There are many factors that determine skin absorption of cosmetics, drugs, and chemicals through the skin. Ingredients must have dermal absorption to have significant long term effects on skin, especially to improve wrinkles and aging skin.

The speed of skin absorption depends on what ingredients are in the skin care product, strength of the skin barrier, humidity levels, and temperature of the skin.

One of the main difficulties in making skin care products effective is getting them to absorb into the skin. Ingredients that are called penetration enhancers are added to speed and increase skin absorption of serums, lotions, gels, oils  and creams.

Yes some skin care ingredients can absorb and penetrate into deeper skin layers but not all. Skin absorption and how fast they get into the skin depends on several factors:

• polarity of the molecule
• size
• hydrophilic vs lipophilic
• condition of skin barrier
• temperature
• skin hydration
• thickness of stratum corneum
• other products layered together

 Scientists have studied how drugs pass through the skin and have shown that the outermost layer of skin, called the stratum corneum, is the main barrier to prevent substances from getting into the skin. How fast a skin care lotion, cream, or serum penetrates and moves through this layer determines how well it will be absorbed by the skin. ⁽²⁾

I have seen patients try to use nasal drops such as Afrin or eye drops such as Visine to treat facial redness. However, treatment made to go in your eyes or nose are very different than topical treatments and are n usually not very effective when use don the skin.

This is why nasal sprays and eye drops do not absorb well into the skin:

Penetration enhancers are not typically found in nasal sprays or eye drops mainly because their roles and effects are different from what is desired in these formulations. Here's why they are generally not used in such products:

1. Different absorption goals : Penetration enhancers are used to facilitate the absorption of active ingredients through the skin or mucous membranes by altering their permeability. In nasal sprays and eye drops, the primary goal is often to deliver the active ingredient to the surface tissues (nasal passages or eye surface), not to enhance its absorption through these tissues into the bloodstream or deeper tissues.

2. Sensitivity of mucosa and cornea : The nasal mucosa and the cornea are sensitive and can be easily irritated. Penetration enhancers often disrupt lipid layers or alter tissue structure to increase absorption, which can lead to irritation or damage in these sensitive areas. For instance, ingredients that disrupt the lipid barrier could cause drying, inflammation, or irritation to the delicate mucous membranes of the nasal passages or the cornea.

3. Risk of systemic absorption : Enhancing the penetration of active ingredients in areas like the nose or eyes could increase the risk of systemic absorption. This could lead to unintended systemic effects, especially if the active ingredient is potent and meant to act locally, not systemically.

4. Formulation stability and compatibility : Penetration enhancers may not be compatible with the formulation of nasal sprays or eye drops. They could affect the stability, solubility, or efficacy of the active ingredients, or they might interact negatively with other formulation components.

5. Specific formulation requirements : Nasal sprays and eye drops have specific formulation requirements to ensure they are isotonic, non-irritating, and stable. Penetration enhancers might interfere with these requirements, affecting the product's safety and effectiveness.

While penetration enhancers are valuable in topical formulations for skin application, their use in nasal sprays and eye drops is limited due to the potential for irritation, the necessity for localized action rather than deep tissue penetration, and the delicate balance required in formulating products for the nasal passages and eyes.

The skin care routine step order is critical when building a safe and effective skin care routine because the ingredients in the skin care routine, which ingredients are used together, and how products are layered affect how well all the products in the skin care regimen pass through the skin.

Different ways that ingredients can affect skin absorption include ⁽⁵⁾ :

• Break down the structure of the outermost layer of skin called the stratum corneum
• Damage the skin barrier
• Interact with proteins in the skin
• Use solvents to change the skin's properties
• Change connections between skin cells
• Affect skin metabolism
• Enter through the hair follicle

Some of these substances work by changing both the water-loving (hydrophilic) and oil-loving (hydrophobic) pathways for skin penetration, while others only affect one of these pathways.

The skin barrier is hydrophobic which means that oils can more easily pass through than water.  This is why oils are often used as penetration enhancers.  However, oils are also occlusives. So if oils are applied before an ingredient they can slow absorption, but when applied after an ingredient they can speed absorption.  The type of oil and the fatty acids it contains are also huge factors in skin absorption speed and depth of penetration.

Factors affecting absorption through skin:

• size
• temperature
• hydrophilic vs hydrophilic'
• entry through heir follicle vs need to transverse the epidermis
• polarity of the molecule
• type of fatty acid present

How deep oils penetrate into the skin is determined by how fast they absorb thought the skin barrier. The type, structure and amount of fatty acids in the oil and the strength of the skin barrier determine absorption speed. The structure of oleic acid, the predominant fatty acid in olive oil, causes tiny holes in the skin barrier which help speed absorption. This is why olive oil penetrates deeper and faster into the skin the most oils. However, it is not a very hydrating oil because the holes it leaves behind allow water to evaporate off of the skin in a process called transepidermal water loss.

Olive oil penetrates the skin best, but coconut oil, almond oil, and tea tree may be better choices depending upon your skin type.

Many essential oils are mixed with ethanol, menthol, or other types of alcohol which can increase skin absorption.

These essential oils increase the skin absorption of other ingredients:

• Anise
• Bisabolol (found in chamomile)
• Cardamon
• Eucalyptus
• Niaouli
• Peppermint
• Sweet Basil
• Ylang-ylang

Eucalyptus, tea tree and peppermint oil can damage the skin ⁽⁵⁾ leading to an increase risk of skin allergy or irritation.

The speed of skin absorption of face creams and body creams depends upon the types of fatty acids and oils in the skin care product.

Unsaturated fatty acids absorb faster into the skin. These fatty acids typically have one or more double bonds in their structure, leading to increased skin absorption.

Examples of Unsaturated Fatty Acids:

• Oleic acid
• Linoleic acid
• Palmitoleic acid
• Alpha-linolenic acid
• Gamma-linolenic acid

Many oils are rich in these unsaturated fatty acids, contributing to their benefits in skin care.

Oils with a Predominance of Unsaturated Fatty Acids:

• Olive oil (high in oleic acid)
• Sunflower oil (high in linoleic acid)
• Safflower oil (high in linoleic acid)
• Grapeseed oil (high in linoleic acid)
• Flaxseed oil (high in alpha-linolenic acid)

These oils are often used in various skin care products for their hydrating and nourishing effects, attributed to their content of unsaturated fatty acids.

Saturated fatty acids do not penetrate the skin as quickly as unsaturated fatty acids. This slows skin absorption because saturated fatty acids have no double bonds in their structure, resulting in straight, tightly packed tails. The shape of the tails and the polarity of the molecules make them less fluid, so they do not absorb into the skin as readily.

Examples of Saturated Fatty Acids:

• Palmitic acid
• Stearic acid
• Myristic acid
• Lauric acid
• Caprylic acid

Oils that contain a high proportion of these saturated fatty acids include:

Oils with a Predominance of Saturated Fatty Acids:

• Coconut oil (high in lauric acid)
• Palm oil (high in palmitic acid)
• Cocoa butter (high in stearic acid)
• Shea butter (high in stearic acid)

These saturated fatty acids and oils containing them are often used in skin care for their emollient properties, but they are not as readily absorbed into the skin due to the structural characteristics mentioned above.