[tpm]

> In Japan, however, it's almost always very humid.

Yeah I realised that's a little bit of an issue in my theory. Of course if it's very humid I would also expect that most buildings have aircon that's keeping the humidity down and can contribute to the allergies.

[Tor3]

Buildings, as in large buildings, maybe. Homes don't use that much aircon. We actually never do, except if we have guests, and in any case it's never used the way I experienced so often in the US - that it's actually cold (and very uncomfortable). When used in Japanese homes it's used sparingly. There's also typically a "dehumidify"-only setting, but that's mostly a losing battle, what with the house being surrounded by humid air (and until very recently even new homes were far from airtight), so mostly you just live with it.