Hair turns gray primarily because hair follicles produce less melanin, the pigment responsible for hair color, as people age. Melanin is created by melanocytes in the hair follicles, and when these cells reduce pigment production or die off, new hair grows in without color, appearing gray or white. This process is largely influenced by genetics and the natural aging cycle of hair follicles

. The biochemical cause involves the accumulation of hydrogen peroxide in hair follicles, which bleaches the hair from within. Normally, the enzyme catalase breaks down hydrogen peroxide, but its levels decline with age, leading to oxidative stress that damages pigment-producing cells

. Stress itself does not directly turn hair gray by changing the color of existing hair strands, since hair color is fixed once a strand grows. However, stress can accelerate hair loss (telogen effluvium), causing hair to shed and regrow more quickly, potentially revealing more gray hair if pigment production has already decreased. Some studies in animals suggest stress might influence graying by affecting melanocyte stem cells, but this is not yet conclusively proven in humans

. Certain health conditions can cause premature graying, including vitamin B12 deficiency, thyroid disease, autoimmune conditions like vitiligo and alopecia areata, and some rare genetic disorders. These illnesses affect melanocytes or hair follicle function, leading to early loss of pigment

. In summary, hair turns gray due to a decline in melanin production in hair follicles caused by aging, genetics, oxidative damage, and sometimes health issues. Stress may indirectly contribute by accelerating hair cycling but does not directly change hair color