In Japan, one of the major causes of seasonal allergic rhinitis and seasonal allergic conjunctivitis is the Japanese cedar pollen. This pollen is one of the most potent known, causing severe symptoms. The Japanese cedar pollen is present from aproximately the end of January until the beginning of April on the north island and mid-April until early May on the south island. The cypress tree is also of importance in Japan, with its pollen scattering a little later than that of the cedar tree.

[1] Allergy Research Group, Jikei University School of Medicine : http://www.tky.3web.ne.jp/~imaitoru/English.html