Along with weather conditions for each city, we display air quality as a small colored number labeled (EAQI). That number is not arbitrary — it is the European air quality index (EAQI), the official scale of the European Environment Agency (EEA).
This article explains what that number means, why it often differs from what you see in American applications like weather.com, and why we display it at all on a pollen site.
What is EAQI
EAQI summarizes current air quality in a single number on a scale from 0 to over 100. A lower number means cleaner air, a higher number means more pollution. The number is calculated from concentrations of several pollutants, and the index takes the worst (highest) sub-index — the logic is that air quality is determined by the substance that currently has the most relative to its threshold.
The index is an orientative indicator of current conditions, not a long-term forecast. We update it roughly once an hour.
Bands and colors
The index is divided into six bands. The color of the label on the site follows exactly this scale:
| EAQI | Band | Meaning |
|---|---|---|
| 0–20 | Good | Air quality is good, risk is very low. |
| 20–40 | Fair | Air is acceptable; very sensitive people may feel mild complaints. |
| 40–60 | Moderate | Sensitive groups may feel symptoms with prolonged outdoor stay. |
| 60–80 | Poor | Sensitive people should reduce prolonged or intensive outdoor stay. |
| 80–100 | Very poor | Health effects possible even in the wider population. |
| > 100 | Extremely poor | Avoid exertion outdoors; sensitive people stay indoors. |
The boundary value belongs to the lower band — for example, 40 is still "fair", and "moderate" begins at 41.
What it is based on
EAQI is calculated from concentrations of major air pollutants:
- Particulate matter PM2.5 and PM10 (fine dust, soot, smoke),
- ground-level ozone (O₃),
- nitrogen dioxide (NO₂),
- sulfur dioxide (SO₂).
Each substance has its own scale of thresholds, and the index takes the highest sub-index among them. For allergy sufferers, suspended particles and ozone are especially important because they directly irritate the mucous membranes of the airways.
Why our number differs from weather.com and other apps
This is the most common source of confusion. If you check weather.com or some American app for the same city and same moment, the number will often be roughly twice as high as ours. This does not mean one data point is wrong — it is a matter of two different scales:
- EAQI (European) — scale 0–100+, which we use and is the standard in Croatia and the EU.
- US AQI (American EPA index) — scale 0–500, used by weather.com, AirNow, and most American apps.
The same pollution concentrations are converted by the two scales into different numbers because they have different boundaries, different scales, and different averaging windows (for example, ozone and particulates are not averaged over the same time period).
A concrete example from Zagreb: at the same air quality, the European index was 27 ("fair"), while the American index for the same air was 53 ("moderate"). So almost double the number — purely because of the scale.
| EAQI (ours) | US AQI (weather.com) | |
|---|---|---|
| Scale | 0–100+ | 0–500 |
| Example (same air) | 27 — fair | 53 — moderate |
| Band boundaries | EEA | US EPA |
For a Croatian and European audience, EAQI is the correct scale — it is the index referenced by the Croatian Meteorological and Hydrological Service and other European services.
Where the data comes from
Air quality data is obtained from CAMS (Copernicus Atmosphere Monitoring Service), the European atmospheric model, via the Open-Meteo service. This is a model estimate for city coordinates, not a reading from an individual monitoring station, so the value may differ from the nearest physical station.
How air quality affects the risk score
Polluted air independently of pollen irritates the mucous membranes of the airways and amplifies the allergic and asthmatic response. This is why worse air quality bands raise the risk score we display for allergy sufferers — clean air does not lower it below the baseline.
We explain the details of how pollen, weather, and air quality together make the final number in the article How the risk score is calculated.
Orientative recommendations by band
- Good / Fair: normal outdoor activities without restrictions.
- Moderate: sensitive people (asthmatics, severe allergy sufferers, children, older people) should monitor symptoms and avoid prolonged intensive exertion outside.
- Poor / Very poor: sensitive people should reduce time outdoors; if symptoms worsen, stay indoors and ventilate during times when the air is cleaner.
- Extremely poor: avoid exertion outdoors; sensitive people stay indoors.
Note
The air quality index and risk score are orientative indicators and do not replace medical advice. If you have asthma, severe allergies, or pronounced complaints, consult your family physician or allergist.