Talking to a good friend of mine, I recently discovered this interesting fact about hay fever in Japan: after World War II, there was an extensive program of reforestation in Japan - wood was needed to build houses. Two particular trees were chosen for this: the sugi (a Japanese cedar) and the hinoki (a Japanese cypress). These two trees are the main cause of hay fever in Japan, which is a real problem for a large part of the population.
Very interesting article on Wikipedia, about how the cheaper imported wood that is now used means that a lot of the trees that were planted 30 years ago haven't been cut. And it is the older trees that produce the most pollen! (photo by ふうけ - ふうけ's file, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=546227)
I feel blessed here in Ireland - I don't seem to be allergic to anything in the springtime. Whereas in Belgium, I used to get really affected, particularly in June - prime exam season too! I suspect the silver birch trees at the back of my parents' house were a major factor, although I also suffered when I was studying in Brussels.