It isn't the heat, it's the humidity
Yes, it's a cliche, but still sadly true.
It's the rainy season here in Tokyo, which doesn't mean Calcutta-style monsoon downpours, but does mean a low heat accompanied by a persistent and clammy humidity. Clothes hung out to dry, don't* ; mold grows in the damnedest places, and spend any time outside and you feel like you need a shower NOW.
God, no wonder I feel so tired and irritable.
*Laundry is, indeed, hung out to dry, since home clothes driers are still uncommon--apartment buildings, even new ones, have clotheslines on the balconies).