one brief peak.
Station location changes the meaning of the data
A roadside station may show different values from a station in a park, residential area, or outside the city.
Results may be affected by:
- proximity to roads
- nearby chimneys
- industrial sources
- sensor height
- surrounding buildings
- terrain
- wind speed and direction
A measurement is primarily representative of the station's immediate surroundings.
Compare several stations
When available, check two or more nearby stations.
If all show the same trend, a broader event is more likely.
If only one station is elevated, the reason may be:
- a highly local source
- sensor placement
- a technical issue
- a different location type
How should NO₂, O₃, SO₂, and CO be read?
These pollutants are shown only for official external stations that measure them.
Check:
- the measurement unit
- the averaging period
- the concentration category
- the measurement time
On AirQuality.city:
- NO₂, O₃, and SO₂ are shown in
µg/m³as one-hour values - CO is shown in
mg/m³as an eight-hour rolling average
These pollutants are not included in the portal's overall AQI.
What does a missing value mean?
Missing data do not mean zero concentration.
Possible reasons include:
- the station does not measure that pollutant
- power or internet failure
- maintenance or sensor malfunction
- delay in the external source
- data not yet published or validated
Common interpretation mistakes
Common mistakes include:
- treating AQI and concentration as the same thing
- comparing portals without checking methodology
- ignoring the latest measurement time
- judging an entire city from one station
- treating a brief spike as a lasting problem
- assuming missing data mean zero pollution
A practical reading order
For a quick and useful assessment:
- check the overall AQI
- identify the dominant pollutant
- review PM2.5 and PM10 concentrations
- check the latest measurement time
- review the trend
- compare nearby stations
- consider station location and weather
Only then does one number gain meaningful context.