Skin is easy to underestimate. We see its surface every day, wash it, moisturise it, expose it to weather and apply increasingly complicated collections of ingredients to it. Cosmetic language encourages us to think about what we might put into the skin: vitamins, botanical extracts, antioxidants, essential oils and an ever-changing vocabulary of active ingredients. Biologically, however, one of skin’s most important jobs is precisely the opposite. It has to keep things out. At the same time, it must prevent something essential from escaping: water.

The structure responsible for much of this work is usually referred to as the skin barrier, although that apparently simple expression encompasses several interacting biological systems. At the centre of the physical permeability barrier is the stratum corneum, the remarkably thin outermost portion of the epidermis. It consists of flattened, terminally differentiated cells called corneocytes surrounded by a highly organised extracellular lipid matrix dominated by ceramides, cholesterol and free fatty acids. This arrangement allows terrestrial life to perform a rather extraordinary trick: carrying a water-rich organism around in a comparatively dry environment without continually losing its water to the atmosphere. Before asking what a skincare ingredient can do, therefore, it is worth understanding what the skin is already doing for itself.

Skin is not a single layer

Human skin is usually described in terms of three major regions: the epidermis, the dermis and the underlying subcutaneous tissue. The dermis contains structures including collagen-rich connective tissue, blood vessels, nerves, sweat glands and the deeper portions of hair follicles. Beneath it, subcutaneous tissue contains substantial quantities of adipose tissue and contributes to cushioning, insulation and energy storage. The epidermis sits above the dermis and contains no blood vessels of its own. Its dominant cells are keratinocytes, which undergo an elaborate process of differentiation as they move from the deeper epidermis towards the surface. Eventually, those living keratinocytes become flattened, highly specialised corneocytes within the stratum corneum. This outer layer is extraordinarily thin. Depending upon body site and measurement, the stratum corneum may be only tens of micrometres thick, yet it provides the principal physical permeability barrier between the body’s living tissues and the external environment. Thin does not mean simple.

The bricks and mortar model

One of the most familiar ways of explaining the stratum corneum is the bricks and mortar model. The corneocytes are the bricks. The extracellular lipids surrounding them are the mortar. Like most analogies, this is imperfect, but it captures something important. A wall cannot function properly simply because its bricks are strong; the organisation of the material between them matters enormously. The same is true of skin. Corneocytes contain keratin and other structural components and are surrounded by a specialised cornified envelope. Between them lies an intricately organised lipid matrix rather than the watery extracellular environment found in many living tissues. Those intercellular lipids form the only continuous pathway across the stratum corneum and consequently play a central role in determining what passes through it. This is why skin-barrier science is, to a remarkable extent, lipid science.

Ceramides, cholesterol and fatty acids

Three major lipid families dominate the extracellular matrix of the stratum corneum: ceramides, cholesterol and free fatty acids. Ceramides account for approximately half of stratum-corneum lipid mass in commonly cited analyses, with cholesterol and free fatty acids making up much of the remainder. Their exact proportions and molecular species are complex, and describing them simply as three ingredients understates the extraordinary structural diversity present, particularly within the ceramide family. What matters is not merely that these lipids are present. They have to be organised correctly. The extracellular lipids form highly ordered lamellar structures whose composition, chain length and physical arrangement influence the permeability of the barrier. Modern research has identified numerous ceramide subclasses and shown that changes in lipid composition and organisation occur in several inflammatory skin diseases. This is an important lesson for skincare. Skin does not simply require oil. Its barrier depends upon a very particular biological architecture.

