There was an article in the paper a couple years ago, which talked about the amount of bacteria in hot tubs vs. swimming pools (millions of particles vs. less than a hundred). Warmer temps allows more things to grow -- talked about millions of nasty fecal particles and other things growing in hot tub water. And also about tubs with hot tub jets, hot they're difficult to clean/maintain, so things tend to live/grow in the jets. Bleah, when I gave birth to DS the hospital room had a hot tub. Ewww!
We used to have a hot tub and used it more in the winter months than summer -- when you're sitting in the water, you tend to inhale a lot of the steam, etc. because your face is just above the water level. Our hot tub had a bromine float, sometimes if we just filled the float, we'd be inhaling bromine fumes. Other times our friends sons would come use our hot tub and we'd end up draining it afterwards as we weren't quite sure if they bothered to take bathroom breaks. Sometimes in the summer, the pump wouldn't run as often 'cuz of the heat, so the water would stagnate -- we'd find a bit of greenish algae? growing on the inside edge of the cover.
There are also several articles out there about a rash people get from hot tubs, which is basically pseudomonas follicultis. I know that's one of the issues with a local indoor waterslide park.
Basically you just don't know how hotels and other people maintain their hot tubs. Some people are meticulous, others aren't. With our we did well with maintenance in the winter, but summertime when we didn't use it as often --- not so great.
