Six genus and about 150 different species belong to this Family. Among them,
the most important allergologically are Birch tree (Betula alba or pendula),
Alder (Alnus glutinosa), and Hazel (Corylus avellana).
They are trees and bushes mainly located in the north and center of Spain
and Europe, where they have greater allergologic relevance (see maps).
Birch tree pollen is spheroidal, with small-medium size (20-25 µm in
diameter), with a somewhat granulated surface, and it has three pores. Its
flowering season is between March and May.