Of the two component parts of the skins, it is the epidermis that provides protection from the environment.
Your skin is the largest organ in the body and the first line of defense against harmful microbes, pollution, and UV rays.
Skin is relatively impermeable to polar molecules such as water, sodium, potassium and other ions in solution but more highly permeable to aliphatic alcohols.
Furthermore, the face forehead and dorsum of the hand is the most permeable, the palms of the hand being the least permeable.
In scientific terms, your skin is technically "dry" when its moisture level is less than 10%. That's when you're most likely to smother yourself in skin care products.
Cracked, flaky, and dry skin — which tends to occur when humidity drops in the chilly months — goes by a mouthful of a scientific name: transepidermal water loss, or TEWL (and that is why facial mask iss made for).
At its simplest, TEWL is a measure of how much water seeps from the inside of the body through the different layers of the skin and out into the atmosphere.
Especially dry, irritated, or inflamed skin is also called xerosis, which is usually a minor and temporary problem that can be solved with good moisturizing products.
Moisture is delivered to the skin via blood vessels, but they only supply moisture to the middle layer of the skin-the dermis. From there, water travels upward and outward through the epidermis before evaporating into the atmosphere. This evaporation causes the skin to crack and flake. This process happens constantly, but the skin isn't always dry. That's because the dryer the air the more moisture it will pull from your skin.
Moisturizers work in one of two main ways: they either trap moisture in your skin to keep it from escaping, or they restore moisture in the outer layer of skin that's already been lost.
There are three types of moisturizers. Each works slightly differently, but most products combine all three.
These are called the "old school" or "first-generation" moisturizers - think petroleum jelly or it's brand name, Vaseline. This class of waxes, oils, and silicones work in a very simple way: They create a barrier over the skin, trapping water in the skin's layers and stopping evaporation. The molecules in these moisturizers contain long chains of carbon atoms that repel water. While occlusives are super effective at minimizing dryness - they cut TEWL by a whopping 98% - they can be sticky, messy, and not very cosmetically appealing.
This class of moisturizer, which exists in the form of creams, ointments, lotions, and gels, are generally preferred over occlusives because they feel less sticky. Whereas occlusives coat the skin, emollients penetrate it, making the skin feel soft and flexible.
Emollient products are made with a variety of chemicals, but their basic building blocks are the same as occlusives - long chains of carbon atoms that repel water. Emollients work a little differently than occlusives, though.
Think of the outer layer of skin as a brick and mortar structure: the dead skin cells are the bricks and the surrounding matrix of fats and proteins are the mortar. Special proteins link the dead cells together, forming a barrier between the inside of the body and the bacteria and chemicals outside.
When the air gets dry, it dries out this matrix, and the links between the proteins and skin cells fall apart and fracture. Emollients are like cement in those gaps, restoring moisture and keeping skin smooth.
Humectants work by attracting moisture to the skin and keeping it there. This is basically the opposite of occlusives and emollients, which don't like water. Humectants penetrate the outer layer of the skin, attract water to it, and lock that moisture in.
This happens because humectants have hydroxyl groups in their chemical structure (oxygen and a hydrogen atom), which loves water. Humectants also prompt the production of ceramides, our body's natural waxy molecules that play a major role in the structure of the skin.
But beware, in dry conditions, humectants can draw moisture from the younger, moist cells in the lower layers of the skin instead of pulling moisture from the air. Over time, this could eventually lead to even dryer skin. Minimize this by pairing a humectant with an occlusive, which seals in the moisture.