Fumes

You know you haven't been drinking enough water when you go to pee, and the smell of your own urine makes you want to pull your shirt up over your nose.

It's almost as bad as stepping up to a urinal and finding that the previous user didn't flush. Nothing quite like having to pre-flush and still getting to soak in someone else's pee-fumes while you try to hold your breath for the duration of your pee. Invariably, this will occur when you haven't peed in about 3 hours, so the duration of your stay is at the longer end of the spectrum.