What is considered a high pollen count?

What a pollen count actually measures

A pollen count is a measurement, not a feeling. The Met Office puts it plainly: the pollen count is literally the amount of pollen per cubic metre observed over 24 hours. The American Academy of Allergy, Asthma and Immunology (AAAAI) describes how that number is made: an instrument, usually on a rooftop, collects particles for 24 hours, and the sample then goes to a lab where the pollen types and their concentration are worked out.

That means a count describes air that has already passed. Most apps and weather pages show something else: a forecast. The AAAAI draws the line clearly. Pollen counts are different from pollen forecasts, and forecasts are predicted from the previous year's counts and current weather conditions. In Europe, many forecasts come from computer models; the Copernicus CAMS European air quality forecast, for example, is published through Open-Meteo in grains per cubic metre for six pollen types, up to four days ahead.

Whichever you are looking at, the direction is the same. In the AAAAI's words, the higher the count, the greater the chance that people with hay fever will have symptoms when they are outdoors or exposed to outdoor air.

Where high starts for grass, birch and other pollen

There is no single number that means high for every plant. The Met Office says a reading between 50 and 150 grains of grass pollen per cubic metre is considered high, while for birch a reading between 81 and 200 is high. It adds that hay fever symptoms usually appear when the pollen count exceeds 50.

Germany's national weather service, the Deutscher Wetterdienst (DWD), publishes its own bands, based on daily average pollen per cubic metre of air. For grasses it counts 1 to 5 as low, 6 to 30 as medium and anything above 30 as high. Birch is low at 1 to 10, medium at 11 to 50 and high above 50. Hazel and alder are low at 1 to 10, medium at 11 to 100 and high above 100. Ash is low up to 50, medium from 51 to 75 and high above 75.

The weed and cereal bands sit much lower. DWD rates rye as high above just 6 grains per cubic metre, and mugwort and ragweed as high above 10. Seen side by side, a count of 40 would be high for grass in Germany, medium for hazel, and far into the high range for ragweed. The number only means something once you know which plant it belongs to.

Why two forecasts can disagree on the same day

Put the British and German grass bands next to each other and the gap is obvious. A day with 40 grains of grass pollen per cubic metre sits below the Met Office's high band, which starts at 50, but inside DWD's high band, which starts above 30. Neither is wrong. They are two organisations drawing the line in different places.

Scales differ too. DWD's pollen hazard index, the Pollenflug-Gefahrenindex, is not given in grains at all but in steps from 0 to 3, with half steps in between: 0 for no load, 0-1 for none to low, 1 for low, 1-2 for low to medium, 2 for medium, 2-3 for medium to high and 3 for high. DWD explains that it uses these intermediate steps because its forecast areas are large, so one level and the next can both occur inside the same area. The index covers eight pollen types in 27 areas of Germany.

Resolution plays a part as well. Open-Meteo lists the CAMS European model at roughly 11 kilometres, so one value covers a wide patch of land, and a park, a hay meadow or a city centre inside that patch is not resolved separately. A local count from a single trap and a model forecast for the surrounding area can easily land on different sides of a threshold.

How to read a pollen forecast for yourself

Start with the plant, not the headline. The Met Office says most people with hay fever are allergic to grass pollen and that tree pollen affects about a quarter of them. A forecast headline of high driven by a tree you do not react to tells you little, and a moderate headline can hide a high reading for the one grass you do react to.

Then check the scale. If the forecast gives grains per cubic metre, compare it with the bands for that plant. If it gives an index such as DWD's 0 to 3, read the step and its label rather than converting it in your head. Note who produced it, because the Met Office and DWD will not always agree.

Finally, remember that thresholds are averages. The Met Office stresses that different people have different tolerance to different pollen types and counts, so some will be more sensitive than others. A band tells you how much pollen is in the air. It cannot tell you how you will feel.

Common mistakes when reading pollen levels

Comparing labels across sources. A high in one app and a moderate in another may describe the same air. Check whether both use the same scale before deciding one of them is wrong.

Treating a forecast as a measurement. The AAAAI separates the two: counts reflect what was collected, forecasts are predictions built from past counts and the weather. A forecast for tomorrow can change once tomorrow's weather is clearer.

Reading low as none. DWD's low band for most pollen starts at a single grain per cubic metre. Low means less pollen, not no pollen.

Watching the total instead of your plant. A single overall level folds every pollen type into one word. Pollergo, for example, can show the level for only the pollen types you pick, with the raw value, its scale and the source on each card, so you can see whether a high comes from your grass or from somebody else's birch.

This guide explains how pollen counts and forecasts are reported. It is not medical advice. If pollen affects your health, talk to a pharmacist, doctor or allergist.

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