Editing Moisturizer (section)

== Mechanism of action ==

In the human body, water constantly evaporates from the deeper layers of the skin through an effect known as [[transepidermal water loss]]. By regulating its water content, human skin naturally maintains a dry, easily shed surface as a [[Innate immune system#Anatomical barriers|barrier]] against pathogens, dirt, or damage, while protecting itself from drying out and becoming brittle and rigid. The ability to retain moisture depends on the [[lipid bilayer]] between the [[corneocytes|dead skin cells]].<ref name="seidel">{{citation | editor=Arza Seidel|display-editors=etal| title=Kirk-Othmer Chemical Technology of Cosmetics | year=2013}}</ref>

Moisturizers modify the rate of water loss, with active ingredients of moisturizers falling into one of two categories: occlusives and [[humectant]]s.<ref name="duffill">{{cite web |last1=Duffill |first1=Mark |last2=Oakley |first2=Amanda |title=Emollients and moisturisers |url=https://dermnetnz.org/topics/emollients-and-moisturisers/ |website=dermnetnz.org |access-date=31 December 2020 |archive-date=25 August 2023 |archive-url=https://web.archive.org/web/20230825045713/https://dermnetnz.org/topics/emollients-and-moisturisers |url-status=live }}</ref>

''Occlusives'' form a hydrophobic coating on the surface of the skin, keeping moisture from escaping. The more occlusive the formulation, the greater the effect. [[Ointment]]s are more occlusive than [[aqueous cream]]s, which are more occlusive than [[lotion]].<ref name="duffill" /> Water loss through the skin is normally about 4–8 g/(m<sup>2</sup>⋅h). A layer of [[petrolatum]] applied to normal skin can reduce that loss by 50–75% for several hours.<ref name="seidel" /> Oils naturally produced by the human body moisturize through this same mechanism.<ref name="your-body" />

''Humectants'' are hydrophilic and absorb water. They absorb water from humid air (when >70% humidity) to moisturize the skin. More commonly, however, they draw out water from the dermis into the epidermis, making deeper skin dryer.<ref>{{cite journal |last1=Harwood |first1=Anne |last2=Nassereddin |first2=Ali |last3=Krishnamurthy |first3=Karthik |title=Moisturizers |url=https://www.ncbi.nlm.nih.gov/books/NBK545171/ |website=StatPearls |publisher=StatPearls Publishing |access-date=31 December 2020 |date=2020 |pmid=31424755 |archive-date=28 January 2023 |archive-url=https://web.archive.org/web/20230128195629/https://www.ncbi.nlm.nih.gov/books/NBK545171/ |url-status=live }}</ref> When used in practical applications, humectants are combined with occlusives.<ref>{{cite web |last1=Publishing |first1=Harvard Health |title=Moisturizers: Do they work? |url=https://www.health.harvard.edu/staying-healthy/moisturizers-do-they-work |website=Harvard Health |date=February 2008 |access-date=31 December 2020 |archive-date=20 September 2023 |archive-url=https://web.archive.org/web/20230920001311/https://www.health.harvard.edu/staying-healthy/moisturizers-do-they-work |url-status=live }}</ref> Moisturizers commonly contain water, which acts as a temporary hydration agent.<ref name="marino" />