Hair turns gray because hydrogen peroxide builds up in the follicles and bleaches it from the inside out.

This biological process was detailed in a landmark 2009 study published in The FASEB Journal by researchers from the University of Bradford in the United Kingdom. The team discovered that hair follicles produce tiny amounts of hydrogen peroxide as a metabolic byproduct. In younger individuals, the enzyme catalase quickly neutralizes this chemical by converting it into harmless water and oxygen molecules.As humans age, the production of catalase significantly declines, leading to a massive buildup of hydrogen peroxide at the hair root. This accumulation creates oxidative stress that prevents the enzyme tyrosinase from functioning correctly. Tyrosinase is essential for the production of melanin, the pigment responsible for hair, skin, and eye color.When tyrosinase is inhibited, the melanocytes in the hair bulb fail to transfer pigment into the growing hair shaft. The result is hair that lacks color entirely, which we perceive as gray or white depending on the light. This discovery debunked older theories that graying was simply caused by the 'exhaustion' of pigment cells, proving instead that it is a chemical bleaching process occurring internally.