From fighting those pesky free radicals, to stimulating skin's natural collagen production, cosmetics companies make some alluring promises—and penetration is an important aspect of their claims.
‘Skin penetration’ is used to describe a characteristic of cosmetic products in which their ingredients migrate from the surface of the skin into the lower layers of the skin cells. The idea is that the deeper a product can reach below the skin’s surface, the more effective and lasting change it can create. As such, you’ll see many cosmetics brands use claims of skin penetration when advertising their anti-aging products.
The truth is that if we could absorb everything that was put on our skin, we wouldn’t have to eat. Instead, we’d just tape some spinach to our inner arm and call it a day.
While that’s obviously not the case, some chemicals can penetrate your skin—as evidenced by transdermal patches that deliver prescription drugs by just sitting on your skin’s surface.
So, what makes a difference in how effectively a product can penetrate your skin—if at all?
To help you navigate cosmetic’s claims, let’s first look at:
How Your Skin Works To Keep Stuff Out
Skin is a fascinatingly complicated system designed to protect us against external harms (bacteria, UV radiation), regulate body heat, manage nutrient levels, and minimize water loss. The barrier provided by skin is composed of three main layers:
This is the part of the skin that you directly interact with. “Epi” is a Greek term that means upon or above—so the epidermis is the layer that’s upon or above the dermis.
The outermost layer of your epidermis, called the stratum corneum, is made thick with dead skin cells (mostly keratin) and various waxy substances. This layer acts as a wall of protection from external moisture, chemicals, UV radiation, and is your foremost guard against the bacterial world. As these dead bits fall off or are exfoliated from the body, the lower layers of skin replenish the surface with additional keratin.
The thickness of the stratum corneum varies based on location; it’s very thin on your eyelids and much thicker on the bottoms of your feet. The lower levels of the epidermis are where new skin cells are made. They are also the layers responsible for making melanin, which gives skin its color and protects against sun exposure.
The next layer, the dermis, is where body hair, sweat, and oil come from. It’s also home to blood vessels and nerve endings, which are responsible for your sense of touch.
The bottom layer of skin is the subcutaneous fat layer, which is also called the hypodermis. This layer basically attaches the upper layers of skin to the bone and muscle below. The fat insulates you from temperature variations as well as physical shock, and it contains even more, larger blood vessels.
As you can see, skin isn’t just one porous layer that allows products to easily deliver their key ingredients deep within.
Instead, think of the skin cells as tiny bricks stacked upon one another, held securely with mortar or cement between them. An ingredient has quite a ways to travel before it can penetrate deeply into the skin, having to find its way in between those layers of bricks and mortar.
What determines how well a product can do so?
The 4 Factors That Affect Skin Penetration
Four different mechanisms are responsible for the degree to which any product can penetrate your skin:
Molecular Size & Weight:
Remember our brick and mortar analogy? Most molecules are simply too large to slip through any cracks in the bricks of dense, dead skin cells. With few exceptions, smaller molecules penetrate better.
Molecular size is measured by Daltons. For example, common allergens also tend to be smaller than 500 Daltons. The rule of thumb is that anything smaller than 500 Daltons can penetrate skin—generally anything larger than 500 Daltons can’t.
A rare exception? Hyaluronic acid, which has a molecular weight of 600,000 Daltons. The FDA-approved ingredient (meaning that there’s plenty of research showing it works) is able to hold up to 1000x its own weight in water. As such, it’s used with increasing popularity as an injectable filler and topical moisturizer.
Oil vs. Water-Soluble:
In general, oil-soluble ingredients penetrate much better than water-soluble ingredients because the skin itself is waterproof. So, why not only make oil-soluble products? Because not everything needs to penetrate past the outer layers of your epidermis.
For example, water-soluble alpha hydroxy acids, like lactic acid, works on the surface of the skin by exfoliating away layers of dead cells—a task that doesn’t require penetration. Compare that to the oil-soluble salicylic acid, which can penetrate into pores to fight acne, to understand that not every product is designed to deliver ingredients deep into the skin.