The barrier is manufactured from within

The stratum corneum may consist largely of cells that are no longer living in the conventional sense, but its construction is the end result of an extraordinarily active process occurring underneath. As keratinocytes differentiate and travel towards the surface, their lipid metabolism changes dramatically. Specialised structures called lamellar bodies accumulate lipid precursors and enzymes. At the boundary between the granular layer and stratum corneum, their contents are secreted into the extracellular space and subsequently processed to create the mature lipid matrix. The epidermis is therefore not waiting passively for us to supply it with skincare ingredients. It manufactures cholesterol, fatty acids and ceramide precursors for itself. When the permeability barrier is experimentally disrupted, epidermal synthesis of several of these lipids increases as part of the repair response; interfering with those synthetic pathways can delay barrier recovery. That makes the skin barrier less like a wall constructed once and left in place and more like a structure under continuous maintenance.

Keeping water where it belongs

One of the barrier’s most fundamental functions is controlling the movement of water. Our tissues contain substantial quantities of water, while the surrounding atmosphere is often considerably drier. Without an effective permeability barrier, water would diffuse much more freely from the body into the environment. A small amount of water continually passes through the epidermis and evaporates from the skin surface. This is known as transepidermal water loss, usually abbreviated to TEWL. TEWL is not the same thing as sweating. Sweat is actively secreted by sweat glands; transepidermal water loss describes passive movement of water through the epidermal barrier. When barrier function is compromised, water can escape more readily and TEWL can increase. This is one reason damaged skin can become dry, rough and uncomfortable. The relationship is not simply that dry skin means no water, however. Hydration depends upon the organisation of the barrier, environmental humidity, natural moisturising factors, surface lipids, cleansing practices and numerous physiological variables. A glass of water and a moisturiser are therefore solving very different problems.

Natural moisturising factor

Lipids are only part of the hydration story. Inside the corneocytes themselves is a collection of water-soluble substances collectively known as natural moisturising factor, or NMF. These include amino acids and their derivatives, lactate, urea, sugars and inorganic ions. Many originate from the breakdown of the protein filaggrin as epidermal cells mature. Natural moisturising factor helps corneocytes bind water and contributes to the flexibility and hydration of the stratum corneum. The lipid matrix, meanwhile, helps control the movement of water through and out of the tissue. Hydrated skin therefore depends upon several systems working together rather than a single magical moisturising ingredient. This distinction becomes useful when we later explore humectants such as glycerin. A humectant can help attract and retain water within the stratum corneum, while occlusive ingredients can reduce evaporation and lipid-containing formulations may interact with the barrier in other ways. Moisturising is an outcome produced through several different mechanisms.

The skin barrier is not completely impermeable

If the barrier prevented everything from entering, topical skincare and transdermal medicines would be impossible. Instead, the stratum corneum is selectively permeable. A substance’s ability to penetrate depends upon factors including molecular size, lipophilicity, concentration, vehicle and the condition of the skin. Formulators can manipulate these properties to influence how ingredients behave at or within the surface. This creates one of cosmetic science’s central challenges. We often want products to deposit ingredients onto or within the stratum corneum while preserving the integrity of the barrier that exists to resist penetration in the first place. The more dramatic claim that a cosmetic ingredient penetrates deeply into the skin should therefore not automatically be regarded as desirable. Sometimes the surface is exactly where we want an ingredient to remain.

Cleansing: necessary, but not biologically neutral

Skin accumulates sebum, sweat residues, environmental material, microorganisms and whatever products we have previously applied to it. Cleansing therefore has an obvious practical role. Yet cleansing is not chemically neutral. Surfactants work partly because they interact with oils and allow oily material to be dispersed into water. Unfortunately, that interaction does not necessarily distinguish perfectly between unwanted material on the surface and lipids belonging to the stratum corneum. Research into cleansing has shown that surfactants can extract endogenous lipids or interact with the lipid bilayers of the stratum corneum, contributing to barrier disruption. This does not mean cleansing is inherently bad. It means that how we cleanse matters. Repeated aggressive washing, strong detergents, very hot water and excessive exfoliation can place considerably greater demands upon the barrier than gentle cleansing appropriate to the person’s skin and circumstances. A feeling of extreme tightness after cleansing is not necessarily evidence that the skin has become wonderfully clean. It may be telling us something rather different.

