A moisturizer, or emollient,[1] is a cosmetic preparation used for protecting, moisturizing, and lubricating the skin. These functions are normally performed by sebum produced by healthy skin.[2] The word "emollient" is derived from the Latin verb mollire, to soften.[3]
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 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 dead skin cells.[4]
Moisturizers modify the rate of water loss, with active ingredients of moisturizers falling into one of two categories: occlusives and humectants.[5]
Occlusives form a hydrophobic coating on the surface of the skin, keeping moisture from escaping. The more occlusive the formulation, the greater the effect. Ointments are more occlusive than aqueous creams, which are more occlusive than lotion.[5] Water loss through the skin is normally about 4–8 g/(m2⋅h). A layer of petrolatum applied to normal skin can reduce that loss by 50–75% for several hours.[4] Oils naturally produced by the human body moisturize through this same mechanism.[2]
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.[6] When used in practical applications, humectants are combined with occlusives.[7] Moisturizers commonly contain water, which acts as a temporary hydration agent.[8]
There are many different types of moisturizers. Petrolatum is one of the most effective moisturizers, although it can be unpopular due to its oily consistency.[9][10]
Other popular moisturizers are cetyl alcohol, cetearyl alcohol, cocoa butter, isopropyl myristate, isopropyl palmitate, lanolin, liquid paraffin, polyethylene glycols, shea butter, silicone oils, stearic acid, stearyl alcohol and castor oil, and other oils.
Moisturizers may also be available as lotions, creams, ointments, bath oils, or soap substitutes.
Mineral oils and waxes are not prone to oxidation or rancidity.[11] For this reason, they have essentially replaced vegetable oils in emollients and topical medication.
Moisturizer cosmetics may additionally contain antioxidants, ceramides, emulsifiers, fragrances, penetration enhancers, preservatives, and solvents. Some products are marketed as having anti-wrinkle and skin enhancement effects. Many plant and animal extracts have been claimed to impart skin benefits, but such claims are presented with little scientific evidence.
Moisturizers are used for the treatment of certain skin diseases, such as psoriasis, ichthyosis vulgaris, xerosis, and pruritus in atopic dermatitis. More often, they are bases or vehicles for topical medication, such as in Whitfield's ointment. They are often combined with humectants, such as salicylic acid and urea.[12]
Moisturizers are also widely used in sunscreens, antiperspirants, skin cleansers, shaving creams, aftershaves, and hair tonics.
Moisturizers are used in disposable diapers to prevent dry skin and diaper dermatitis.
A Cochrane review noted that moisturizers show some beneficial effects in eczema.[13] The same review did not find evidence that any one moisturizer was better than another.[13] Furthermore, because none of the different types of moisturizers are more effective than the others, consumers need to choose one or more products according to their age, affected body site, climate/season, and personal preference.[14]
Moisturizers can improve skin comfort and may reduce disease flares such as related to atopic dermatitis.[15] They can be used as leave-on treatments, bath additives, or soap substitutes. There are many different moisturizer products, but the majority of leave-on treatments (from least to most greasy) are one of the following: lotions, creams, gels, or ointments.
Persistent moisturization to the skin via water contact may contribute to an allergic reaction or contact dermatitis.[citation needed] This could allow foreign objects to penetrate the skin.[ambiguous]
Changes in the skin's normal ecological environment–either atop or within the skin–can also allow for the overgrowth of pathogens.[8]
Aromas or food additives in moisturizers may trigger an immune reaction, including development of an allergy.[16][17]
There is currently no regulation over use of the term "hypoallergenic". In fact, some pediatric skin products marketed as hypoallergenic contained allergens.[18][19]
Those with eczema are especially vulnerable to allergic reaction with lotions and creams, as their compromised skin barrier allows preservatives to bind with and activate immune cells.[20]
The American Academy of Allergy, Asthma, and Immunology released a warning in 2014 that natural lotion containing ingredients commonly found in food (such as goats milk, cow's milk, coconut milk, or oil) may introduce new allergies, potentially causing an allergic reaction upon later consuming such foods.[16] A paper published in 2021 noted that frequent skin moisturization in early life might promote the development of food allergy, even when skin conditions such as eczema are taken into account.[17]
Paraffin-based skincare products and contaminated clothing can pose a serious fire hazard.[21] Between 2010 and 2018, paraffin was linked to 50 fire incidents (49 of which were fatal) in the U.K.[22] A West Yorkshire Fire and Rescue Service study found that clothing contaminated with cream containing only 21% paraffin, when set alight, was fully engulfed in flame in 3 seconds.[21] The Medicines and Healthcare products Regulatory Agency (MHRA) released a warning in 2008 about the flammability of paraffin-based products.[23] MHRA recommends that regular paraffin users change their sheets regularly, refrain from smoking and/or bringing open flames near paraffin-coated people or objects.[24] MHRA also recommends that skin creams containing any paraffin have a flammability warning on the packaging.[25]
