Chemistry and Product Knowledge
pH and the acid mantle
Skin's surface pH is mildly acidic (about 4.5–5.5). Products far outside this range can disrupt the barrier. pH affects exfoliant strength: free-acid availability rises as pH drops, so a low-pH AHA is more aggressive than a neutralized one.
Key ingredient families
- Humectants (glycerin, hyaluronic acid) attract water.
- Emollients (oils, esters) soften and smooth.
- Occlusives (petrolatum, dimethicone) prevent water loss.
- Surfactants cleanse by bridging oil and water; they can be harsh if overused.
- Antioxidants (vitamin C, vitamin E) neutralize free radicals.
- Exfoliants: AHAs (glycolic, lactic), BHAs (salicylic), enzymes, and physical scrubs.
- Retinoids accelerate turnover and increase photosensitivity.
Emulsions and preservatives
Most creams and lotions are emulsions of oil and water held together by emulsifiers; because they contain water, they require preservatives to prevent microbial growth. Anhydrous products (ointments, balms) generally need fewer or no preservatives.
Traps
- Confusing humectant and occlusive roles.
- Assuming "natural" means safe or preservative-free products are safer - water-containing products without preservatives are a contamination risk.
- Mixing strong actives (e.g., retinoid plus high-strength acid) without guidance, which increases irritation.
Rule of thumb
"Water in the formula means a preservative is needed; low pH means a stronger acid; match the ingredient to the function."