Petroleum jelly forms a seal that reduces water loss from skin by more than 98% — one of the most effective occlusives known.
Purity
Cosmetic-grade petrolatum is highly refined; concerns relate to impurities in unrefined grades.
Evidence & sources
FDA permits petrolatum at 30–100% as an over-the-counter skin protectant. Evidence: regulatory. Known: US OTC skin protectant monograph. Unknown: — Source: US FDA, Skin Protectant Drug Products for OTC Human Use (21 CFR 347) (Current revision).