Exfoliation and the temptation to remove the barrier

The outermost corneocytes are continually shed through desquamation, an enzymatically regulated process through which connections between cells are gradually broken down. Cosmetic exfoliation can accelerate the removal of surface cells mechanically or chemically. Used appropriately, exfoliating ingredients can have useful cosmetic and dermatological applications. The problem arises when smoothness becomes confused with health and the objective becomes removing as much of the stratum corneum as possible. The stratum corneum is not dead material that the body has forgotten to dispose of. It is functional tissue. A product that makes skin feel dramatically different after one application may sometimes be doing something useful. At other times, it may simply be disrupting the structure we are supposedly trying to improve.

What does a damaged skin barrier actually mean?

The phrase has become ubiquitous on social media, where almost any episode of redness, dryness, acne, stinging or sensitivity can be diagnosed as a broken barrier. Barrier impairment is a genuine biological phenomenon. Changes in stratum-corneum lipid composition and organisation are associated with skin disorders including atopic dermatitis, and experimental barrier disruption produces measurable changes in water loss and epidermal repair responses. But not every skin problem can be diagnosed as barrier damage from appearance alone. Redness can have many causes. Stinging can result from irritation, allergy or underlying dermatological disease. Acne has a complex pathophysiology. Persistent scaling, inflammation or itching may require diagnosis rather than another barrier serum. Support the skin barrier is a useful formulation concept. It is not a universal medical diagnosis.

Moisturisers and barrier support

A well-designed moisturiser can interact with the stratum corneum in several ways. Humectants help increase water within the stratum corneum. Occlusives reduce water loss by forming a relatively water-resistant surface layer. Emollients improve smoothness and flexibility by filling spaces between shedding corneocytes and altering surface feel. Some formulations additionally provide physiological or physiologically related lipids such as ceramides, cholesterol and fatty acids. These categories overlap, and real cosmetic formulations frequently contain ingredients performing several functions simultaneously. This is where ingredient lists become more useful when read as systems rather than beauty shopping lists. The question is not simply whether a cream contains a fashionable ingredient. It is what the entire formulation is designed to accomplish.

Where vitamin E fits

Our Vitamin E, Tocopherols, Botanical Library Monograph No. 003 provides a useful example of this distinction. Alpha-tocopherol is naturally present in human skin and forms part of its lipid-soluble antioxidant system. The skin is continually exposed to oxidative challenges including ultraviolet radiation, and vitamin E has consequently attracted considerable interest in experimental dermatology. A major review of vitamin E physiology in human skin describes experimental evidence relating to antioxidant activity, photoprotection and barrier stabilisation, while also emphasising the relative shortage of controlled clinical evidence defining specific dermatological indications.

Vitamin E therefore belongs naturally in a discussion of skin biology. But it would be misleading to describe it as the ingredient that repairs the skin barrier. The physical permeability barrier depends upon corneocytes and an organised extracellular matrix of ceramides, cholesterol and fatty acids. Tocopherol participates in a different but interconnected part of skin physiology: the antioxidant environment surrounding lipid-rich structures. This is exactly why understanding the barrier makes ingredient claims easier to evaluate. Explore Vitamin E, Botanical Library 003 →

And where do lavender and rose fit?

This is where the distinction between interesting botanical ingredient and barrier-essential ingredient becomes particularly important. Neither lavender nor rose essential oil is required to construct the human skin barrier. That does not make them irrelevant to skincare. It simply means that their role needs to be described accurately.

