A tube to insulate a CPAP hose so that the heated humidity doesn't condense inside the hose, then drain into your nostrils in the middle of the night. Very disconcerting when that happens. You can buy hose cozys, though I don't know what they're actually, technically, called. Nor do I know how much they cost. Possibly not much, possibly quite expensive. More fun to crochet one, though.

So, yes, a narrow tube.