Frequently asked questions about air quality

This page brings together short answers to the questions that most often arise around AirQuality.city data. More detailed explanations are available on the dedicated pages about AQI, particulate matter, methodology and monitoring.

What is AQI?

AQI is an index that converts a pollutant concentration into a simpler number or category. AirQuality.city currently applies U.S. EPA concentration breakpoints to PM2.5 and PM10, with the less favourable of the two PM components driving the overall result.^[1]

It is not a universal global AQI. The European index uses different thresholds and covers up to five key pollutants.^[2]

Why does AirQuality.city use US rather than European thresholds?

The SensorBox network currently measures PM2.5 and PM10. The U.S. EPA scale provides a clear numerical index for those two fractions, which fits the portal's existing architecture.

The European Air Quality Index is designed around a broader dataset: PM2.5, PM10, NO₂, O₃ and SO₂, with additional data-availability rules for different station types.^[2] A European category can be calculated for PM alone, but that is not the same thing as a complete European index. AirQuality.city therefore currently retains its PM index based on U.S. EPA breakpoints and states clearly what enters the calculation.

Is AQI the same as concentration?

No. PM2.5 and PM10 concentrations are reported in µg/m³ and represent the measured amount of particles in the air. AQI is an interpretation created from that concentration using a defined scale.

Two services can therefore show the same concentration but a different colour if they use different indices.

What do AQI 50 and AQI 100 mean?

On the U.S. EPA scale, 50 is still Good and 100 is still Moderate. Unhealthy for Sensitive Groups begins at 101.^[1]

The boundary does not mean the air changes abruptly between 100 and 101. Categories are a communication tool applied to a continuous measurement.

What are PM2.5 and PM10?

They are not two chemicals but two particle-size fractions. PM10 includes particles with an aerodynamic diameter up to 10 micrometres, while PM2.5 covers the finer fraction up to 2.5 micrometres.

Particulate matter can contain material from combustion, traffic, dust, industry, fires and secondary atmospheric reactions. Its chemical composition cannot be identified from PM concentration alone.^[5]

Why is PM2.5 important for health?

Fine particles can penetrate deep into the respiratory system, and exposure to PM2.5 is associated with cardiovascular and respiratory disease and other adverse health outcomes. WHO identifies air pollution as one of the major environmental risks to health.^[5]

A single short-lived reading is not equivalent to long-term exposure. Duration, frequency and cumulative concentration matter when considering health risk.

Does SensorBox measure NO₂, O₃, SO₂ or CO?

Current SensorBox stations primarily measure PM2.5 and PM10 together with meteorological data. NO₂, O₃, SO₂ and CO shown on AirQuality.city come from official stations that measure and publish those pollutants.

Those gases do not enter the portal's overall PM AQI, so a good PM result does not automatically mean every other pollutant is low.

Why does AQI differ between services?

Common reasons include different index methodologies, different monitoring stations, update times, averaging periods and pollutant coverage.

Croatia's official portal uses the European index, while AirQuality.city currently applies U.S. EPA breakpoints to PM. The Croatian system uses 24-hour moving averages for the PM component of its displayed index, while AirQuality.city uses the latest processed PM value available to the portal.^[3]

Are SensorBox and official stations equally accurate?

They do not use the same technology or quality-control process. SensorBox is a low-cost optical system designed for local trends and supplemental monitoring, while official stations use instruments and procedures intended for regulatory monitoring.

The US EPA notes that humidity, temperature, calibration, ageing and other conditions can affect low-cost sensor performance.^[4] That does not make such sensors useless, but it does define how their data should be interpreted.

Can fog raise a PM sensor reading?

Yes. High relative humidity and fine water droplets can influence optical PM sensors, particularly in very humid or foggy conditions.^[4]

At the same time, fog often occurs during stable atmospheric conditions with weak mixing, when real pollution can also remain trapped near the ground. A high value during fog should therefore not automatically be dismissed as an error; check nearby stations, the trend and weather conditions.

Does rain clean the air?

Rain can reduce concentrations of some airborne particles by removing them from the atmosphere, but the effect depends on rainfall intensity and duration, particle type and whether local sources continue emitting.

During rain or very high humidity, an optical sensor can also respond differently to the changed conditions, so the whole event is more informative than a single point.

Why does air quality often worsen on winter evenings?

Several things can happen at once: domestic-heating emissions increase, wind weakens and vertical mixing becomes less effective. During a temperature inversion, colder air remains near the surface and pollution can accumulate instead of dispersing.

That is why winter charts often show evening increases in PM2.5 and PM10. One chart alone, however, cannot prove the exact source.

Does missing data mean the concentration is zero?

No. Missing data means the portal does not have a valid measurement for that time. The reason may be a power or network interruption, maintenance, instrument failure, source delay or the fact that a particular pollutant is not available.

Zero is a measured value; missing data is the absence of a valid reading.

Can official data change later?

Yes. Croatia's official system explicitly states that real-time values are not final and may be changed during validation.^[3]

Regulatory analysis, official reporting and long-term conclusions should therefore rely on final validated data from the competent authorities.