How Pollen Forecasts Work

What Is a Pollen Count?

A pollen count measures the number of pollen grains in a given volume of air over a 24-hour period, reported as grains per cubic metre (grains/m³). It tells you how much allergenic pollen is floating around and whether it’s likely to trigger symptoms.

Pollen counts are retrospective: they tell you what was in the air yesterday. Forecasts combine these counts with weather predictions and historical data to estimate what’s coming next.

The Three Pollen Types

Not all pollen seasons are the same. There are three major categories, each peaking at different times of year:

Tree Pollen: Spring (February–May)

Trees are the earliest pollen producers. Common allergens include birch, alder, olive, ash, and plane trees. In warmer regions, tree pollen can start as early as January. In Mediterranean areas, olive tree pollen is a major spring allergen. In northern Europe, birch dominates.

Grass Pollen: Late Spring to Summer (May–July)

Grass pollen is one of the most common triggers of hay fever. Even low counts can cause symptoms; grass pollen is particularly potent. Key species include timothy grass, ryegrass, and meadow grass.

Weed Pollen: Late Summer to Autumn (August–October)

Ragweed and mugwort are the dominant weed allergens. A single ragweed plant can produce up to a billion pollen grains in a season, and they can travel hundreds of kilometres on the wind.

The exact timing varies by region and climate.

Pollen Levels

Pollen concentrations are classified using clinically relevant thresholds established by the European Academy of Allergy and Clinical Immunology (EAACI) and used by the Copernicus Atmosphere Monitoring Service (CAMS). The thresholds differ by pollen type because some types are far more potent than others: a grass count of 3 grains/m³ is already clinically relevant, while tree pollen needs to reach 10 grains/m³ before the season is considered active.

Threshold	Trees & Mugwort¹ (grains/m³)	Grass & Ragweed (grains/m³)
Season onset	≥ 10	≥ 3
Peak period	≥ 100	≥ 50

¹ Alder, birch, and olive share these thresholds with mugwort. Although mugwort is a weed, CAMS groups it with tree pollen because it has similar concentration levels.

Below the season onset threshold, concentrations are unlikely to cause symptoms in most people. Once the peak threshold is reached, even those with mild sensitivities are likely to be affected.

How Pollen Is Collected

Most pollen monitoring stations across Europe use a device called a Burkard trap (or Hirst-type sampler). It works by drawing air through a narrow slit onto an adhesive-coated tape that slowly rotates over seven days. The tape is then cut into daily segments, mounted on slides, and examined under a microscope where a trained analyst identifies and counts the pollen grains by type.

This process is labour-intensive, which is why most pollen counts arrive with a 1–2 day delay. Newer automated systems using digital holography and image recognition are being deployed, particularly in Switzerland and Germany, that can deliver near real-time readings every few hours.

The European Aeroallergen Network (EAN) coordinates a network of monitoring stations across Europe that provide the observational data used to validate pollen forecasts.

How Forecasts Are Made

The Copernicus Atmosphere Monitoring Service (CAMS) produces four-day pollen forecasts for six allergenic species across Europe: alder, birch, grass, mugwort, olive, and ragweed. Forecasts are updated daily with hourly concentration data at around 10 km (6.2 mi) resolution and are freely available.

CAMS forecasting combines several inputs:

1. Monitoring data: Current and recent pollen counts from EAN ground stations establish what’s actually in the air.
2. Plant phenology: Knowing which plants are flowering and at what stage of their cycle helps predict when pollen release will ramp up or wind down.
3. Weather models: Temperature, wind, rain, and humidity all directly affect how much pollen gets into the air and how far it travels.
4. Historical patterns: Multi-year data for each region provides baselines for what to expect at a given time of year.
5. Atmospheric transport models: The SILAM (System for Integrated modeLling of Atmospheric composition) model simulates pollen release, dispersal, and removal across large areas, including cross-border transport of pollen from hundreds of kilometres away.

How Weather Affects Pollen

Weather is the biggest day-to-day variable in pollen levels:

Warm and dry: Pollen release increases. Plants actively shed pollen in warm, dry conditions. These are the worst days for allergy sufferers.

Wind: Spreads pollen over long distances. Dry, windy days see the highest airborne concentrations.

Rain: Washes pollen out of the air. Counts typically drop during and immediately after rainfall. However, the day after rain often sees a spike as plants release fresh pollen once conditions dry out.

Humidity: High humidity makes pollen grains heavier, so they don’t travel as far. Very dry air keeps pollen airborne longer.

Thunderstorms: A special case. Cold downdrafts concentrate pollen at ground level, and moisture can rupture pollen grains by osmotic shock into tiny fragments small enough to penetrate deep into the lungs. This can trigger severe reactions even in people who don’t normally have asthma, a phenomenon known as “thunderstorm asthma”.

Tips for Allergy Sufferers

• Check the forecast before planning outdoor time.
• Keep windows closed on high pollen days. Use air conditioning set to recirculate.
• Shower before bed to wash pollen out of your hair and off your skin.
• Don’t dry laundry outside during pollen season; it collects on fabric.
• Wear sunglasses outdoors to keep pollen out of your eyes.
• Start medications early: antihistamines and nasal sprays work best when started 2–4 weeks before your season begins.
• Rinse your nose with saline solution to flush out pollen after being outside.

How Airpult Shows Pollen

On Airpult, pollen data is displayed alongside air quality on the forecast page. You can see the current level for each of the three pollen types: tree, grass, and weed. To find a location, use the explore page to search for any city or town and jump straight to its forecast, including current pollen levels.