The skin, our body’s largest organ, constantly interacts with the external world. Substances applied to its surface can enter the body through skin absorption, also known as percutaneous or dermal absorption. Understanding this process helps explain how products like moisturizers and topical medicines work and highlights the importance of safe handling of other substances.
The Skin’s Protective Barrier
The skin serves as a primary protective barrier, preventing harmful external substances from entering the body and minimizing water loss. This function is largely attributed to its outermost layer, the stratum corneum. The stratum corneum, often called the “brick and mortar” layer, consists of flattened, dead skin cells (corneocytes) acting as bricks.
These corneocytes are tightly bound by a lipid-rich matrix, which serves as the mortar. This unique structure creates a highly effective, yet not entirely impermeable, defense. It regulates the movement of molecules, enabling some to pass while restricting others.
Pathways of Skin Absorption
Despite its protective nature, substances can penetrate the skin through several distinct pathways. One significant route is the transcellular pathway, where molecules pass directly through the corneocytes. This involves substances dissolving into and diffusing across the alternating lipid and water-rich layers within these cells. Small hydrophilic or moderately lipid-soluble molecules often utilize this route.
Another common mechanism is the intercellular pathway, where substances navigate the tortuous, lipid-filled spaces between the corneocytes. This pathway is particularly favorable for lipid-soluble compounds, as the extracellular matrix of the stratum corneum is highly lipophilic. Together, these constitute the transepidermal pathway, the primary means of absorption.
A third, less dominant pathway involves skin appendages like hair follicles and sweat ducts. Though a small fraction of skin surface, they can provide direct conduits for certain substances, especially larger molecules.
What Influences Skin Absorption
Numerous factors determine how readily a substance absorbs through the skin. The molecular size of a substance is a key determinant, with smaller molecules generally absorbing more effectively. Substances with a molecular weight below 500 Daltons tend to absorb better.
The lipid solubility of a compound also plays a significant role; lipophilic substances typically penetrate the skin more easily due to the stratum corneum’s lipid-rich nature. The substance’s concentration and duration of contact also directly influence absorption; higher concentration or longer exposure increases uptake.
The condition of the skin itself heavily impacts absorption rates. Intact, healthy skin provides a robust barrier, but damaged or inflamed skin (e.g., cuts, burns, skin diseases) has compromised function, increasing absorption. Skin hydration levels can also alter permeability. Temperature and the substance’s formulation (e.g., cream, gel, patch) also modify absorption.
Everyday Examples of Skin Absorption
Skin absorption is central to the function of many common products. Transdermal medication patches are a prime example, delivering drugs like nicotine, hormones, and pain relief directly into the bloodstream. This method allows for controlled, sustained release, bypassing the digestive system. Many cosmetic ingredients, like those in moisturizers, are formulated to absorb and deliver active compounds.
On the other hand, inadvertent or harmful absorption can occur with various chemicals. Certain pesticides, industrial solvents (e.g., toluene, benzene), and heavy metals (e.g., lead, mercury) can absorb through the skin, potentially leading to systemic health effects. Cleaning products and other household chemicals also pose absorption risks if not handled carefully.
Safeguarding Against Unwanted Absorption
Minimizing unwanted or harmful skin absorption involves several practical measures. Carefully read and follow instructions and warnings on labels for all chemicals, medications, and personal care products, including recommended application methods.
When handling hazardous substances, wear appropriate protective gear like gloves and long sleeves. This creates a physical barrier, reducing exposure. After contact with chemicals, thorough handwashing removes residues.
Avoid applying irritating or hazardous substances to broken, irritated, or damaged skin. Compromised skin allows greater absorption, increasing adverse effects. Maintaining skin health and hydration also preserves the skin’s natural barrier.