Applying lotion initiates a complex process involving physics, chemistry, and biology. Lotion is typically an emulsion of water, oils, and compounds designed to deliver moisture and protective agents to the skin. Where it goes is complicated because the skin is a sophisticated, multi-layered organ that acts as an effective barrier.
The Skin’s Outer Layer: A Protective Barrier
The primary destination and first point of resistance for any topical product is the stratum corneum, the outermost layer of the epidermis. This layer functions to keep foreign substances out and prevent the loss of water from the body, a process called transepidermal water loss. Structurally, the stratum corneum is often described using a “brick and mortar” analogy.
The “bricks” are flattened, protein-rich skin cells called corneocytes, which are dead cells filled with keratin. The “mortar” surrounding these cells is a complex, lipid-rich matrix composed of ceramides, cholesterol, and free fatty acids. This specialized lipid arrangement forms a dense barrier that is difficult for most molecules to penetrate. This architecture explains why lotion does not instantly vanish upon contact, but rather sits on the surface momentarily, interacting with this protective structure.
The Two Ways Lotion Works: Surface Protection and Deeper Delivery
Lotion works through two simultaneous actions: protecting the surface and delivering specific ingredients deeper into the skin. Most bulk ingredients, such as heavy oils and thickeners, remain on the top of the stratum corneum. These occlusive agents, like petrolatum or mineral oil, form a physical film that reduces the evaporation of water from the skin’s surface, which is their main moisturizing effect.
Other ingredients, known as humectants (such as glycerin or hyaluronic acid), are small, water-loving molecules that penetrate the stratum corneum to a shallow depth. Inside the upper layers of the skin, they attract and bind water from the air or deeper epidermal layers, holding it in place to increase hydration. This action accounts for the immediate softening and smoothing effect of lotion.
To travel beyond the stratum corneum into the deeper, viable layers of the epidermis, a lotion ingredient must be small enough and possess the correct balance of fat and water solubility. These smaller, lipid-soluble compounds, such as certain vitamins or active pharmaceutical ingredients, typically penetrate via the intercellular route. This pathway involves the molecule moving through the lipid “mortar” that surrounds the corneocytes. This winding path through the lipid matrix is the major route for cosmetic ingredient absorption.
What Happens to Ingredients After Absorption
Ingredients that successfully navigate the stratum corneum and reach the deeper, living layers of the skin begin metabolism and clearance. The skin is metabolically active and contains various enzymes, including the cytochrome P450 (CYP) family, primarily located in the viable epidermis. These enzymes act as a second line of defense, converting penetrating lipophilic compounds into more water-soluble forms.
This local metabolism helps to detoxify and process the absorbed chemicals near the site of application. Once transformed into water-soluble metabolites, these compounds are cleared from the skin. A very small fraction of absorbed lotion ingredients may enter the body’s systemic circulation via localized blood vessels.
From the bloodstream, these trace amounts are carried to the liver (the body’s main processing center) and the kidneys. The liver metabolizes the compounds further, and the kidneys filter and prepare them for excretion, primarily through urine. For standard cosmetic lotions, the amount reaching systemic circulation is minimal, ensuring effects remain localized to the skin.