The difference between percutaneous absorption and penetration of cosmetic ingredients

The complex mechanism of percutaneous absorption

It is said that various substances are absorbed through the skin. However, the mechanism of skin absorption of chemicals and other substances is not simple. Not all substances are absorbed in the same way, and there are differences in the amount and time that they remain in the body, as well as in how the body reacts to them.

In reality, the methods for measuring the percutaneous absorption of cosmetic ingredients are still unstable, and it is difficult to accurately measure the percutaneous absorption of each ingredient.

Slowly but surely, more reliable measurement methods are being proposed . However, because they are easily affected by skin conditions and usage conditions, it is not yet possible to reliably predict the risk of percutaneous absorption.

Based on this, we have summarized what is currently known about percutaneous absorption of cosmetics.

Differences from skin penetration

Even people who are afraid of percutaneous absorption still want cosmetic ingredients to penetrate the skin properly.
What is the difference between permeation of ingredients into the skin and transdermal absorption?

What is skin penetration?

The passage of a substance through the stratum corneum (the outer layer of the skin) into the deeper layers of the skin

What is percutaneous absorption?

Entry of a substance into the bloodstream

While penetration only occurs through the skin, transdermal absorption means that the substance enters the body via the bloodstream.

When it comes to skincare, it's ideal for ingredients to penetrate the skin, because by getting deeper, they can do their best to hydrate and condition.

On the other hand, ingredients that penetrate deep into the skin cannot be washed away. So, do all of the ingredients that penetrate into the skin eventually enter the bloodstream? The answer is no. Because the size and conditions of each substance are different, not all of them are absorbed through the skin.

Three routes of percutaneous absorption

There are three ways that substances can get through the skin and into the bloodstream:

Intercellular absorption

It is absorbed via the "glue" that connects the cells of the stratum corneum. This "glue" is called intercellular lipids and is composed of ceramides, cholesterol, cholesterol esters, and fatty acids.

Lipids are oily and do not mix well with water, which means that oily ingredients are more likely to be absorbed this way than water-based ingredients.

Intracellular absorption

The second absorption pathway is intracellular absorption. The outermost layer of the skin is the stratum corneum. This stratum corneum is made up of 15 to 20 layers of "dead cells" that have completed their role.

If an ingredient is made up of very small molecules, it can pass through this layer and be absorbed.

Absorption through pores and hair follicles

Pores and hair follicles pass through the epidermis and reach the dermis, where blood vessels nourish the skin. Pores and hair follicles, which make up only about 0.1% of the skin surface, are not the main route of percutaneous absorption, but some substances are easily absorbed via hair follicles.

Conditions that facilitate percutaneous absorption

Whether cosmetic ingredients are absorbed into the skin is also affected by the condition of the skin and the duration of contact.

When the skin is damaged, such as when some of the stratum corneum cells are peeled off, the condition of the stratum corneum is disrupted, which has been shown to increase permeability and significantly increase the rate of absorption into the skin .

This is especially important for people with skin problems such as acne , eczema , or psoriasis, or who have sensitive skin , as it could mean an increased risk of potential side effects from any ingredients that may be of concern.

In addition, the frequency, time, and amount of cosmetics used are also factors that affect percutaneous absorption. Using cosmetics multiple times a day or using a large amount at once may increase the absorption rate.

Substances absorbed through the skin

Natural essential oils and synthetic fragrances are small oily molecules, so they are generally believed to have the potential to be absorbed through the skin via intercellular absorption.

The same is true for other oil-based ingredients, which pass through intercellular lipids and hair follicles and are therefore more likely to be absorbed into the skin than water-based ingredients.

A study on the skin absorption of natural oils examined the blood of babies who received oil massages.

Newborns were massaged with safflower oil and coconut oil four times a day for five days. The results showed that babies who were massaged with safflower oil had higher levels of unsaturated fatty acids in their blood, while babies who were massaged with coconut oil had higher levels of saturated fatty acids.
This suggests that oil applied to the body may change the baby's fatty acid composition.

In addition, a US survey of 20 teenage girls who use cosmetics reported that the following substances were detected in the subjects' blood and urine:

• Phthalates
• Triclosan
• Parabens
• Synthetic Musk

These are suspected carcinogens and hormonal compounds that could have long-term effects on women whose bodies and reproductive organs are still developing.

What happens if it is absorbed through the skin?

What happens to chemicals absorbed through the skin, how long they stay in the body, and how they act varies widely, and there's still a lot we don't know.

In most cases, they are excreted from the body relatively smoothly through the liver, lungs, kidneys, etc. However, some can remain in bones and fat for several years. They may become toxic when they come into contact with certain organs, tissues, or other substances that have entered the body.

In addition to the chemicals themselves, factors such as a person's age, sex, genetic background, past exposure history, diet, etc. may also affect the effects of these substances. It is extremely difficult and complicated to obtain data that clearly shows the extent to which many substances are harmful and under what conditions.

Simply reading the full ingredient list of a cosmetic product may not always reveal the exact effects of percutaneous absorption.
"Don't choose anything suspicious"
It might be a good idea to make a selection based on that criteria.