The polarity, or charge, of the molecule is important in determining how well each penetrates. For example, both sugar and salt are water-soluble, but only one is polar—a bond between two molecules when the electrons are identical on both atoms. The chemists who create cosmetics understand that a molecule’s polarity affects many aspects of its behavior, including how it interacts when coming up against the brick and mortar of skin.
The Condition of Your Skin:
Skin on some parts of your body is thinner than others. For example, the skin under your eye is very thin, which is one of the reasons dark circles show up so much. Thin skin is more prone to penetration than thick skin—because there’s less distance to penetrate, it makes sense that more product could get through.
Additionally, abraded skin is more susceptible to penetration than intact skin. Meaning, if you apply a product to an area that’s been recently shaved, such as your face, underarms, or legs, that product can penetrate deeper into the skin in those areas. The same goes for applying products after heavy exfoliation.
Now you have a clearer understanding of what a product has to go through to effectively penetrate your skin, as well as what factors determine its effectiveness. However, ingredients labels don’t really list molecular size or polarity! So, as a consumer, how are you supposed to determine whether or not a product can penetrate your skin?
Consider the Type of Formula
A formula is a delivery vehicle, and how effective one is at any given job depends a lot on what it’s built for.
If it’s a rinse off product, it’s unlikely to result in very much penetration. That’s because penetration is measured in milligrams per square centimeter per unit of time. That unit of time is frequently hours—not minutes and certainly not seconds. Therefore, it is extremely unlikely that anything that is rinsed off the skin will have time to penetrate.
However, a leave-on product, like a moisturizing lotion, gives the ingredients much more time to penetrate. This is why when you see sets of products claiming to harness the power of a miracle ingredient, you don’t need to invest in toners and washes to give their claims a trial run. Instead, just try out their moisturizer, lest any of your money gets washed down the drain.
Look At the Label
We mentioned hyaluronic acid as an exception to the rule that smaller molecules equal better penetration, making it something you might look for on a label. Because of its proven effectiveness, hyaluronic acid is an increasingly popular ingredient in anti-aging products—but, how to tell if a product contains effective levels of the ingredients you want?
Sephora sells Peter Thomas Roth’s “VIZ-1000™ 75% Hyaluronic Acid Complex.” That sounds like an abnormally high concentration of an A-list anti-aging ingredient without a prescription.
However, a glance at the ingredients list will show a bunch of extracts listed first, and then hyaluronic acid listed as the 25th or 26th ingredient. In fact, it’s almost last in the list of ingredients on the product's label.
If this is truly a 75% hyaluronic acid product, then wouldn’t hyaluronic acid have to be the first or second ingredient?
A second glance at the product’s name and you’ll realize that it’s a “75% hyaluronic acid complex”—meaning that they’ve taken hyaluronic acid, mixed it with a bunch of extracts that don’t do anything, and used 75% of this mixture to make the product.
What does that mean for shoppers interested in Roth’s Hyaluronic Acid Complex? It might be a perfectly fine product, but the name doesn’t mean it contains a useful level of hyaluronic acid.
That’s the kind of shady approach to marketing you should be aware of when looking at cosmetics labels, even when you know which ingredients can penetrate deep enough into your skin to be effective.
Bottom Line? “Cosmetics” Remain Just That
The design of any cosmetic formula is to benefit the outer layers of skin, which is why they’re not subjected to the same regulations as pharmaceutical drugs—those formulas designed to effect change within your body.
Some prescription-strength products use specially developed ‘penetration enhancers’ to deliver their key ingredients, like vitamin C or retinol, past the skin’s outer layers. Products that use penetration enhancers are engineered by a chemist who must create a molecule that is soluble in skin and small enough to penetrate the body.
By their very nature, anything considered a cosmetic won’t be able to penetrate past the skin’s layers to interact with your cells. If these products could, they’d probably be classified as drugs and require FDA approval.
The bottom line is that many variables affect a product’s ability to penetrate your skin that are difficult to discern without a chemistry lab. One easy tell? If you can purchase the product without a doctor’s prescription, those penetrating ingredients can’t effect lasting change.