Birch (Betula spp.)
Flowering: Late March - May
The tree may reach up to 30 meters height. The alternate leaves are doubly toothed.
The trunk, typically white and black, is smooth. The male and female flowers are grouped in distinct catkins. They produce high amounts of allergenic pollen grains (one third of the total pollen amounts in the air).
The most common species in Belgium are the silver birch (Betula pendula syn.: B. verrucosa and B. alba), and the downy birch (B. pubescens). These trees are adapted to full light exposure and are therefore among the first trees colonizing clear areas. They are present on soils that are poor, sandy, dry to humid, and acidic.
In Belgium, birch pollen is considered as the major cause of allergy due to tree pollen. Its concentration in the air highly fluctuates from one region to another in our country. By the sea, pollen density in the air is usually lower. People sensitized to birch pollen will thus benefit from a stay at the seaside. Fluctuations in pollen grain concentrations are observed from year to year. Indeed, the pollen concentrations in the air depend on the biorythm of the plants (a year of high production for birch pollen is often followed by 2 years of lesser production), and on meteorological conditions during the inflorescences development and at the time of pollen dispersal. Rainy conditions usually lower the pollen amounts in the air.
Cross-reactive allergy: there is cross-reactive allergy between the pollens of the different Betulaceae species (alder, hazel and birch). Therefore, individuals allergic to birch pollen may also experience allergy symptoms at the beginning of the year, when the concentrations of alder and hazel pollens are very high. Some individuals sensitized to birch pollen may experience cross-reactive allergy with some foodstuffs (apples, cherries, almonds, hazelnuts, kiwis). Consumption of these raw fruits may cause symptoms such as pruritus and mouth swelling.