Pollens are one of the most important causes of respiratory allergy including allergic rhino-conjunctivitis, and can even cause food allergies.123 Recently, the prevalence of both pollinosis and pollen-food allergy syndrome has increased in Korea, which may reflect climate changes4 as well as increases in the growth of tree stocks in Korean forests. From 2005 to 2015, the growth of tree stocks doubled from 79.2 to 146 m²/ha,5 and the planting density of trees in the order Fagales (including oak, birch, and alder) increased steadily, in contrast to those of pines. The types of pollens responsible for allergic reactions differ geographically, as the major inhabitant trees, grasses, and weeds markedly differ from region to region. To be termed a “causative pollen,” several conditions must be met. In the 1930s, August Thommen6 suggested a list of postulates for causative pollens, which is still used today. First, the pollen must contain allergens to induce pollinosis, as it is not true that every type of pollen is allergenic. Secondly, the pollen should be windborne. Although pine tree pollen is windborne and abundantly distributed in Korea, its allergenicity is minimal and does not induce clinical symptoms. Thirdly, the pollen must be produced in sufficiently large quantities to cause symptoms. Fourthly, the pollen must be sufficiently buoyant to be carried considerable distances. Trees that distribute windborne pollen produce large amounts of pollen with buoyant shapes. For example, a single birch tree produces about 5.5 billion pollen grains annually, whereas an oak produces 0.6 billion pollen grains. Finally, the plant producing the pollen must be abundantly distributed. Species of trees that meet these criteria are limited in each country, and thus a pollen calendar should focus on tree species that meet Thommen's postulates.