Hair turns gray because the hair follicles stop producing melanin, the pigment responsible for hair color. As people age, the cells in hair follicles called melanocytes gradually reduce the production of melanin, leading to hair growing in gray or white. This is a natural part of aging, usually beginning in the 30s or 40s, but genetics and other factors like stress, smoking, vitamin deficiencies, and certain medical conditions can influence when and how quickly hair turns gray.

The process involves the depletion or dysfunction of melanocyte stem cells in the hair follicles, which causes new hairs to grow without pigment. Stress can accelerate gray hair by causing these pigment-producing cells to move out of the follicles. Additionally, accumulation of hydrogen peroxide in hair follicles can bleach the hair, contributing to gray hair.

In summary, gray hair occurs when the pigment cells in hair follicles reduce or stop melanin production, leading to the loss of hair color due to aging and other contributing factors.