What causes hair to turn white or gray?

Hair turning white or gray primarily stems from a decrease in melanin production, which is the pigment responsible for hair color, in the cortex of the hair shaft. As people age, the melanocytes—cells that produce melanin—gradually become less active. When these cells significantly reduce or cease melanin production, the hair loses its original color and appears white or gray. This process is natural and is most commonly associated with aging.

The other options presented do not accurately account for the primary cause of hair color changes. While sun exposure can cause damage to hair and contribute to dryness or other issues, it does not directly reduce melanin production. Hormonal changes during puberty might influence hair growth and development, but they are not responsible for the graying of hair, which is more closely related to aging. Increased blood flow to hair roots can support hair growth and health but does not affect the pigment levels in the hair. Therefore, the correct answer highlights the essential biological mechanism behind the change in hair color as people age.