What does a high pollen count mean?
A pollen count is a measurement of how many pollen grains are in a cubic metre of air. Forecasts turn that number into a level from none to very high, because the raw count means little on its own. The thresholds also differ for grass, tree and weed pollen.
The levels
- None 0/4
- Effectively no pollen of that type in the air. Out of season, or washed out by rain.
- Low 1/4
- Most people notice nothing. Highly sensitive people may get mild symptoms on a long day outdoors.
- Moderate 2/4
- Many allergy sufferers start to react. A good day to take antihistamines before going out rather than after.
- High 3/4
- Most people allergic to that pollen will have symptoms. Windows shut, medication on board.
- Very high 4/4
- Severe symptoms are likely for sufferers, including people who normally cope. Uncommon. Usually the peak of the season on a warm dry breezy day.
What the numbers behind them mean
For grass pollen, the trigger for the large majority of British hay fever, the traditional Met Office bands are roughly: low below 30 grains per cubic metre of air, moderate 30 to 49, high 50 to 149 and very high 150 or more. Tree and weed thresholds sit at different numbers because it takes far more nettle pollen than birch pollen to produce the same misery.
This site shows a level for each of the three pollen types separately rather than a single blended number, because a "moderate" overall day made up entirely of grass pollen is a bad day if grass is your trigger and a non-event if it is not.
Why the count changes through the day
Pollen is released in the morning and rises with warming air. Counts are typically highest in the middle of the morning, drop as the warmest air lifts pollen high above head height, then rise again in the early evening as the air cools and grains sink back down. Early morning and early evening are the two windows to be careful about.
Weather matters as much as season
Warm, dry, breezy days give the highest counts. Rain clears pollen from the air. A properly wet day can drop a high count to low. Thunderstorms are the exception: the downdraught can concentrate pollen at ground level and humidity can rupture grains into finer fragments that reach deeper into the airways, which is the mechanism behind so-called thunderstorm asthma.
How accurate is a pollen forecast?
It is a model, not a measurement of the air outside your door. Forecasts come from atmospheric models fed by monitoring stations, land cover and weather, so they describe the general level across an area rather than your street. Treat a forecast as a guide to whether today is a take-precautions day, alongside how you actually feel. Our data comes from the Google Pollen API, which is derived from models including the Copernicus Atmosphere Monitoring Service; we are not affiliated with the Met Office.
Today's levels near you
Related: the UK pollen calendar and when hay fever season starts and ends.