+1. Though there are common allergens (like sulfates, parabens, many essential oils, etc.), that doesn't mean that the common allergens are causing a problem for you. It can, literally, be anything. I'd start by cross-referencing the ingredients in products you were using previously so you can rule out a bunch of common ones. Then make a note of which product has what, where the rash occurs (if it's only on your hands, then it could be just in a styling product), and introduce one new product at a time. Also, if you have any known food allergies, look for related ingredients in your products (eg, if you're sensitive to wheat, then wheat germ oil, wheat protein, and so forth will probably cause problems for you).
Rather than washing with everything and risking full-body hives, do a patch test. Take a little bit of product and rub it into your forearm. Don't rinse it off - just leave it for about 24 hours (or less if it starts itching). If it begins itching, especially if you get hives in the area, then that'll be your problem product. I'd test ALL the new stuff, though, just in case there's a common ingredient in them causing you problems.
3b/c. Fine, med-high porosity, normal elasticity. Hair loves oils in summer, protein in winter. Constantly battling the hard water monster.
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American literature PhD student in the UK...