In all higher plants, it is the job of pollen to transfer the male genetic material to the ovary during fertilisation. Individual pollen grains are virtually invisible to the naked eye, depending on type, each measuring from 10 to 100 thousandths of a millimetre. As they take on varying shapes, they can be identified under the microscope. Pollen contains protein compounds that are regarded by the body as foreign substances and are therefore combatted. Hay fever is an overreaction of the body's own immune system to contact with these allergens.
Which pollens trigger hay fever?
In order for pollen to be able to trigger hay fever, two conditions must be fulfilled. Firstly, the pollen must contain allergens that are attacked by the body as intruders. Not all pollens have such aggressive allergens. Secondly, the pollen must be present in the air in large quantities. Only anemophilous plants produce pollen in sufficient quantities from their flowers. Anemophilous plants (i.e. those which spread their pollen in the wind) include grasses and many trees. The flowers of a single blade of grass contain approximately 4 million grains of pollen. A single birch catkin has a similar quantity of pollen. Insect-pollinated plants produce only small amounts of pollen, as the flowers are picked out specifically by certain insects and this results in pollination. Therefore insect-pollinated plants, and in our country that mostly means plants with brightly coloured, sweetly scented flowers, have no significance where hay fever is concerned. At most they trigger allergic reactions in the event of close contact with the flower.
There are also examples of plants that produce large quantities of pollen, but despite this do not trigger pollen allergies. This is because they contain no allergenic substances. This includes the pollen of coniferous trees. In the spring, cars and window sills sometimes get covered in a kind of yellow dust, which is actually the pollen of spruce and pine trees. The worst of its effects is an irritation of the conjunctiva, much in the same way as normal dust.
Important pollen types in Switzerland
In Switzerland, six types of pollen are responsible for about 95% of pollen allergies: grass, birch, ash, hazel, alder and mugwort pollen. Other plant types that can also trigger hay fever are described in the picture section. However new plants can be added to the list. Hogweed or ragweed, the worst pollen allergen in North America, was introduced into Europe at the beginning of the 20th century. It has spread above all in Hungary, Vienna, the Po valley and around Lyon and since then has caused allergies in Europe. In Switzerland it is only to be found isolated pockets in Geneva and Ticino. However, people who are allergic to plants in the Asteraceae genus were already observing allergies. It is the job of the scientists to keep a careful eye on the spread of ragweed in Switzerland so that they can prevent allergies.
Most people do not have a reaction to just one type of pollen, but to pollen from related plant species. For instance, the birch and hazel families are closely related and have similar allergens. Therefore, we talk about "cross reactions". There can also be cross reactions between pollen and food allergies.
Those who are allergic can get a hay fever attack if the pollen concentrations in the air exceed a specific threshold, which varies from plant to plant. In the case of rye pollen, 10 pollen grains per cubic metre of air is sufficient, but with grasses the threshold is 50 pollen grains. However, on a sunny day, the grass pollen content of the air can rise to in excess of 500.
Airborne pollen and the weather
The amount of pollen in the air and consequently hay fever symptoms are heavily dependent on the weather. Temperature, sunshine, air humidity and wind all have an important role to play. Plants only release their pollen if the weather is dry and relatively warm. There are, however, differences from plant to plant. Just an hour after it has stopped raining, a birch can start releasing pollen into the air again. In the case of grass pollens on the other hand, there will be hardly any pollen in the air on rainy days. Pollen concentrations also fluctuate according to the time of day, generally with values being higher during the day than the night. However, during periods of good weather, there is hardly ever a pollen-free time. There can even be pollen in the air during the night. This means that bedroom windows should also stay closed at night, unless it has rained heavily during the daytime.
The start of the pollen season also varies from year to year: so, for example, birch trees can start flowering as early as the middle of March or it can be as late as the middle of April. Flowering gets later the higher you are above sea level. In the Alps, birches may even not start flowering until May. Therefore, it is useful to keep yourself informed about the current pollen situation.
Text from: aha! It's in flower... Swiss pollen guide. Published by aha! and MeteoSwiss (booklets are available free-of-charge from the publishers)
