system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is happening when someone uses skin lotion/moisturizer?

I guess I’m asking how skin cells interact with the chemicals in the lotion and where those chemicals go once they’re “absorbed”.
gpt
While the answer depends on several factors (including what's getting applied to the skin, where on the body it's being applied, what vehicle (or topical agent type - think cream, lotion, ointment, foam, etc) is being used, and what the composition/active ingredient(s) are in that vehicle), let's start with the basics and move from there.  

If we're talking about things applied for the purpose of moisturizing, the goal is less about direct hydration and more about preventing excess water loss. Preventing water loss from the skin is super important: in fact, it's a primary function of the stratum corneum, the topmost layer of skin that is made of protein-rich cells that have lost their nuclei (colloquially referred to as "dead skin cells"). You can think of these cells as the 'bricks' of our skin: holding them together like 'mortar' is a lipid-rich layer full of things like ceramides, sphingolipids, and more.  

On the daily, we run into tons of stuff that can disrupt either the bricks or the mortar of this layer: dry weather, friction, caustic topicals or agents, skin diseases, you name it. When the barrier's weakened, there's more points through which water can exit this skin. This escape, called 'transepidermal water loss (TEWL)', is ultimately what results in dry skin.  

By applying moisturizing agents, you're essentially putting an external 'mortar' on the stratum corneum: this decreases TEWL, and helps to 'rehydrate' skin by making sure excess water isn't lost to the environment.  

As for 'chemical absorption', it all depends on the properties of those chemicals. The rate-limiting step for absorption from the skin is the stratum corneum: if something has a hard time passing through a lipid-rich environment, it's not going to pass through here so easily. Meanwhile, if you're trying to treat a condition that starts deeper down in the skin, having agents that can readily diffuse through the skin's top layers to exert their effect would be ideal.  

Hope this helps!