Seventy years or so ago Japan decided it needed low cost timber to rebuild after the war. And so it cut down much of its native woodland and replanted it with cedar which grows quickly and is relatively good for construction. Then someone thought "why not import cheap foreign cedar instead?". And so they did but they left the old trees to grow. And grow. And grow.
The problem here is that cedar produce vast quantities of pollen and a significant number of people are sensitive to it. Each spring face masks and eye washes become almost a necessity as the air becomes saturated with a quite visible threat. You can see it literally falling from trees as it fills the atmosphere. Almost all forest surrounding Tokyo are now Cedar or the equally problematic Cypress.
So why not cut the trees down? It would be a simple solution and they could be replaced with non-allergenic varieties if the concern is less trees means more concrete. The answer is economics and age-old redundant land laws forcing negotiation with each and every tree owner. And today the government announced 2015 will be a bumper crop. A lot of people will be having a hard time in six weeks from now. Time to lace your face masks up.