Yes please!
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Okay! You asked for it!
I've revised a little of it - but it gets a bit ropey towards the ened because I ran out of time - I might refine it with edits during my train journey later - but can't make any promises!
The Long Version...Skin is designed to be mostly impermeable - it's main purpose is to stop pathogens entering the more sensitive underlayers - it's secondary purpose is thermal regulation.
As you know, it accomplishes this by sweating, and in mammals, having a subcutaneous layer of fat and hairs that stand on end to trap an insulating layer of air underneath the hairs (except in humans we don't have enough hair to do that anymore so our hairs just stand up and make us look very slightly fuzzier.)
As part of it's primary purpose in being a barrier to pathogens (and other toxicants), it produces sebum - which does lots of stuff. It is secreted by exocrine sebaceous glands, which are found wherever there is hair; most notably on the scalp, face, and shoulders. Sebum also helps waterproof you - and can help distribute some of your pheremones so that other people can smell you (did you know that what we call "love" has a worryingly large basis in smell?)
Sebum contains wax esters and some fatty acids and oils and stuff - and as a result, there is a concentration gradient between it and the blood vessels supplying the sebaceous gland - and as a result, a small, usually tiny amount of whatever is in your blood will dissolve into the sebum.
Most of this doesn't matter - the concentration is so miniscule that you only really notice it if something particularly pungent ("smelly", to use the technical term) is excreted by this pathway, in which case you notice it on the skin.
If you have more oil-soluble (non-polar) molecules in your blood, say, from drinking oil every day (don't try that, by the way), the concentration gradient is slightly higher, and this results in more of it diffusing across - the larger the molecule the harder it is to get into the sebum (because there is a barrier between the blood and the excretory side of the sebaceous gland... to stop you bleeding through your hairs - which would be pretty scary)
And that is how certain oily substances that you eat (and digested) end up on your skin: some examples include certain drugs, oils, and esters - often smelling sweet or musky.
Next up we have the water soluble (polar) stuff - this one's easy - it comes out in your sweat. Your body produces salty water (perspiration) to act as an evaporative cooling mechanism - this makes it slightly less pleasant for microbes to grow in, and makes it easier to evaporate - which means it can cool you quicker.
Once again, as it's secreted, water-soluble molecules can partition across your membranes into it - but, as I mentioned, most of anything in your blood ultimately exits through your urine (except carbon, which exits through your lungs as CO
2)
Anything water soluble can come out through your sweat - the most obvious example is "allicinic acid" - the chemical that makes garlic smell... garlicky. Well, one of the chemicals. It's also a minor antibacterial (only minor though - if you have no access to any medicine it might help you a little... but even the worst antibiotics these days are still orders of magnitude more effective).
And that's why you smell garlicky when you eat garlic - well - one of the reasons, some of it also comes out in your saliva and you swallow it - and likewise, some of it ends up in your lungs - both of which contribute to garlic/anti-vampire breath after eating garlic bread.
NOW - the reverse is also possible; if you smear something onto your skin -
some of it
can end up in your blood. Certain chemicals are better at this than others - but generally speaking your skin is good at keeping chemicals out - this is why you don't swell up and die when you take a shower; this is why you can wash the dishes without poisoning yourself with dish soap; and this is why froliking in a field of grass doesn't cause you to have a massive allergic reaction to everything around you - it's physically isolated from your immune system. Obviously some people are more sensitive to certain proteins than others - and some people are orders of magnitude more sensitive to certain things - case and point - try rubbing some peanuts on somebody with a peanut allergy and see how quickly they start to panic once they realise what you're doing.
SO - now we have a plausible mechanism for absorbing some chemicals through your skin to varying degrees: good - you've got chemicals in you. Now what?
Well, that's all we need to sell you something: "your absorbs it" - and unless you know a lot of biochemistry, you'll have no idea what it's going to do - if anything. For instance, let's take a simple example - cyanide. We all know cyanide is bad - it's a poison... well, what defines a poison is the dose required to kill you - the smaller the dose required to kill you, the more poisonous it is. Cyanide doesn't require a whole lot to kill you - but guess what... your body makes it. OOOH NO! So do many plants. Some plants (like clover) produce a lot of it as a defence mechanism to ward off predators (like bunnies, who avoid it like the plague) - while other plants, like us, only produce traces as a byproduct - but that still means there is (trace) cycanide in food - just because something is detectable does not mean it has an effect; the dose makes the poison - and likewise, the dose makes the medicine.
One of my favourite examples of this is "Pantenne Pro-V" - you know, pro-vitamins! I have no idea what a pro-vitamin is... I know what a pro-drug is - it's a molecule that you eat - and when you eat it, it's not the drug - but your body digests it and makes it INTO the drug (like codeine - which turns into morphine when you metabolise it - thanks liver!).
Except... that doesn't make sense - does that mean a pro-vitamin is something you take that's not a vitamin but your body makes it into a vitamin? No, it can't be - a vitamin is, by definition, something your body
cannot make itself, and thus
must be acquired (eaten) from the environment!
Not that it matters, because you're plastering this stuff on your hair - where it doesn't get absorbed (much).
So what is it actually? Probably "just a protein" that sticks to your hair, maybe it grabs water (hygroscopic), maybe it's waxy (makes your hair shiny), or maybe it's a random vitamin that's been added - either deliberately or by accident as a result of some side reaction or other ingredient.
As cyberdragon mentioned, Essential Oils are another good example: an essential oil (literally an oil that is the essence of something - for instance, essence of rose, garlick, bluebell, whatever - it's just a distilled oil, usually from a plant, and it usually smells nice if you're going to put it on your skin, but there's no requirement for it to smell nice)
How this ties into BS is related to psychology, ignorance of science in general, and ignorance of how to do basic research.
Consumers know that some plants have
magical medicinal properties - so in their mind, there is now a coupling of the two ideas; that of "a chemical is on/in my skin" and "medicines are chemicals" - and that is the premise of the various tinctures/oils/lotions/solutions/treatments. On top of that, you have a placebo effect - which can be amazingly powerful - like, performing surgery with placebo anaesthetics and the patients are fine with it kind of amazingly powerful.
Ultimately you end up with two ingredients to BS - a plausible mechanism, and, ideally, ignorance of the subject. Unfortunately, ignorance is the default position on every subject; so there's a lot of BS out there! As James Randi said; "Ignorance is both hereditary and contagious" - a most poignant point in this day and age.
Anyway, I have ran out of time - I'll no doubt continue my ramblings at some point
Any questions - feel free to ask!