Lavender essential oil is a complex mixture of volatile aromatic compounds. It has an extensive history of aromatic and traditional use and a developing experimental literature, which we explore in Lavender, Lavandula angustifolia, Botanical Library Monograph No. 001. But essential oils are not interchangeable with the lipids of the stratum corneum. Indeed, fragrance materials can cause irritation or allergic contact dermatitis in susceptible individuals. In a retrospective Australian patch-testing study, lavender was an uncommon but documented cause of allergic contact dermatitis, with personal-care products and essential oils among the principal exposure sources. That does not mean lavender is universally harmful to skin. It means natural does not mean biologically invisible. Explore Lavender, Botanical Library 001 →

Rose requires the same precision. Rose water, Damask rose essential oil, rose absolute and rosehip oil are chemically different materials despite sharing the word rose. None should automatically be described as repairing the barrier merely because it comes from a plant. If we eventually formulate with rose, the relevant questions will be which rose material, at what concentration, in what formulation, for what purpose and with what evidence. That is a much more useful question than asking whether rose is simply good for skin. Explore Rose, Botanical Library 002 →

Fragrance and a compromised barrier

There is a broader principle here that becomes particularly relevant to botanical skincare. Essential oils contain chemically active fragrance compounds. Some are capable of sensitisation, and oxidation during storage can alter allergenic potential. For someone with healthy, tolerant skin, a properly formulated fragranced cosmetic may cause no difficulty at all. Someone with compromised or highly reactive skin may have quite a different experience. That makes concentration, formulation, oxidation stability and safety assessment particularly important when botanical fragrance materials are used. The goal should not be to defend essential oils because they are natural, nor to reject them because they contain allergens. It should be to understand them well enough to use them intelligently.

The microbiome belongs to the barrier story too

The physical barrier is only one component of the skin’s relationship with its environment. Human skin supports diverse communities of microorganisms, while surface lipids, acidity, moisture, immune function and antimicrobial compounds help determine which organisms can thrive there. Some products of epidermal lipid metabolism themselves contribute to antimicrobial defence. This is why the increasingly popular expression microbiome-friendly deserves careful treatment. The skin microbiome is real and biologically important. Demonstrating that a particular cosmetic meaningfully benefits it is another matter. As with botanical ingredients, scientific vocabulary should not become a substitute for evidence. We will return to the skin microbiome separately because it deserves considerably more than a paragraph.

Skin has an acidic surface

Healthy skin generally maintains an acidic surface environment, often referred to as the acid mantle. This acidity participates in several processes relevant to barrier function, including lipid-processing enzyme activity, desquamation and interactions with the skin’s microbial communities. This is one reason cosmetic pH matters. It is also why simplistic advice to alter skin pH dramatically with household acids or alkaline preparations deserves caution. Skin chemistry is not improved merely because a kitchen ingredient happens to be described as natural. The skin already regulates an extraordinarily sophisticated chemical environment. Our interventions should have a reason.

The barrier changes throughout life

The skin barrier is not identical in every person or at every age. Infant skin continues to develop after birth. Sebum production changes dramatically with hormones. Ageing alters epidermal turnover, lipid production and hydration. Body sites differ substantially in stratum-corneum thickness, follicle density and sebaceous activity. Environment matters too. Cold, low humidity and indoor heating can increase the challenge of retaining moisture, while occupational exposure to water, detergents, solvents and repeated handwashing can repeatedly disturb the barrier. There is therefore no single skincare routine that represents biological perfection for everyone. Good skincare responds to the skin that actually exists.

More skincare is not necessarily better skincare

Once the barrier is understood, one of the stranger features of contemporary skincare becomes apparent. Consumers are routinely encouraged to apply long sequences of cleansers, acids, toners, essences, serums, retinoids, botanical extracts, oils and masks to an organ whose outer surface has evolved partly to resist chemical intrusion. Some of those products can be useful. Using all of them simultaneously is not necessarily more useful. Repeated exposure to surfactants, exfoliating acids, retinoids and fragrance materials can increase irritation in susceptible skin, particularly when several active products are introduced together. Once irritation develops, another collection of products may then be sold to repair the barrier. Sometimes the most rational skincare intervention is subtraction.

What does the skin barrier actually need from us?

