Skin absorption: how does it actually work?
There is a lot of misinformation regarding skincare being spread over the internet these days. A couple of those skincare myths that I see regularly are: "60% of everything you apply to your skin ends up in your bloodstream" and “It takes only 26 seconds for a cosmetic ingredient to reach your bloodstream”. Luckily, these statements are untrue and are spread around by people who are ill-informed and repeat information they read on the internet. It is true that our skin has an absorbing effect, but it is by no means close to that 60%. How, when, and how much the skin absorbs, depends on various factors. I will explain the most important ones to you in this blog.
On a daily basis our skin touches many different types of harmful and non-harmful substances. These substances can penetrate our skin and then enter our bloodstream through skin absorption. The rate of skin absorption largely depends on the outer layer of the skin; this is called the stratum corneum. This has an important function, because it forms a barrier to the outside world and other skin layers, helping to prevent molecules from entering and leaving the skin, thereby protecting the lower skin layers. Not all molecules or substances can be blocked in this way. Only a small part can penetrate the skin. To be absorbed through the skin, a chemical must pass through the epidermis, hair follicles or glands. Although small amounts of chemicals can enter the body quickly through the glands or hair follicles, they are mainly absorbed by the epidermis. The epidermis consists of several different layers, chemicals must first pass through these layers before they can enter the dermis, after which they can penetrate the bloodstream or lymph and circulate to other parts of the body. The extent of absorption depends on a number of factors, the most important ones being:
Duration of the contact
Molecular weight of the molecule
Duration of contact How much and whether or not certain substances are absorbed through the skin may also depend on the duration that the substance is in contact with the skin. A moisturizer stays on your skin for almost all day, while a cleanser stays on the skin for a few minutes before you remove it. The substances that are in a cleanser will therefore have much less chance of penetrating your skin than, for example, a serum or moisturizer. Solubility The stratum corneum, the outermost layer of the skin and outer layer of the epidermis, is mainly composed of fatty acids, cholesterol and ceramides. These are lipophilic substances. Lipophilic substances are those that do not dissolve in water but can only dissolve in other fatty substances. This makes it possible for those fatty substances to penetrate the stratum corneum more easily than water-soluble substances. Examples of fatty substances that are great for your skin are: CoQ10, vitamin E, and plant and essential oils.
Skin condition With damaged skin such as eczema, inflammation or a disrupted skin barrier, molecules can also be absorbed better and faster by the skin. For example, you should always watch out for skincare products that contain alcohol, as these can damage the skin, which means that other ingredients can penetrate the skin faster and deeper. In some cases, you want ingredients to penetrate your skin deeper, but in other cases you want to prevent it. This includes preservatives, emulsifiers, thickeners, etc.
Molecular weight of the molecule Whether a molecule can penetrate the stratum corneum depends on the size of the molecule. The molecular weight of a compound must be under 500 Dalton in order for it to allow skin penetration. Larger molecules cannot pass the stratum corneum. An example of a molecule that cannot penetrate the skin, because it is too large, is hyaluronic acid. The molecular weight can vary from 5 thousand Daltons to even 7 million Daltons. It is therefore much too large to penetrate the skin and instead forms a thin film on the skin.
Concentration The concentration of an ingredient is also a factor that has an influence on skin penetration. For example, when you get a massage, a greater percentage of a massage oil might penetrate the skin, because your skin is warm and the product is applied in a higher dose. Compare it to a facial oil which may only be applied in a few drops and gently massaged into the skin for a shorter period of time.
Consider carefully what you are putting on your skin There are a lot of other factors that come into play when it comes to skin absorption, but at least now you have a global idea of what it takes for a molecule to be able to penetrate the skin. It is also nice to know that not everything that you put on your skin is absorbed by your skin. Unfortunately, there are still plenty of harmful substances that can penetrate your skin and end up in your bloodstream. That is why it is always important to pay close attention to what you put on your skin, always read the ingredient list carefully before you buy or use something, and don’t forget to wear gloves when using detergents when you clean your house, because those are chemicals that you most definitely do not want to get on your skin.
1. Eaton, DL and Klaassen Curtis D. Principles of Toxicology. in Cassarett & Doull's Toxicology, The Basic Science of Poisons. 5th edition. 1996. McGraw-Hill.
3. Formula botanica. “Percutaneous Absorption”. Advanced organic cosmetic science. 2016