Drugs and other substances can be absorbed through the skin. While the skin acts as a protective barrier, it is not entirely impenetrable. This process, known as transdermal absorption, allows compounds to pass through the skin’s layers and enter the bloodstream, potentially leading to systemic effects.
The Skin’s Barrier Role
The skin’s outermost layer, the stratum corneum, acts as a primary protective barrier. It is composed of flattened, dead skin cells (corneocytes) embedded within a rich lipid matrix. This structure is often described as a “brick-and-mortar” arrangement, with corneocytes as the bricks and lipids as the mortar.
The stratum corneum’s lipid matrix, primarily ceramides, cholesterol, and free fatty acids, forms a highly organized, stacked structure. This barrier prevents excessive water loss and restricts the entry of many foreign compounds. Despite this defense, various factors can influence the skin’s permeability, allowing certain substances to bypass it.
Factors Governing Absorption
Several characteristics of both the substance and the skin determine whether a compound can be absorbed and to what extent.
- Molecular size: Optimal absorption occurs with compounds having a molecular weight of 400 to 500 Daltons, ideally around 400 Daltons or less.
- Lipid solubility: Substances need to be sufficiently soluble in both lipids and water to navigate the skin’s layers. A log partition coefficient (Log P) between 1.0 and 3.0 is suitable for effective permeation.
- Concentration: Higher concentrations generally lead to increased absorption.
- Formulation or vehicle: The way a drug is delivered impacts its release and penetration.
- Skin integrity: Damaged skin has increased permeability.
- Skin thickness: Thinner areas, like the face, absorb substances more readily than thicker areas.
- Skin hydration: Hydration can enhance absorption by softening the stratum corneum.
- Skin temperature: Increased skin temperature can boost absorption.
Intentional and Unintentional Absorption
Drugs are intentionally delivered through the skin for localized and systemic effects. Topical medications, such as creams, ointments, and gels, treat specific skin conditions or provide pain relief directly at the application site. These formulations primarily act locally, with minimal amounts entering the bloodstream.
For systemic effects, transdermal patches deliver medication through the skin into the bloodstream. These patches offer advantages like controlled release and bypassing the digestive system. Examples include patches for pain relief (fentanyl), hormone therapy (estradiol or testosterone), and smoking cessation (nicotine).
Unintentional absorption can occur from accidental skin contact with potent substances. Pesticides and industrial chemicals pose risks through dermal exposure, potentially leading to systemic toxicity. Highly potent pharmaceutical drugs, like fentanyl, can also be absorbed through the skin, especially with prolonged contact, on compromised skin, or in certain solutions like alcohol-based hand sanitizers. While dry powder fentanyl typically has low skin absorption, direct contact should be avoided.
Minimizing Risks and Responding to Exposure
Preventing unintentional absorption involves implementing safety measures when handling chemicals and medications. Always wear appropriate personal protective equipment (PPE), such as gloves, safety goggles, and protective clothing. Ensure adequate ventilation in work areas to minimize airborne exposure. Familiarize yourself with safety data sheets (SDS) for any chemicals you encounter. Practicing good hygiene, including thorough hand washing with soap and water after handling chemicals, is important.
In the event of accidental skin exposure, immediate action is important. Move away from the source of exposure if safe. Promptly remove any contaminated clothing, shoes, and jewelry, cutting clothing if stuck. Immediately flush the affected skin area with copious amounts of cool, running water for at least 15 minutes. Avoid rubbing the affected area, and do not apply ointments or creams unless instructed by medical professionals.
Seek immediate medical attention if symptoms persist or worsen, or if the exposure involves a toxic substance. If fentanyl exposure is suspected, wash the skin with soap and water, and avoid using alcohol-based hand sanitizers, as these can increase absorption.