Recently, I’ve heard a few arguments about skin permeability and basically that chemicals cannot pass through the epidermis. Some argue that topical applications, including skincare implements and make up, do not transmit through the skin. Yeah, right. If the skin did not absorb molecules, we wouldn’t have dozens of medicines with systemic effects that are delivered via patches, creams, and gels. Yes, the skin is your body’s outermost protective layer. But no, it is not a suit of impenetrable armor. Just like any other part of your body, cells make up the skin. Like, actual living cells that are susceptible to chemicals and contaminants just like all other body cells. Let’s have a look at the science behind the skin and how this protective barrier works.

A Closer Look at Skin

The skin is our body’s largest organ, with an average surface area of 2 square meters. This multilayered, complex system’s job is to protect our bodies from external harm, regulate body temperature, and manage nutrient levels and water loss.

The outermost part of the epidermis, the stratum corneum, is where all the action takes place. Dead corneocytes make up most of this layer. The stratum corneum arrangement is analogous to a wall of “bricks and mortar” where corneocytes represent the bricks and intercellular lipids represent the mortar. Considering its barrier characteristics and water resistance, the stratum corneum is the main layer that limits absorption through the skin (1). But absorption does occur, and there are 3 basic mechanisms.

3 Methods of Skin Penetration

Transcellular route

Also known as intracellular, this route of penetration involves molecules passing through the corneocytes. This path is difficult because chemicals must first cross the lipophilic membrane of each cell, then the hydrophilic center of the cell, and then back out through the lipophilic membrane. So a molecule must be both hydrophilic and lipophilic in order to take this route of penetration. That’s uncommon.

Intercellular route

This is when a molecule passes between the corneocytes in the stratum corneum. This is the most common method for molecules to permeate the skin. Because of the lipid content of these spaces between cells, this route favors lipophilic molecules.

The barrier function of the lipid-rich epidermis prevents most hydrophilic and large molecular weight chemicals from penetrating intact skin (2). However, when the corneocytes become saturated with water, they take on the wrinkled “raisin skin” look. At this point, they become vastly more permeable (3).

Intrafollicular route

This absorption occurs through hair follicles and sweat glands. Because the ratio of these skin appendages is low in comparison to the surface area of skin, it is the least common method of absorption (4).

Let’s Not Forget Other Factors

While it seems that our skin is an excellent barrier to the outside world, with mechanisms to protect against both lipophilic and hydrophilic particles, there are other factors to consider. We humans have devised many ways to thwart the skin’s every effort to protect our bodies.

Other chemicals that enhance penetration

There are several mechanisms by which other chemicals act as penetration accelerators. They can break down corneocytes, increase the distance between corneocytes, and act as solvents that solubilize the intercellular lipids (4).


This is an obvious one, folks. The more concentrated the molecules are in a solution, the more that will find a way through.

Presence of Nanoparticles

Studies show that molecules larger than 500 Dalton are unable to penetrate the skin effectively (5). Unfortunately, not all skin preparations are that large. Alarmingly, it is common practice to include nanoparticles in many cosmetic and skin care formulations. These microscopically tiny molecules can pass through the brick wall of epidermis with ease (6). The ability for molecules to cross the skin barrier has given rise to the booming cosmeceuticals industry that claims to solve every skin issue from wrinkles to age spots.

Location of Application

Certain areas of the skin, such as the neck, inner forearms, face, genitalia etc., are thinner and more permeable. When you apply products to these areas, they are more likely to end up in your bloodstream.

Skin Health and Condition

All of the protective factors described above assume healthy skin. Skin health is compromised by aging, exposure to solvents, skin care routine, health condition, and environmental factors. And as skin health declines, so does its protective ability.

The moisture content of the skin affects permeability as well. Dry skin loses some of its protective features. Even the temperature of skin plays a role. The warmer the skin at the time of application, the more fluid the lipid barrier becomes, making it easier for molecules to pass (4). The obvious problem here is that most people apply skin care products right after a hot shower.

Studies are Lacking

Concrete scientific evidence on human subjects is lacking. Most studies focus on animal or in vitro studies. Thus, it is really hard to know for certain exactly which chemicals pass through the skin into the bloodstream. We can only base our decisions on the understanding of the way skin works and what type of particles can theoretically override this beautifully designed system.

So What Do We Do?

The bottom line is, there are too many factors to be considered. If your skin is perfectly healthy and well moisturized and you only apply one type of product to your skin—which you are sure contains no solvents, accelerators, or nanoparticles—there is a good chance you’ll be okay. But if your skin is a bit dry, or you are using multiple products (as most of us do on a daily basis), or if one or more of those products is a bit concentrated, or your skin is saturated and heated from a nice bath. . . you get the idea.

Let’s face it people, we can’t eliminate all the variables. We’d not only have to read every single label and research all the ingredients, we’d still have to evaluate when and how we use each and every product and make sure our skin stays perfectly healthy.

So why not just choose products that don’t contain questionable chemicals? Sure, we can buy cheap crap make up and lotions that smell like a scented candle. But what are the long-term consequences? We simply can’t be too careful about what we slather, smear, and dab on our body’s largest organ.

Want a recipe to love on your skin? Try this Skin Nourishing Ointment DIY.

Looking for more great education about keeping your body healthy inside and out? Come check out The YL Education Club!