1. Petrolatum (White Petroleum Jelly) — 98% TEWL Reduction
Petrolatum is the undisputed king of cutaneous occlusivity. Clinical evaporimeter studies consistently demonstrate that pure white petrolatum reduces transepidermal water loss by over 98% — a performance unmatched by any other cosmetic substance.
Petrolatum consists of a purified mixture of semi-solid saturated hydrocarbons (C25+). It does not penetrate into viable epidermal layers; instead, it forms an impermeable, semi-crystalline hydrophobic barrier on the surface.
Crucially, petrolatum is non-comedogenic (its hydrocarbon molecules are far too large to enter follicular ostia). However, because it traps 98% of moisture, it also traps heat, sweat, and leave-on active retinoids, compounding retinoid irritation if applied directly over prescription creams.
2. Shea Butter (*Butyrospermum Parkii*) — 50–60% TEWL Reduction
Shea Butter is a natural plant lipid rich in stearic and oleic triglycerides, phytosterols, and triterpene esters. It provides moderate occlusivity (reducing TEWL by approximately 50% to 60%) alongside rich emollience.
However, its high oleic acid content (~45%) can perturb intercellular lipid packing in eczema-prone barriers and serve as a carbon food source for *Malassezia* yeast, making it risky for fungal acne sufferers.
3. Dimethicone (Polydimethylsiloxane) — 20–30% TEWL Reduction
Dimethicone is a synthetic siloxane polymer. Unlike petrolatum's dense hydrocarbon seal, dimethicone forms an elastic, permeable, breathable lattice over the stratum corneum.
It reduces TEWL by approximately 20% to 30% while permitting gas and oxygen exchange. Dimethicone imparts an unmatched velvety, non-greasy matte finish, making it the premier daytime occlusive for oily and acne-prone skin.
4. Plant-Derived Squalane — 10–15% TEWL Reduction
Squalane is a fully saturated, hydrogenated hydrocarbon modeled after human sebum squalene. It functions primarily as a biomimetic emollient rather than a true occlusive.
It integrates into intercellular lipid bilayers, softening stiff, dry corneocytes without forming a heavy surface seal. Squalane is 100% non-comedogenic, completely oxidation-stable, and 100% Malassezia-safe.
Takeaway: Slugging with Petrolatum is unrivaled for overnight recovery of raw, cracked barriers. Use Dimethicone for breathable daytime protection without shine. Use Squalane for lightweight, fungal-acne-safe emollience.