One number is not the whole picture of air quality

The quickest way to check air quality is to open a map, look at the colour and decide that the air is good or bad. That is often adequate for a glance, but meaningful interpretation starts when the number is placed in context.

On AirQuality.city, a few additional checks prevent most common mistakes: which pollutant is driving the result, how fresh the reading is, whether the change has lasted for hours, and whether nearby stations show the same pattern.

AQI is an entry point into the data, not the end of it

The overall AirQuality.city AQI is currently based on PM2.5 and PM10. A separate index is calculated for each fraction, and the less favourable value becomes the station result.

If a station shows AQI 120, the next useful step is to see whether PM2.5 or PM10 is responsible. The context can be different: fine PM2.5 is often strongly associated with combustion, while PM10 can also rise sharply during dust events. The ratio between them, however, is not enough on its own to identify a source reliably.

AQI categories also depend on methodology. AirQuality.city uses U.S. EPA concentration breakpoints for PM2.5 and PM10, while Croatia's official portal and the EEA use the European index with different thresholds and a broader pollutant set.^[1][2][3]

Concentration shows what was measured

PM2.5 and PM10 are reported in µg/m³. That is the underlying measurement, while AQI is an interpretation built from that concentration.

The distinction is particularly useful when comparing two services. If both show roughly 20 µg/m³ of PM2.5 but use different colours, the index methodology is probably the reason. If one shows 20 and another 45 µg/m³, check whether they are using the same station, hour and averaging period at all.

Measurement time can change the meaning completely

A reading from three hours ago is not necessarily wrong, but it may no longer describe the air outside now. During a wind shift, a fire, the evening start of domestic heating or the passage of a weather front, concentrations can move quickly.

The timestamp is therefore one of the first things worth checking. If a station has stopped reporting, its last known value should not be interpreted as the current state.

A chart separates a brief incident from a sustained episode

A single peak can be caused by a short local source, resuspended dust or an unusual event immediately beside the sensor. If the value returns to normal after a few measurements, that event does not carry the same meaning as elevated pollution lasting several hours.

When concentrations rise gradually, remain high through the evening and a similar trend appears at several stations, the evidence for a wider episode becomes stronger. A chart can therefore say more than the latest point alone.

Station location determines what the measurement can represent

A roadside station answers a different question from a background station on the edge of a city. One measures air in an immediate traffic environment; the other is intended to describe broader urban or regional background conditions.

The same applies to a SensorBox in a residential area. If it sits a few dozen metres from an active chimney, it may capture local smoke very clearly, but that does not mean the entire city is experiencing the same concentration.

Nearby stations reveal the scale of an event

Comparing several sites is one of the fastest ways to judge whether a change is local or regional. A simultaneous rise across distant stations points to a broader influence, while an isolated spike deserves a closer look at the immediate surroundings and the instrument itself.

This does not prove the source, but it is a useful first step. Stronger source attribution requires meteorology, a longer data record, emissions information and, in some cases, satellite or chemical measurements.

Particulate matter is not the whole of air quality

Official stations may also measure NO₂, O₃, SO₂, CO and other pollutants. Where available, AirQuality.city displays those values separately; they do not currently enter the portal's overall PM AQI.

A green particulate index therefore does not automatically mean every other pollutant is low. In summer, when ground-level ozone can be more important than particles, the full set of measurements at a station becomes particularly useful.

Reading air-quality data well is not complicated, but it takes a few seconds beyond the colour: check the AQI, concentration, timestamp, trend, location and nearby stations. That is when a single number begins to carry real meaning.