Almost every time you listen to the weather forecast in spring and summer, there seems to be a mention of the pollen count. Unless you have hay fever or another allergy, you may not pay much attention to this; and in fact a lot of people don’t really know what ‘pollen count’ means although they may realise that a high count means problems for hay fever sufferers.

In fact you measure the pollen count by counting the number of grains of pollen that you can find in a cubic metre of air. Pollen itself is a powder, usually but not always quite fine. It contains the material necessary to produce seed plants, which in turn produce the male plant cells. If you examine a single grain of pollen you will find that it has a cover; a kind of shell that protects the cells when they are carried from their place of production, the anther (a part of the stamen) in one flower to the stigma, a receptive place in another flower of a compatible type. This work of carrying and spreading is called pollination and is carried out either by the wind or by insects, especially bees.

Most pollination occurs in warm spring or summer weather, and the Met Office, for example, describes the pollen count season as being from March to August. However, high pollen levels can even occur in January or November with the right combination of conditions. The type of pollen will vary according to the season, with tree pollen occurring in early and mid spring, grass in summer and weed pollen in late summer and early autumn.

To measure the pollen count, you usually use a special trap which is set in a high position, like a building roof, so that it will get maximum exposure to wind and the air caught in it will contain a large variety of the pollens in the area. Being high up the trap will also collect pollens from further away than a ground level trap could do.

Pollen particles are sucked into the trap, which contains a slit to let the air in, and then caught on a sticky substance inside the trap, such as adhesive tape. The tape is on a round drum which slowly turns, so that fresh pollen can be caught continually,  or on a microscope slide. Every day the tape is changed and then it is treated with a special substance, a staining agent which makes it possible for the different pollen types to show up. It is then examined under a microscope and the grains are counted, as well the different types being recorded.

The count is usually based on grass pollen, and the number recorded is simply the number of grains of such pollen caught in the trap that day (a full day of 24 hours). The count is reported, and the pollen forecast for next day is based on the pollen count as well as other factors that may affect pollen levels, such as temperature and wind.

A high pollen count would usually be reckoned as over 150 grains of grass pollen per cubic metre of air, going down to a low count which is 30 grains or less. On the weather forecast the count is usually described as being low, moderate or high (moderate is around 35 to 49 grains).

Anything above about 30 grains is likely to cause some problems to allergy sufferers, though most symptoms aren’t too severe unless the count is high. However, different people are allergic to different allergens which may occur in different pollen types.

You can learn more about pollen counting here.