Japan belongs to the temperate zone with four distinct seasons, and has six principal climatic zones: Hokkaidō (北海道) with long, cold winters and cool summers; Central Highland (中央高地) with typical inland climate and large temperature differences between summers and winters and between days and nights; Seto Inland Sea (瀬戸内海): with mild climate; Pacific Ocean (太平洋): with cold rainy winters and hot humid summers; Ryukyu Islands (南西諸) with warm rainy winters and hot humid summers. Heat and high humidity can lead to possible food poisoning, fatigue and heat stroke.
More than 5,000 species of plants find home in these diverse climatic conditions and more than 60 types produce pollen that can cause allergic reactions. In Tokyo, common allergy-causing plants and their pollen seasons are as follows:
Japanese Cedar is one of the major causes of seasonal allergic rhinitis and seasonal allergic conjunctivitis (itchy eyes). Its pollen is one of the most potent known and may cause severe symptoms. In Tokyo, about one in 3.5 people is believed to suffer from sugi allergy. The Japanese cedar pollen is present from approximately 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 flowers a little later than that of the cedar tree and could also contribute to allergies. The pollination of Ragweed begins in late summer, around August, but its impact is less significant than in North America.