For healthy skin, often less than the beauty industry suggests. Gentle cleansing when cleansing is needed. Protection from excessive ultraviolet exposure. A suitable moisturiser when the skin needs additional hydration or reduction in water loss. Avoidance of repeated unnecessary irritation. And treatment of genuine skin disease when it occurs. Beyond that, skincare becomes partly about preference: texture, fragrance, ritual, sensory pleasure and the enjoyment of particular botanical materials. There is nothing wrong with that. A rose-scented cream does not have to reconstruct the epidermis to justify its existence. Lavender does not need to become a dermatological treatment before its fragrance can be enjoyed. Vitamin E does not need to erase scars before its antioxidant chemistry becomes interesting. Perhaps botanical skincare becomes more credible when we stop demanding miraculous behaviour from every ingredient.

What this means for Botanicals and Co

Understanding the skin barrier changes the way we will approach the ingredients in the Botanical Library. When we study Lavender 001, the question is not simply what benefits lavender has for skin. We need to ask what material is being used, what compounds it contains, at what concentration it is appropriate, what evidence exists and what sensitisation risks need to be considered. With Rose 002, we need to distinguish rose essential oil from rose water, absolute and the entirely different fixed oils commonly sold under the rosehip name. With Vitamin E 003, we need to distinguish antioxidant chemistry from barrier architecture and Tocopherol from Tocopheryl Acetate. And when we reach Sea Buckthorn 004, the discussion becomes more interesting again because its seed and pulp oils introduce different fatty-acid profiles, tocopherols, carotenoids and other lipid-soluble constituents.

Slowly, these individual pieces begin to connect. That is the point of the Library. We are not collecting ingredients because they sound beautiful on a label. We are trying to understand how they fit within the biology and chemistry of the thing to which they will eventually be applied. And skin is not an empty canvas waiting for botanicals. It is an active, renewing, lipid-manufacturing, water-regulating, microbially inhabited organ with several hundred million years of evolutionary engineering behind it. The best skincare should probably begin with a little respect for that.

References and further reading

  • Bouwstra, J. A. et al. (2023). The skin barrier: an extraordinary interface with an exceptional lipid organization. Progress in Lipid Research, 92, 101252. doi:10.1016/j.plipres.2023.101252
  • Wertz, P. W. (2018). Lipids and the permeability and antimicrobial barriers of the skin. Journal of Lipids, 2018, 5954034. doi:10.1155/2018/5954034
  • Feingold, K. R. (2009). The outer frontier: the importance of lipid metabolism in the skin. Journal of Lipid Research, 50(Suppl), S417-S422. doi:10.1194/jlr.R800039-JLR200
  • Feingold, K. R. & Jiang, Y. J. (2011). The mechanisms by which lipids coordinately regulate the formation of the protein and lipid domains of the stratum corneum. Dermato-Endocrinology, 3(2), 113-118. doi:10.4161/derm.3.2.14996
  • Jungersted, J. M. et al. (2008). Lipids and skin barrier function, a clinical perspective. Contact Dermatitis, 58(5), 255-262. doi:10.1111/j.1600-0536.2008.01320.x
  • Madison, K. C. (2003). Barrier function of the skin: la raison d’être of the epidermis. Journal of Investigative Dermatology, 121(2), 231-241. doi:10.1046/j.1523-1747.2003.12359.x
  • Thiele, J. J. & Ekanayake-Mudiyanselage, S. (2007). Vitamin E in human skin: organ-specific physiology and considerations for its use in dermatology. Molecular Aspects of Medicine, 28(5-6), 646-667. doi:10.1016/j.mam.2007.06.001
  • Ananthapadmanabhan, K. P., Mukherjee, S. & Chandar, P. (2013). Stratum corneum fatty acids: their critical role in preserving barrier integrity during cleansing. International Journal of Cosmetic Science, 35(4), 337-345. doi:10.1111/ics.12042
  • Bingham, L. J. et al. (2019). Contact allergy and allergic contact dermatitis caused by lavender: a retrospective study from an Australian clinic. Contact Dermatitis, 81(1), 37-42. doi:10.1111/cod.13247