How does the forecast differ from measurement?

Measurement shows how much pollen the air held over the past few hours - that is the result from the microscope. The forecast is a prediction of future concentration based on historical data, the weather forecast, plant phenology (when they flower) and atmospheric models. The forecast is less accurate than measurement but useful because it allows you to prepare.

Why do the levels run from 1 to 5 and not from 0?

Level 1 ("Low") is not zero pollen - it means there is so little pollen in the air that it will not cause symptoms in most allergy sufferers. Measuring methods have a lower limit of detection, and even at level 1 sensitive individuals can react. The scale is designed to answer the practical question "what will my symptoms be today?", not a strictly scientific zero.

How far ahead is the forecast reliable?

The result for today is the most accurate. Tomorrow's forecast has an accuracy of about 70-80% for most allergens. Three days ahead, reliability falls to 50-60%, because the weather (rain, wind, temperature) significantly affects pollen concentration. alergija.hr provides a 4-day forecast with this limitation in mind.

What if I am very sensitive even at level 2?

Levels 1-5 are standardised to "most allergy sufferers". Sensitised individuals - especially those with a polyvalent allergy (several allergens at once) or those with asthma - can react even at a lower level. Set your personal email-alert threshold to 1 or 2 if you fall into that group, and talk to an allergist about preventive treatment.

Why is the forecast sometimes wrong?

Rain washes pollen out of the air, wind carries it over long distances, and warm, dry days encourage flowering. Atmospheric conditions change faster than the forecast can track. Another reason: in Croatia all measurements are done manually with a 1-2 day delay, which means today's forecast partly follows data from the day before yesterday.

Can I get a forecast for my individual allergens?

Yes. On alergija.hr/profil you select your own allergy profile (e.g. birch and grasses only), and the system shows only the forecast for your allergens. Email alerts are sent only when your selected allergens exceed the chosen threshold.

Glossary

Pollen forecast

Predicting the concentration of pollen in the air

A prediction of the concentration of pollen in the air for the next few days, expressed as a level from 1 to 5 or as the number of grains per cubic metre of air.

What it is

The pollen forecast is an estimate of the amount of pollen in the air over the coming days. The goal is to give allergy sufferers enough time to prepare - to take medication, avoid spending long periods outdoors and plan their daily activities. It differs from real-time measurement: the forecast projects future levels, while measurement shows how much pollen the air held over the past few hours.

How it is measured

Pollen is measured with a volumetric Hirst-type trap. The device draws in a known volume of air across a slowly moving tape, and the pollen grains stick to an adhesive surface. A specialist biologist counts the grains under a microscope and sorts them by species. The count produces a concentration expressed in grains per cubic metre of air, which is then mapped onto levels 1-5 depending on the type of allergen.

Sources and data

The main data source for Croatia is the National Institute of Public Health "Dr. Andrija Štampar", which coordinates a national network of pollen measuring stations in 25 Croatian cities. Štampar publishes daily measurement results and 3-day forecasts for 21 allergens. alergija.hr uses this data as its primary source, with the Copernicus CAMS atmospheric model as a backup source for 6 allergens.

Pollen levels - what they mean

Low

No symptoms in most allergy sufferers. Sensitised people may feel a mild itch in the nose or itchy eyes.

Moderate

Mild symptoms - sneezing, a slight runny nose, occasional itching. An antihistamine as needed.

High

Likely symptoms in most allergy sufferers - more intense sneezing, nasal congestion, itchy eyes. Regular use of an antihistamine is recommended.

Very high

Strong symptoms - significantly impaired nasal breathing, heavy congestion, swollen eyes. Reduce time outdoors, close windows, add treatment (nasal corticosteroids) on an allergist's recommendation.

Extreme

Extreme reactions are possible in all allergy sufferers - asthma attacks, severe swelling of the airways. Avoid being outdoors, control household dust, and contact an allergist.

Measurement methodology

Counting pollen grains is time-consuming and depends on the specialist, so results are published with a 1-2 day delay. Modern solutions - automatic real-time pollen counters (Rapid-E, Hund BAA500, Poleno) - are only beginning to be rolled out in Europe. In Croatia all measurements are still done manually.

Context in Croatia

alergija.hr provides forecasts for 6 Croatian regions (Continental, Coastal, Highland, Dalmatia, Istria and Kvarner, Slavonia), with a special focus on the 21 most common allergens in Croatia. We send email alerts when the level exceeds a threshold the user sets themselves (1-5).

Frequently asked questions

How does the forecast differ from measurement?: Measurement shows how much pollen the air held over the past few hours - that is the result from the microscope. The forecast is a prediction of future concentration based on historical data, the weather forecast, plant phenology (when they flower) and atmospheric models. The forecast is less accurate than measurement but useful because it allows you to prepare.
Why do the levels run from 1 to 5 and not from 0?: Level 1 ("Low") is not zero pollen - it means there is so little pollen in the air that it will not cause symptoms in most allergy sufferers. Measuring methods have a lower limit of detection, and even at level 1 sensitive individuals can react. The scale is designed to answer the practical question "what will my symptoms be today?", not a strictly scientific zero.
How far ahead is the forecast reliable?: The result for today is the most accurate. Tomorrow's forecast has an accuracy of about 70-80% for most allergens. Three days ahead, reliability falls to 50-60%, because the weather (rain, wind, temperature) significantly affects pollen concentration. alergija.hr provides a 4-day forecast with this limitation in mind.
What if I am very sensitive even at level 2?: Levels 1-5 are standardised to "most allergy sufferers". Sensitised individuals - especially those with a polyvalent allergy (several allergens at once) or those with asthma - can react even at a lower level. Set your personal email-alert threshold to 1 or 2 if you fall into that group, and talk to an allergist about preventive treatment.
Why is the forecast sometimes wrong?: Rain washes pollen out of the air, wind carries it over long distances, and warm, dry days encourage flowering. Atmospheric conditions change faster than the forecast can track. Another reason: in Croatia all measurements are done manually with a 1-2 day delay, which means today's forecast partly follows data from the day before yesterday.
Can I get a forecast for my individual allergens?: Yes. On alergija.hr/profil you select your own allergy profile (e.g. birch and grasses only), and the system shows only the forecast for your allergens. Email alerts are sent only when your selected allergens exceed the chosen threshold.

Related terms

Allergy season