The subject of skin permeability to various materials is complicated Essential oils are not the only external input to the skin that should be considered. This topic of the interaction of skin and hair with ingredients
and the environment is a very broad and can impact the safety and efficacy of your products.

It requires a very simplistic view to discuss its properties initially, but more focused discussions are needed to understand the individual components. The skin is primarily intended to control the passage of water in and out of the body. The skin membrane is composed of oil like polymers in a simplistic view. This oily membrane will not allow water and other polar materials to pass through easily, but it is a leaky bag. Pores are the most common mechanism of water transport out of the skin, but other cellular mechanisms
exist that allow active transport of specific compounds in and out of cells. These transport mechanisms are complicated by the thin layer of dead skin that the water borne items must penetrate before living cellular mechanisms are available to begin active transport. However as we know from sitting in the Jacuzzi too long, our skin can change its water content

Oily and oil soluble materials are a different matter. Oily materials dissolve in oil and thus can permeate the skin easily. But penetration rate is relatively slow because the dead cells absorb the oily materials and
become softened (This is good). As the top skin layers absorb the maximum amount that they will hold, oily materials will begin to penetrate to the living cells.

Larger molecules penetrate slower and polymers will not even pass the outer layer of skin. This effect increases the safety of preservatives like Cosmocil CQ because its molecule is simply too large to pass through upper dead skin cells. If it can’t pass though the upper dead layer, it can’t affect the living layer. This is the effect of the skin barrier protecting the underlying cells. Materials that prevent trans dermal water loss or actas skin protectives lie in this category too.

All of these effects influence our exposure to materials that are placed on our skin. Keeping the dead layer properly plasticized has benefits. It helps keep water borne polar materials out, while improving the penetration of oily nutrients that are delivered in oil or water. It also helps protect the skin against damage from everyday movement and stress.

This means that water soluble antioxidants like vitamin C will not penetrate the skin, but oil soluble ones like Vitamin E will. converting a water-soluble nutrient to an oil soluble form often allows deep absorption, but the nutrient must be freed from its carrier to be available to the cell. Tocopherol acetate is more stable form of vitamin E, but it displays no antioxidant properties until it is converted to free vitamin E. while cells
can accomplish this easily for tocopherol acetate, there is growing evidence that the conversion of materials like retinyl palmitate to the parent material (vitamin A) is not efficient and this raises other issues. Ensuring
that the delivery system provides the nutrients in a form that the cells can use is important.

The problem for cosmetic makers is to ensure that our ingredients reach the target areas. Ingredients that soften the skin or prevent trans dermal water loss are most effective when confined to the upper layer. Nutrients do need to be delivered to the growing cells, but they generally should not be allowed to penetrate to the capillary bed.

Water soluble ingredients present the widest variation on how the skin handles them. Generally materials in rinse off products must have great affinity for the skin to stick around long enough to be absorbed. This is
why we see large amounts of Kojic acid in soaps, but the recommendation for creams is to use a low content to avoid irritation. The small amounts of Kojic acid in leave on products should be much more effective though since it will be given time to absorb.

Soaps and cleaners tend to complicate the absorbtion of materials because their natural tendency is to capture oily materials to allow them to be carried away in the cleaning process.

Since it is only possible to look at general principle to determine what ingredients are likely to do, each ingredient should be evaluated individually before use. These topics need to be discussed more broadly to
allow us to create better products.

The hair has a much different ingredient interaction profile than the skin and probably use a separate thread to avoid confusion keeping in mind that scalp health is also involved.