Air Quality Frequently Asked Questions
This page provides concise answers to common questions about data displayed on AirQuality.city.
What is AQI?
AQI is the Air Quality Index. It converts measured pollutant concentrations into a simpler numerical value and category.
AirQuality.city uses the US EPA AQI methodology for PM2.5 and PM10.
How is a station's overall AQI determined?
PM2.5 and PM10 each have a separate AQI value.
The station's overall AQI is the higher of the two:
max(AQI PM2.5, AQI PM10)
This means the overall result is determined by whichever particulate pollutant falls into the less favorable category at that moment.
Are NO₂, O₃, SO₂, and CO included in the overall AQI?
No.
NO₂, O₃, SO₂, and CO are displayed separately for official external stations that measure them, but they are not included in the overall AirQuality.city AQI.
Do SensorBox stations measure every pollutant?
No.
SensorBox stations measure:
- PM2.5
- PM10
NO₂, O₃, SO₂, and CO are available only for official external stations that measure and publish them.
What does an AQI of 50 mean?
Values from 0 to 49 belong to the “Good” category.
A value of 50 falls into “Moderate”.
What does an AQI of 100 mean?
A value of 100 falls into “Unhealthy for sensitive groups”.
This does not mean everyone will experience health effects, but sensitive people should pay more attention to the duration of outdoor exposure and intense outdoor activity.
Who belongs to sensitive groups?
More sensitive people may include:
- children
- older adults
- people with asthma
- people with chronic lung disease
- people with heart disease
- people who spend long periods exercising outdoors
Individual response depends on health status, concentration, exposure duration, and physical activity.
Is one high reading a reason to panic?
Not necessarily.
Check:
- the latest measurement time
- whether the increase persists
- the trend over the previous hours
- nearby stations
- possible local sources
A brief spike may be caused by a passing vehicle, smoke, local dust, or another temporary event.
Why do different portals show different AQI values?
Common reasons include:
- different AQI methodology
- different monitoring station
- different update time
- different averaging period
- different pollutant set
- operational versus validated data
AirQuality.city uses the US EPA AQI methodology for PM2.5 and PM10. Another portal may use the European index or custom categories.
Is AQI the same as concentration in µg/m³?
No.
µg/m³ is a concentration unit representing the measured amount of a pollutant in the air.
AQI is an index derived from concentration according to a selected methodology.
What does PM2.5 mean?
PM2.5 refers to very fine airborne particles with a diameter of up to 2.5 micrometres.
Because of their small size, they can penetrate deeper into the respiratory system.
What does PM10 mean?
PM10 refers to airborne particles with a diameter of up to 10 micrometres.
It includes PM2.5 and larger particles such as some dust, pollen, road particles, and material produced by combustion.
Why does a station have no data?
Possible reasons include:
- power failure
- internet connection failure
- maintenance or sensor malfunction
- delay in the external source
- temporary instrument unavailability
- data not yet published or validated
Missing data do not mean zero concentration.
Why do values often rise in winter?
Fuel combustion for heating increases in winter, while calm weather and temperature inversions can trap pollution near the ground.
PM2.5 and PM10 may therefore remain elevated for hours or days.
Can rain improve air quality?
Rain can temporarily reduce some airborne particles by washing them out of the atmosphere.
The effect depends on:
- rainfall intensity
- duration
- particle type
- whether local sources continue emitting pollution
Light or brief rain may not improve conditions significantly.
Does fog affect air quality and measurements?
Fog and high humidity may occur during weather conditions in which pollution disperses poorly.
With optical PM sensors, water droplets and high humidity may also affect particle detection.
During fog, it is especially useful to compare trends and several nearby stations.
Why does station location matter?
A roadside, residential, park, or industrial station represents a different micro-environment.
The nearest station is not always the most representative of your exact location.
Are all data on the portal official?
No.
AirQuality.city displays two source groups:
- its own SensorBox measurements
- official external station data from Croatia's AZO/ISZZ system
SensorBox measurements are operational sensor data. Official values may later undergo additional validation in the source system.
Can official data change later?
Yes.
The official source may later:
- correct a value
- mark data as invalid
- remove an obvious instrument error
- add a missing value
AirQuality.city may reflect the change after it is published by the source system.
What is the quickest way to check current conditions?
A practical order is:
- check the latest measurement time
- review the overall AQI
- identify the dominant pollutant
- review PM2.5 and PM10 concentrations
- check the trend
- compare nearby stations
One number without time, location, and trend provides only part of the picture.