Thank you, SoLa-san. I know that. I just failed to understand what ないではないか means so I thought ぬ might have other functions.
No, it's not a double negative. The seeming negative じゃないか is not a negative but emphasizes the forgoing sentence whether it is affirmative or negative. You would want to translate it by "Do you realise that" or "Don't you see that" or something to that effect. Here, the speaker is wondering why the listener does what they see as an obstacle for wiping sweat off.ない ＋じゃないか（ではないか） っていうのは二重否定と言います