Skincare glossary

Emollients fill in the micro-spaces between corneocytes (dead skin cells) in the stratum corneum, creating a smoother surface and reducing the rough, flaky feel of dry skin. They differ from humectants (which attract water) and occlusives (which block water loss) — emollients primarily improve skin texture and flexibility. Many emollients also provide a secondary occlusive effect. Plant oils, fatty acids, and esters are the most common emollient classes in cosmetics.

Examples Jojoba oil · Caprylic/Capric Triglyceride · Isopropyl Myristate · Cetyl alcohol · Squalane

See also humectant · occlusive

Humectants are hygroscopic molecules that attract and bind water molecules, drawing moisture into the outermost layer of skin (the stratum corneum). They work by forming hydrogen bonds with water, pulling it from the dermis or from humid ambient air into the epidermis. Common cosmetic humectants include glycerin, hyaluronic acid, and panthenol. Because humectants can draw water out of skin in very dry environments, they are often paired with an occlusive to seal in the moisture they attract.

Examples Glycerin · Hyaluronic acid · Panthenol · Urea · Sodium PCA

Dossiers hyaluronic acid

See also occlusive · emollient

Occlusives work by sitting on top of the stratum corneum and physically blocking transepidermal water loss (TEWL). Unlike humectants, they do not attract water — they seal it in. Common occlusives include petrolatum, dimethicone, and plant-based waxes. They are most effective when applied to damp skin, layered over humectants so there is moisture to trap. Occlusives are the dominant mechanism behind thick overnight moisturizers and healing ointments.

Examples Petrolatum · Dimethicone · Shea butter · Beeswax · Squalane

See also humectant · emollient

Surfactants (surface-active agents) are amphiphilic molecules — they have a water-loving (hydrophilic) head and an oil-loving (lipophilic) tail. When applied to skin with water, their tails surround and encapsulate oil, sebum, and particulate dirt into micelles, which are then rinsed away. In cosmetics they serve as cleansing agents, emulsifying agents (stabilising oil-in-water or water-in-oil emulsions), and foam boosters. Harshness varies significantly: sodium lauryl sulfate (SLS) is a strong anionic surfactant that can disrupt the skin barrier; gentler options like cocamidopropyl betaine and sodium cocoyl isethionate are preferred for sensitive skin.

Examples Sodium Lauryl Sulfate (SLS) · Sodium Laureth Sulfate (SLES) · Cocamidopropyl Betaine · Sodium Cocoyl Isethionate

See also ph · emollient

Antioxidants protect cells by intercepting free radicals — unstable molecules generated by UV exposure, pollution, and metabolic processes — before they can cause chain reactions that damage lipids, proteins, and DNA. They donate one of their own electrons to stabilise the free radical, becoming mildly oxidised themselves in the process. In skincare, antioxidants applied topically (vitamin C, vitamin E, ferulic acid, niacinamide) form an additional layer of photoprotection and help reduce cumulative photodamage. They work best in combination: vitamin E regenerates vitamin C after it donates an electron, and ferulic acid stabilises both.

Examples L-Ascorbic acid (vitamin C) · Tocopherol (vitamin E) · Ferulic acid · Niacinamide

Dossiers l ascorbic acid · tocopherol · ferulic acid

See also free radical

Free radicals are atoms or molecules that carry one or more unpaired electrons, making them highly reactive. They stabilise themselves by stealing electrons from nearby molecules — lipids in cell membranes, collagen fibres, or DNA — triggering chain reactions of oxidative damage. In skin, the primary sources of free radicals are UV radiation (generating reactive oxygen species) and environmental pollutants. Cumulative free-radical damage is a major driver of photoaging: collagen breakdown, loss of elasticity, and hyperpigmentation. Antioxidants in skincare interrupt this chain by donating electrons without becoming damaging radicals themselves.

See also antioxidant

pH (potential of hydrogen) measures the concentration of hydrogen ions in a solution, running from 0 (most acidic) to 14 (most alkaline), with 7 as neutral. Healthy skin surface pH sits between 4.5 and 5.5 — slightly acidic — which supports the acid mantle and a balanced microbiome. Many actives require a specific pH range to function: vitamin C (L-ascorbic acid) needs a pH below 3.5, AHAs work optimally below 4.0, and retinoids are most stable near neutral. Products formulated outside the correct pH for their active can be ineffective or irritating.

Examples Skin surface: pH 4.5–5.5 · Effective vitamin C serum: pH 2.5–3.5 · AHA exfoliant: pH 3.0–4.0

Dossiers l ascorbic acid · glycolic acid · salicylic acid

See also active ingredient · surfactant

In cosmetics, an active ingredient is one added to deliver a specific functional benefit — brightening, exfoliating, firming, or treating a skin concern — as distinct from base ingredients like emollients and preservatives that primarily carry or stabilise the formula. In OTC drug products (sunscreens, acne treatments, anti-dandruff shampoos), 'active ingredient' has a regulatory meaning: it is the drug substance recognised by the FDA as providing the therapeutic action and must be listed separately on the label with its concentration. In cosmetic-only products, the term is used loosely by brands to highlight key ingredients.

Examples L-Ascorbic acid (brightening) · Niacinamide (pore + tone) · Salicylic acid (acne, OTC drug)

Dossiers l ascorbic acid · niacinamide · salicylic acid

See also inci · ph

Comedogenicity refers to the tendency of a substance to promote the formation of comedones — clogged follicles that appear as blackheads (open) or whiteheads (closed). Comedogenicity ratings (0–5 scale) were originally derived from rabbit ear occlusion models and have limited direct applicability to human use at real-world concentrations. An ingredient rated comedogenic in isolation may not cause breakouts in a dilute rinse-off formula. Acne-prone skin types are most susceptible, but individual response is highly variable.

See also non comedogenic · patch test

INCI (International Nomenclature of Cosmetic Ingredients) is a system of standardised, internationally recognised names for cosmetic ingredients developed by the Personal Care Products Council and adopted by regulatory bodies in the EU, US, and many other markets. INCI names are based on Latin botanical names for plant-derived ingredients and on IUPAC chemical nomenclature or common names for synthetics. Cosmetic products sold in the EU must list all ingredients on pack using INCI names in descending order of concentration. The same system is used in the US and many other markets, making ingredient-label reading consistent across borders.

Examples Aqua (water) · Glycerin · Tocopherol (vitamin E) · Ascorbic Acid (vitamin C)

See also active ingredient

Non-comedogenic is a marketing label claiming that a product is unlikely to block hair follicles and cause comedones (blackheads and whiteheads). There is no FDA or EU regulatory standard for this term — brands self-certify. The underlying concept comes from comedogenicity ratings developed in animal models (rabbit ear assays) that have limited predictive value for human skin. Individual response varies by skin type, formulation concentration, and product layering. The term is a useful rough guide but should not be taken as a guarantee.

See also comedogenic · patch test

A patch test is a precautionary trial used to detect contact allergies or sensitivities before a product is applied to the face or a larger skin area. A small amount of the product is applied to an inconspicuous site — typically the inner forearm or behind the ear — and left for 24 to 48 hours without washing. If redness, itching, swelling, or blistering occurs, the product should not be used. Patch testing is especially recommended before using high-concentration actives (retinoids, AHAs, vitamin C), products containing fragrance or essential oils, and anything applied to compromised skin. It does not replace a clinical patch test conducted by a dermatologist to diagnose specific contact allergens.

See also non comedogenic · comedogenic