Most people use these two words interchangeably, when they are actually quite different.
Dry skin is caused by a lack of the lipids within the stratum corneum, the outermost portion of our epidermis. In general, this lipid content by weight is made up of 10% free fatty acids, 25% cholesterol and 50% ceramides. These lipids are essential in the prevention of water loss. Dehydrated skin is caused by a lack of water within the skin.
Our skin is a part of our excretory system, and approximately 4-6 ounces of water pass through and out of our epidermis every day – known as transepidermal water loss or TEWL. In order for the skin to maintain optimal health, a delicate balance is required in order to prevent excessive TEWL, which can lead to impaired barrier function. Impaired barrier function is characterized by rough, red and flakey skin that is easily irritated and sensitive to topical products.
Maintaining this delicate balance requires drinking plenty of water to keep our dermis hydrated, using humectant ingredients topically so the epidermis is able to attract water from the dermis, and the use of skin-friendly occlusives to hold that moisture within the epidermis to prevent it from evaporating. Examples of humectant ingredients are hyaluronic acid, urea, glycerin, sodium PCA and honey. Occlusive ingredients that are beneficial to the skin are dimethicone, cyclomethicone, plant oils, shea butter and niacinamide. Occlusives that should be avoided are petrolatum, lanolin and mineral oil.
Because our epidermis only allows a small amount of water to penetrate from the outside in, it is up to us to apply beneficial ingredients to keep our skin healthy and hydrated every day.
Article from PCA Skin