Andrew Huberman· PhD
Next to every hair root within the dermal layer of the skin, so this is below the epidermis, there is a gland called the sebaceous gland, and the sebaceous gland makes oily stuff, and the oily stuff is called sebum. The sebum does two things. First of all, it forms a little bit of a seal right at the place where the hair exits the skin. And that seal is very important, actually, for waterproofing of your skin.