Allergenic pollens that cause rhinoconjuctivitis and/or asthma are those from trees or plants that pollinate through the air (anemophilic pollination) and not through insects (entomophilic pollination). Although pollen grains would seem to be too large to easily reach the intrapulmonary airways, the relationship between pollen counts and the presence of asthmatic symptoms is only too evident. This is probably because the allergens inducing seasonal asthma are not only found within pollen grains but also outside the grains in particles of less than 10 mm that are freely found in the atmosphere. The most important pollens producing pollinosis in Spain are those from cypress trees from January-March, birch trees in April (macizo galaico), Platanus hispanica (March-April), grasses and olive trees from April-June, Parietaria from April-July and Chenopodium and/or Salsola from July-September. By geographical areas, the main cause of pollinosis are grasses in the center and north of the peninsula, olive trees in the south (Jaén, Sevilla, Granada, Córdoba) and Parietaria in the Mediterranean coast (Barcelona, Murcia, Valencia).