The hood of course (mostly because it was closest to a gas mask).
None of the others seemed hot to me.
It would be hot for me to see someone wearing a gas mask, but since you said it's open mouth, idk.. it still looks close enough that I would be imagining a mouth piece over it, lol
It wouldn't be hot for me to wear one, though. I'd probably feel trapped/ I like my glasses as an accessory