Your hair can get dry because your scalp doesn’t make enough oil to moisturize your hair, or your hair lets moisture escape.
Your scalp isn’t making enough moisture. Hair has no natural lubrication. It relies on oils made in the hair root to keep your hair moisturized and looking lustrous. Sometimes, hair doesn’t make enough oil, which leads to dry hair. (Likewise, roots in overdrive lead to oily and greasy hair.) As you age, your hair naturally makes less oil. Since the roots are under your skin, dry scalp goes along with dry hair. Dry scalp peels and sheds, leading to dandruff flakes on your shoulders.
The moisture you have is escaping. Each strand of healthy hair has a protective layer called the cuticle. Just as shingles shield your home from rain and sun damage, the cuticle shields your hair from heat and sun damage. In a healthy cuticle, the layers lie tightly together and keep moisture in. When a cuticle’s layers separate and peel away from hair, its ability to hold moisture is compromised – some oil escapes.