Aquagenic pruritus. Eg. Chlorine molecules are much smaller/less mass than water, yet a single molecule could trigger their histamine responses. With some people, a single drop of water, distilled or not, could set them off, with itchy hives all over their body, in some cases, even a single drop of rain, or humidity could send them into a potentially-lethal shock reaction. (I saw a show on Opera about a girl with this condition, fog could set her off and she could only shower 30 secs each week and drink milk or juice only because of the chemical composition) bound water isn't a problem, it's the water itself that's why they're only allergic to themselves if they sweat. On 1,000 ways to die, a woman had this condition, the fire sprinklers went off and she died because it made contact with her skin. So is it because the molecule is big, it can bind to the receptor site(s)? I was wondering is it something do do with the OH- and H+ ions? example: one of my friends cannot touch rainwater, or it literally peels his skin off, his throat closes up and gets extremely sore if he drinks water. the girl I mentioned cannot even touch sweat, saliva or blood, and uses air conditoners constantly to stop perspiration.