I have read (but I have NO idea where/when) that depending upon the 'hotness' of the shower/bath/hot tub and the length of time spent soaking...that the skin absorbs as many toxins as drinking anywhere from 1-10 GALLONS of tap water.
(If anybody has those researched/tested figures, please post them!)
And of course, that's all even worse in a brominated/chlorinated hot tub :(