Blond hair turns green due to the absorption of copper, if present in the water and if the hair is previously damaged. The concentration of copper must be greater than 0.3 parts per million and the hair cuticle must have been damaged due to physical factors (brushing, hot drying, and sun exposure) and/or chemical factors (peroxide bleaching, permanent waving, use of alkaline, and tar shampoos, or exposure to chlorinated water in swimming pools). The damaged cuticle opens up a pathway for the copper in the water.
A study has shown that damaged hair will adsorb 3 times more copper than undamaged hair. The green discoloration is more apparent in people with blond, gray, or white hair. People with dark hair could have the same copper concentrations as those with blond hair that has turned green, but the color change would not be as noticeable. Contact a hair care professional for suggestions on how to return your hair to the desired color.