For an allergic person, hay fever ruins weeks of spring and summer. It comes from pollen grains, ten to fifty micrometres across, which the wind carries over long distances and which the nose cannot stop. Knowing the day's level lets you adjust your behaviour rather than simply suffer.
Why this indicator matters
Pollen is not spread evenly through the day. Plants release it mainly when the air is dry and warm, around the middle of the day; the loaded air then settles on surfaces. Rain washes it out: after a shower, the concentration drops for a few hours, and that is the best time to air the house without sneezing.
The concentrations come from a European air quality model, which simulates the emission and transport of six pollens: alder, birch, grass, mugwort, ragweed and olive. It covers Europe; outside its area, we show no pollen indicator rather than make one up.
How to read it
The box names the day's dominant pollen, its level (low, moderate, high, very high) and its maximum value in grains per cubic metre. The levels are specific to each pollen: a few grains of ragweed are enough to trigger symptoms, whereas it takes hundreds of grains of birch.
If rain is forecast in the next twenty-four hours, the slot suggested is the three hours after the rain ends. Otherwise, the sentence advises keeping the windows closed during the warm hours of the day and airing the house late in the evening.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- The indicator speaks up from level 1 out of 4 (low pollen); it switches to “caution” from level 3 (high).
- An hour of rain counts from 0.2 mm; the slot covers the 3 hours after the rain ends.
- Warm hours to avoid when there is no rain: from 11:00 to 16:00.
- Bands for each pollen, in grains/m³ (low, moderate, high): birch and alder 10, 50, 500; grasses 20, 50, 200; mugwort 10, 50, 500; olive 10, 50, 200; ragweed 5, 20, 50.
A worked example: Antwerp
In Antwerp, with the forecasts of 2026-11-15 14:48, this indicator stays silent: none of the conditions above is met at the moment.
Its limits
The concentration is calculated for a point representative of the region, not for your street, and by a model: a neighbouring park or a field of grass mown the day before changes the picture. No sensor measurement is used. This is not medical advice: if symptoms persist, see an allergy specialist.
Frequently asked questions
What is the best time of day to air the house in the pollen season?
Just after rain, which has washed the air, or late in the evening. Avoid the middle of the day, when pollen release peaks in dry, warm weather.
Why is the level not the same for every pollen?
Because their allergenic strength differs: five grains of ragweed per cubic metre are enough to bother many sensitive people, whereas birch takes hundreds of grains.
Is light rain enough to wash the air?
Rain, even moderate, brings a large share of the grains back down. Simple drizzle is less effective. The site counts hours above 0.2 mm.