Pollen is one of the major causes of respiratory allergy (i.e. asthma and rhinitis) and conjunctivitis. Pollen is produced by plants during the flowering period and it can be dispersed in the environment carried by insects (entomophil pollen) or transported by wind (anemophil pollen). Pollen able to induce symptoms in allergic people is usually anemophil and small enough to enter the upper airways. Plants producing allergenic pollen can be grasses, weeds or trees.

Grasses - Grass is the most common source of allergen in Europe, since it is widely distributed and the different species are highly cross-reactive. Grasses flowers during spring, usually from April to June, but pollen can be detected also in September in southern countries. In northern, central, and eastern areas, the main grass flowering period starts at the beginning of May and finishes at the end of July. In the Mediterranean area, flowering usually starts and ends one month earlier. Pollination occurs about 2-3 weeks earlier at sea level than in mountainous regions. On the whole, in Europe, grass flowering notoriously peaks in June.
Weeds - Ragweed (Ambrosia) and mugwort (Artemisia) are the Compositae (Asteraceae) most involved in pollinosis. Artemisia grows mainly in southern Europe and it flowers from late July to the end of August in northwest Europe.

The genus Ambrosia (A.), which includes both A. artemisiifolia (short or common ragweed) and A. trifida (giant ragweed) has long been recognized as a significant cause of allergic rhinitis. The pollen of A.artemisifolia is produced in enormous amounts and one single plant alone may produce millions of pollen grains. The most representative species, A.artemisifolia, was first signalled in Europe in 1860 and ragweed pollen is increasingly important from an allergological point of view in parts of Central and Eastern Europe. Ragweed and mugwort have nearly identical flowering seasonal periods and high degree of cross-reactivity.

Parietaria (wall pellittory) is the main allergenic genus of the Urticaceae (nettle) family. The most important species are Parietaria judaica, which grows mainly in coastal Mediterranean areas and Parietaria officinalis. The extraordinarily long persistence in the atmosphere of Parietaria pollen in the Mediterranean area is responsible for a multiseasonal symptomatology. In some areas, like southern Italy, some patients have year-long symptoms.

Trees - Birch allergy is very important especially in central and northern European countries in which it represents the main source of allergenic pollen. In Western Europe, the main flowering period usually starts at the end of March, and in central and Eastern Europe, from the beginning to mid-April. Going northward, the flowering season start, depending on the latitude, from late April to late May (northern Europe). Far shorter or longer periods, with yearly alternating low and high pollen production, have been observed in various European regions.

The Corylaceae trees hazel and alder are the first (December-April) to shed pollen in the outdoor air in Europe.

Olive (Olea europaea) pollen is considered as one of the most important causes of respiratory allergic disease in the Mediterranean region. In Spain, southern Italy, Greece and Turkey, olive pollen is an important cause of pollinosis. The main pollen season is from April to June.

The genus Cupressus is widely spread in Mediterranean area. It releases an enormous amount of anemophilous pollen in the late winter, a period of the year when no other allergenic plants are flowering. Cypress allergy is characterized by higher prevalence of dry cough and a lower prevalence of conjunctivitis compared to grass pollen allergy.