Hair turns white (or grey) when pigment-producing cells called melanocytes in our hair follicles slow down or stop making melanin, the natural color that gives hair its shade. This process is mostly driven by genetics and aging—as oxidative stress builds up over time, melanocytes get damaged and eventually produce less pigment, so new hair grows in lighter and finally white. Factors like stress, nutritional deficiencies (B12, iron, copper), smoking, thyroid issues, and autoimmune conditions can speed up this process, but they don’t usually cause permanent greying on their own. Once a follicle loses its pigment production, the change is typically permanent.