The stratum corneum, the outermost layer of the epidermis, is the primary layer responsible for absorbing substances that penetrate the skin. However, the rate of absorption depends on the substance's ability to pass through this layer. The stratum corneum acts as a rate-limiting barrier, meaning how quickly a substance passes through this outer layer largely determines the overall absorption into the body. Other layers of the skin, like the dermis, also play a role in absorption, but the stratum corneum is the first and most significant barrier.
Dermal Absorption: A Closer Look
Dermal absorption, as defined by the CDC's NIOSH (https://www.cdc.gov/niosh/topics/skin/default.html), is the process where chemicals pass through the skin and into the body. Many workplace chemicals can cause damage through this route.
Several factors influence absorption:
- Substance properties: The chemical makeup of a substance directly impacts how easily it penetrates the skin.
- Stratum corneum integrity: A healthy, intact stratum corneum offers better protection against absorption than one that is damaged or compromised.
- Skin hydration: Hydrated skin may absorb substances more readily than dry skin (https://www.fau.eu/2014/02/13/news/research-into-skin-that-holds-water/). The outermost layer swells when it absorbs water.
What Gets Absorbed?
While some skincare ingredients remain on the surface, others penetrate deeper (https://www.bbc.com/future/article/20240306-what-can-you-absorb-through-your-skin). The absorption process is complex and depends on the specific substance. For example, the ozone layer absorbs UV-B radiation, preventing much of it from reaching the skin (https://ozone.unep.org/ozone-and-you, https://www.epa.gov/ozone-layer-protection/basic-ozone-layer-science). Melanin, a pigment in skin, also absorbs UV radiation, protecting the underlying layers (https://my.clevelandclinic.org/health/body/22615-melanin, https://pmc.ncbi.nlm.nih.gov/articles/PMC2671032/).
Two main pathways for absorption are via skin appendages or directly through the stratum corneum (https://pmc.ncbi.nlm.nih.gov/articles/PMC2835875/).
