system
Given the following post, respond as if you're commenting on Reddit:
human
Progesterone Cream (USP) is rubbed into skin to be absorbed into the bloodstream in an hour. With this understanding of skin permeability, is it known if chemicals/bleach used in hair dying could also get into the bloodstream, or react with skin in some way to be absorbed into the body?

I recently learned that many women rub Bio-identical (USP) Progesterone cream into their skin, to get it quickly into the bloodstream, blowing my mind, as I thought skin was way more impenetrable than that. Now, all the various strong chemicals people (especially women in pursuit of beauty) put on skin seem scary. Which ones can be absorbed into the body?
gpt
Rule of thumb is that chemicals comprised of small-ish fat soluble molecules penetrate the skin more easily when applied with a fat-based medium, like cremes and pastes. The more water soluble a substance is, the larger the molecule is and the less fat based the medium is (either water based or solid/dry/powdery), the less likely it is to actually penetrate the skin, enter the blood stream and act throughout the entire blood system. The larger the skin surface you rub it in the more will penetrate, and the more damaged the skin is (wounds, inflammation, infection for example) the more will penetrate, the thinner the skin is (skin at the back vs skin on the scalp) the more will penetrate. That being said, we're still talking about trace amounts entering the central blood stream in either case, as the skin doesn't actively resorb substances, instead substances have to diffuse throughout the skin into the blood stream. Rubbing a large surface of your entire body with progesterone cream (progesterone being entirely fat soluble and a small enough molecule; the body doesn't need much overall progesterone at all) is a different story than putting a bit of dry make-up on a couple of select few surfaces on your face. The main component in bleach based hair dying products, hydrogen peroxide, is constantly produced and metabolized in the body in trace amounts already, and the miniscule amount added by resorbing through the skin (which can lead to skin irritation and chemical burn) is metabolized before it can even hit the blood stream in meaningful quantities. Additionally, in some legislations cosmetic products have legal limitations in how deep their main components are allowed to be able to penetrate the skin.