Occlusives
An occlusive delays TEWL by forming a film over the stratum corneum. Occlusives are often greasy. They dissolve lipids and slow TEWL while on the skin. However, TEWL returns to the original level once the occlusive is removed. Overuse of occlusives is problematic as excessive lowering of TEWL leads to maceration and a higher risk of bacterial infiltration. Most occlusives are oily, with the most widely used formulations being petrolatum and mineral oil. Other types of occlusives include paraffin, squalene, dimethicone, soybean oil, graspeseed oil, propylen glycol, lanoline, beeswax, and sunflower oil, etc.
Image 3. Actions of occlusive agents in the skin
Petrolatum
Petrolatum inhibits water loss of the skin 170 times more effectively than olive oil. Since 1872, it has been one of the most widely used occlusive agents. Contrary to general notions, petrolatum is non-comedogenic and has a low risk of contact dermatitis. However, it is greasy and causes excessive glossiness when applied alone and should be combined with other agents. As many tend to dislike its heavy texture, it is not often used for aesthetic purposes.
Lanolin
Lanolin is a naturally occurring substance obtained from sheep sebaceous glands. Its complicated chemical structure prevents artificial synthesis. Despite originating from sheep sebaceous glands, lanolin has very different composition from that of human sebum. Lanolin contains cholesterol, an important component of skin lipids. At human body temperature, lanolin and skin lipids coexist in the solid and liquid forms, respectively. However, lanolin has a high risk of contact dermatitis and can be used as a sensitizer. Therefore, its use in cosmetics is rapidly declining. The phrase ‘lanolin-free’ can be often found on the labels of many moisturizers for this reason.
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Mineral Oil
Mineral oil is one of the most widely used oils in the last 100 years. Mineral oil exist in various levels from industrial to cosmetic, with cosmetic mineral oil being non-comedogenic. In the last decade, there have been several reports on the association between mineral oil and cancer. However, the means of exposure to mineral oil used in these reports were direct contact with the dermis or aspiration. On the other hand, cosmetic products containing mineral oil have extremely low carcinogenicity.
Vegetable Oil
Sunflower seed (Helianthus annuus) oil is a safe and affordable substance with excellent epidermal barrier functions. It is reported that the skin barrier is restored within an hour of application of sunflower seed oil and the moisturization continues for 5 hours after application.
Evening primrose (Oenothera biennis) oil is rich in omega-6-FA, linolic acid, and gamma-linoleic acid (GLA). Linolic acid plays an important role in maintaining the cohesiveness of stratum corneum and reducing TEWL. It is also an important ingredient in GLA synthesis. Oral ingestion of evening primrose has been recently reported to have good moisturizing effect. It is used as an effective oral and topical treatment of atopic dermatitis.
Olive (Olea Europaea) oil was used for bathing and therapeutic purposes in Ancient Greece, Egypt and Rome. It is rich in powerful antioxidants such as polyphenol and other beneficial substances such as squalene, fatty acid, TG, tocopherol carotenoid, sterol, and chlorophylls, etc. Olive oil is very safe with rare side effects.
The use of jojoba increased recently and it can be used as a humectant as its composition resembles that of human sebum. It has analgesic, antibacterial, anti-inflammatory, and anti-oxidant effects but it is also reported as a common allergen.
-To be continued-
