Epidermis is your first barrier of defense against the environment. At the bottom of the epidermis lies the basal layer where millions of new epidermis cells are produced every day through mitosis or cell division. These cells get flattened and hardened as they climb up the epidermis. This takes close to a month. When they reach the corneal layer, the cells are lifeless.
Sebum (皮脂) is an oily substance secreted by the sebaceous glands (皮脂腺) in mammalian skin. Its main purpose is to make the skin and hair waterproof and to protect them from drying out. An excess of sebum, however, can make the skin or hair oily, and it can begin to smell when broken down by bacteria outside the body.