Severe allergic symptoms are primarily triggered by the tall grasses growing in pastures, various cereal species such as maize and rye.
Sweet grasses are the most important fundamentals of the human diet. However, they are also the main triggers of hay fever. Around 70% of pollen allergy sufferers react to grass pollen. It is an almost impossible undertaking to avoid them because they are found worldwide and, as wind pollinators, they produce huge amounts of pollen.
Yet not everything that looks like a grass is actually a problem for hay fever sufferers. Sedges (Carex), rushes (Juncus), galingale (Cyperus) or cotton grass (Eriophorum) look very similar to true sweet grasses but, for instance, belong to other plant families and therefore have no appreciable allergenic potency for sufferers of grass pollen allergy.
The family of sweet grasses (Poaceae) comprises around 9500 different species. Although it is not the biggest family of flowering plants, economically it is still the most important plant in the world. Cereal species such as rice, wheat and maize form the foundation of the human diet, sugar cane supplies the bulk of the world’s sugar and many grasses serve as a source of pasture feeding for domestic and wild animals. Grasses are found all over the world and occur in nearly all climatic regions. Fossil finds prove that the first grasses appeared on the earth about 60 million years ago, at a time when the primitive dinosaurs were dying out. Although all sweet grasses have the same flower structure, they could not be more varied in their appearance. As well as one-year grasses only a few centimetres tall, there are some that grow to enormous heights and reach well over 100 years of age. This kind of giant grass thrives in tropical Asia, for instance: the giant thorny bamboo (Bambusa bambos) reaches just under 40 metres and grows up to 91 centimetres a day.
Sweet grasses are pollinated by means of the wind. This means pollen transmission is far less targeted than it is by animal pollination. The wind pollinators make up for this deficit by mass producing flower pollen. This leads to regular "dust clouds" during the flowering season. Efficiency is enhanced by the fact that grass plants themselves often appear in masses. In the case of rye (Secale cereale) the average quantity of pollen per stamen is around 19,000 pollen grains. Thus an average stem will produce around seven million pollen grains.
There are around 220 different species of sweet grasses that grow in Switzerland. They differ considerably in respect of pollen allergies. Thus a number of grasses are not very allergenic and only rarely trigger cross-reactions with foods, such as the common reed (Phragmites australis), various oat grasses (Avena sp.) or the Bermuda grass (Cynodon dactylon). Severe allergic symptoms are primarily triggered by the tall grasses growing in pastures, such as timothy grass (Phleum pratense), orchard grass (Dactylis glomerata), false oat grass (Arrhenatherum elatius) and perennial and Italian ryegrass (Lolium perenne and L. multiflorum). However, various cereal species such as maize (Zea mays) or rye are allergenic as well. Rye probably has the highest allergenic potency among all grasses. The highly allergenic grasses thrive almost everywhere: in our meadows and pastures, at roadsides or in forest clearings, from the colline zones up to the subalpine level. The flight of their pollen starts in April and ends in September, with the main flowering season between May and July.
In Switzerland around 1.2 million people, roughly 20% of the population, are affected. The flower of a single blade of grass contains around 4 million flower pollen grains. Conifers are among the flower pollens that do not trigger any allergy. Visible as a yellow dust-fall in springtime, at most they cause irritation of the conjunctiva.
Author: Beat Fischer, BAB Office for Applied Biology, Berne