How can some toxins or chemicals be absorbed through the skin when some others can’t?

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Some chemical compounds (the one that triggered this questions for me was LSD) can be absorbed through the skin into the bloodstream and should be handled with gloves, while some others are harmful (e.g. ammonia) but aren’t absorbed into the bloodstream? Is it a matter of solubility?

In: 5

In this case size matters. Some molecules are tiny and are not repelled by the natural oils and so they can enter the stretchy pores of our skin. I don’t know the role of the sweat and the solubility.

It depends on what the lipid layer of the skin allows to pass through. As lipids are mostly nonpolar, polar substances like water or ammonia aren’t absorbed easily through the lipid layer. As for LSD, chemist manufacturers have been quoted to say that it can’t actually be absorbed by the skin: [https://www.reddit.com/r/LSD/comments/9ub4vs/how_easily_can_it_be_absorbed_thru_the_skin/](https://www.reddit.com/r/LSD/comments/9ub4vs/how_easily_can_it_be_absorbed_thru_the_skin/)

That is quite logical, as the LSD molecule is polar.