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Do cosmetic ingredients penetrate skin?
Chris says…Can you guys talk a little bit about ingredient penetration? Why do some ingredients penetrate skin while others won’t? What mechanisms and conditions are involved?
Let’s start our discussion about the truth of skin penetration by mentioning a few popular misconceptions on the subject.
One of the reasons we wanted to tackle this topic is that there’s so much misinformation out there. I think most consumers of beauty products have seen the fear mongering headlines like the following:
- Our skin absorbs __X__ number of pounds of cosmetics each year.
- A “high percentage” of everything we apply to our skin every day penetrates.
- EVERYTHING that we apply to our skin penetrates into the blood stream.
- If a chemical is found in human urine, that means the cosmetic it came from is dangerous.
- If skin didn’t absorb everything we apply to it, then why does a medication patch delivery work so well?
If these headlines were true we wouldn’t have to eat because we could absorb nutrients just by rubbing food on our skin. That’s obviously not the case, although it’s true that SOME chemicals can penetrate skin.
Now, in a single 30 minute show we can’t do a deep dive into the data for every cosmetic ingredient used in every product but we can explain…
- How skin penetration works
- The 4 main factors that control skin penetration
- And perhaps the most relevant question – is penetration necessarily a bad thing.
How skin penetration works
Let’s do a very quick review of skin biology. Skin has evolved to be a protective mechanism. It is literally a barrier to separate us from the outside world. It’s composed of 3 main layers…
“Epi” is a Greek term is used as a prefix meaning upon or or above so the epi-dermis is the layer that’s upon or above the dermis. This is the part of skin that you directly interact with. It’s important to understand that the outer layer, called the stratum corneum, is dead. The thickness of this layer does vary based on location. It’s very thin on your eyelids, for example, and much thicker on the bottoms of your feet. The lower levels of the epidermis are where new skin cells are made. It’s also the layer responsible for making melanin which gives skin its color. And as we already said, its responsible for protecting your body.
The next layer, the dermis, is where body hair, sweat and oil come from. It’s also home to nerve endings which are responsible for your sense of touch. There are also blood vessels in the dermis.
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.
So, as you can see, an ingredient has quite ways to travel before it can penetrate completely though your skin. Next let’s talk about exactly what determines how well that penetration will work.
We talk about this level of skin using the “brick and mortar” analogy because you can think of the skin cells as tiny bricks stacked up on one another with some sort of mortar or cement in between them. So this is where we start our discussion of skin penetration because to get into the skin the first thing an ingredient has to do is find its way in between those layers of brick and mortar.
Now let’s talk what mechanisms are responsible for this penetration and what conditions control the degree of penetration.
4 Factors that control skin penetration
1. Size/molecular weight
An important factor is the size of the molecule which is tied to its molecular weight. Most molecules are simply too large to slip between those cracks between the “bricks” of the dead skin cells.
There are some surprising exceptions for example in Episode 75 we talked about the discovery of that hyaluronic acid can make its way through the skin. But by and large the smaller molecules will penetrate better. The rule of thumb is that anything smaller than 500 Daltons can penetrate skin while anything larger than 500 Daltons can not. A Dalton, by the way, is is the standard unit that is used for indicating mass on an atomic or molecular scale. Common allergens also tend to be smaller than 500 Daltons.
2. Oil soluble vs water soluble
In general, oil soluble ingredients penetrate much better than water-soluble ingredients because the skin itself is water proof. In technical terms we describe this as the hydrophile or lipophile balance of the ingredient.
A classic example here is a water soluble alphahydroxy acid like lactic acid which works on the surface of the skin. Compare that to a more oil soluble beta hydroxy acid like salicylic acid which can penetrate into pores to fight acne.
This makes sense because much of the intercellular space is filled up with lipids like ceramides.
Lastly the polarity or the charge of the molecule is also important. For example, both sugar and salt are water soluble but one is polar and one is not so you’d expect them to penetrate differently.
Collectively these properties help determine how likely an ingredient is to penetrate skin. Another factor to consider is the condition of skin itself.
4. The condition of the skin
As we explained a minute ago, 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. When you consider the kinetics, if there is less distance it has to penetrate through it makes sense that more could get through.
Also, abraded skin is more susceptible to penetration than intact skin. That means if you’re shaving your face or your armpits, then ingredients are more likely to penetrate deeper into skin in those areas. It also makes sense that this would apply to skin that is heavily exfoliated.
Consider the delivery vehicle
It’s important to consider the formula from which the ingredient is being delivered for two reasons. First, 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. A leave on product, like a moisturizing lotion, gives the ingredients much more time to penetrate. (Note: an exception could be ingredients which are highly substantive to skin.)
Second, there may be other ingredients in the formula that enhance penetration. These “penetration enhancers” are typically these either oily materials or ingredients with polyol (OH) groups. They can be chemicals that are either synthetic or natural in origin and they’re thought to work by changing the way the lipids in-between the skin cells are packed together. (Like the “brick and mortar” model we talked about.) Some well known examples include ethanol, some PEGs , methyl pyrrolidone, jojoba oil and peppermint oil. We found a couple of interesting articles on penetration enhancers and we’ll put links to those in the show notes.
Is penetration always a bad thing?
Setting aside for the moment the question of whether or not an ingredient does or does not penetrate the skin there is another important question to ask. If it penetrates the skin is that always bad?
If it just penetrates through the upper layers of skin and is not absorbed into the bloodstream it will eventually just be sloughed off as part of the dead skin cells. To present a systemic hazard it has to not only penetrate the skin but it has to be absorbed from the skin into the bloodstream.
Once in the bloodstream our bodies have a very efficient filtering mechanism in place to remove toxins. This is another reason why concept of the dose makes the poison is so important. Low levels of contaminants will be filtered out by the kidneys and you’ll either come out in your P or feces.
Of course some contaminants can overwhelm the body’s natural filtering system and be hazardous to your health. Lead is a good example – very small amounts of lead are filtered out of your body (that’s one of the reasons it’s okay to have lead in your lipstick) but higher doses of lead do build up in your body and cause health problems.
My point is that the difference between skin penetration and skin absorption is an important distinction.
Its also important to realize that the risk of an ingredient penetrating skin is is factored into the safety assessment done on cosmetic ingredients.
The Beauty Brains bottom line
It’s not easy for ingredients to penetrate through the skin because of it’s “brick and mortar” structure.
An ingredient has to have the right size and the right compatibility with skin lipids to “slip through.”
There are other ingredients called penetration enhancers that boost penetration (especially in transdermal drug patches).
The myths are NOT true about your body absorbing large amounts of chemicals through your skin.
The regulatory bodies that determine the safety of cosmetic ingredients certainly factor skin penetration and absorption into their assessment.
This week we played a special “wake up and smell the bacon” edition of our game. Which of these bacon-themed personal care products is fake? (Listen to the show for the answer.)
- Bacon scented toilet paper
- Bacon bit exfoliating scrub
- Bacon scented deodorant