Educational Resource

The Definitive Skincare Ingredient Glossary

Bridging the gap between laboratory science and your bathroom shelf. Explore over 200 terms, ingredients, and biological processes defined with clinical precision by cosmetic chemist Chloé Fournier.

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Why Ingredient Literacy Matters in 2026

The modern skincare landscape is saturated with marketing buzzwords like "clean," "non-toxic," and "clinical strength." However, these terms often lack regulatory definitions. True empowerment comes from understanding the International Nomenclature Cosmetic Ingredient (INCI) system.

As a cosmetic chemist, my goal is to strip away the hype and focus on the molecular efficacy of ingredients. Whether you are curious about the difference between AHAs and BHAs or wondering how to identify a stable form of Vitamin C, this glossary serves as your scientific compass.

Understanding formulation isn't just about spotting "hero" ingredients; it's about recognizing the supporting cast—the emulsifiers, preservatives, and penetration enhancers that make a product safe and effective.

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Expert Categorized Glossary

The Acid Hierarchy (Exfoliants & PH)

Alpha Hydroxy Acids (AHAs)

Water-soluble acids derived from sugary fruits. They work by ungluing the bonds holding dead skin cells to the surface. Examples: Glycolic, Lactic, Mandelic.

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Beta Hydroxy Acid (BHA)

Oil-soluble acid that penetrates deep into pores to dissolve excess sebum and dead cell build-up. Salicylic acid is the most studied BHA — the gold standard for oily and acne-prone skin. Works most effectively at concentrations of 1–2% and a pH of 3–4.

Polyhydroxy Acids (PHAs)

Next-generation exfoliants (gluconolactone, lactobionic acid) with larger molecular structures than AHAs, limiting penetration depth. This makes them exceptionally gentle — ideal for sensitive, rosacea-prone, or post-procedure skin that cannot tolerate traditional AHAs.

Hyaluronic Acid

A naturally occurring polysaccharide that binds up to 1,000× its weight in water. Despite the name, it is not an exfoliating acid — it functions as a humectant. Multi-weight formulas combine high-molecular-weight HA (surface hydration) with low-molecular-weight fragments (deeper penetration) for comprehensive plumping.

Azelaic Acid

A dicarboxylic acid derived from grains, effective at 15–20% prescription strength for acne and rosacea. Inhibits tyrosinase to reduce post-inflammatory hyperpigmentation without melanocyte cytotoxicity — making it safe for all skin tones, including darker complexions that are prone to PIH.

Ascorbic Acid (Vitamin C)

The most potent and researched form of Vitamin C. A powerful antioxidant and tyrosinase inhibitor that brightens skin and stimulates collagen synthesis. Requires low pH (below 3.5) and airtight, opaque packaging to prevent oxidation — a brown or orange colour indicates degradation.

Biological Processes & Skin Science

Stratum Corneum (The Skin Barrier) ↓

The outermost layer of the epidermis, composed of 15–20 layers of flattened, dead keratinocytes embedded in a lipid matrix of ceramides, cholesterol, and fatty acids. This "brick and mortar" architecture is the skin's primary barrier against water loss and environmental aggressors. When compromised, TEWL increases and skin becomes sensitive and reactive. See our barrier-repair routine guide for formulation strategies.

Transepidermal Water Loss (TEWL) ↓

Transepidermal Water Loss is the passive diffusion of water through the skin to the environment. A healthy barrier maintains TEWL at approximately 5–10 g/m²/h. Harsh surfactants, over-exfoliation, or genetic factors like filaggrin mutations dramatically increase TEWL. Occlusives (petrolatum, mineral oil, waxes) reduce TEWL by forming a physical seal; humectants require an occlusive layer in low-humidity environments to prevent them from drawing moisture out of the skin.

Microbiome ↓

The collection of trillions of microorganisms — bacteria, fungi, and mites — that coexist on skin. A healthy microbiome, dominated by Staphylococcus epidermidis, maintains a slightly acidic pH (4.5–5.5) that inhibits pathogenic colonisation. Alkaline cleansers, antibiotics, and over-cleansing create dysbiosis linked to acne, eczema, and rosacea. Prebiotics and postbiotics in formulations support microbiome balance. More on the biology in our science guide.

Oxidative Stress ↓

An imbalance between reactive oxygen species (ROS — free radicals from UV, pollution, and metabolism) and the skin's antioxidant defences. ROS attack lipids, proteins, and DNA, accelerating collagen degradation and hyperpigmentation. Antioxidants (Vitamin C, Vitamin E, niacinamide, resveratrol, green tea polyphenols) donate electrons to neutralise free radicals. Formulation stability is critical: antioxidants degrade rapidly if unprotected from oxygen and light.