What is pollen?
In order for plants to reproduce, they must exchange male and female reproductive cells. The female cells remain on the plant, and pollen, a fine powder that contains microscopic grains containing the male cells, is transported around. Some plants self-pollinate, some rely on the wind to spread the pollen and some need a carrier, such as bees and other insects.
Pollen carried by the wind has to be tiny and lightweight and is therefore easily inhaled by humans and pets. In some cases, this causes an allergic reaction known as hay fever. There are around 30 different types of pollen that cause hay fever and it is possible to be allergic to more than one type.
What is the pollen count?
The “pollen count” is literally the amount of pollen per cubic metre observed over 24 hours. It is sometimes used interchangeably with the term “pollen forecast” but the pollen count is used, along with what the weather is up to, to forecast pollen levels over the coming days. The 'forecast' is actually a forecast of the risk of the level of pollen over the coming days.
How is the pollen count measured?
There are various monitoring sites across the UK which measure pollen between March and September. Most sites have been in operation since the 1960s, and they form part of a much bigger European Aeroallergen Network based in Vienna.
Pollen is physically collected using a Burkard trap. Inside the trap is a spindle with sticky paper wrapped around it. This rotates slowly and air is drawn through a slit in the casing surrounding the spindle. As air flows through the trap pollen particles are left on the paper ready for counting.
Traps are usually placed on roofs in order to sample air that has been well mixed up. Too close to the ground and you’ll get all sorts of pollutants and bits of dust; too high and you’ll only collect tree pollen. The paper is removed and the pollen particles are counted using a microscope.
When is the pollen count considered high?
As you’d expect, this very much depends on the type of pollen, but once counted, the number of particles is represented per cubic metre. Hay fever symptoms usually appear when the pollen count exceeds 50.
- Grass - a reading between 50 and 150 grains of grass pollen per cubic metre is considered high.
- Birch - a reading between 81 and 200 is high
While this is how we define a 'High' reading, it's worth noting that different people have different tolerance to different pollen types and counts, so some will be more sensitive than others.