How copper leaves a green tint
Pool water can carry tiny amounts of dissolved copper, often from copper-based treatments or from water moving through copper plumbing. The metal may be invisible in the water, but it can cling to hair. As copper-containing residue accumulates and reacts with air or other chemicals, it can produce a blue-green stain on the hair’s surface.
Hair is made largely of keratin, a protein with a rough, layered outer surface. Those layers can trap mineral deposits, especially when hair is dry or damaged. Repeated exposure gives more copper a chance to stick, so the tint may become more noticeable over time.
Why the color shows up on blondes
The green comes from the deposit, not from blonde pigment changing into green. Darker hair can conceal a small amount of discoloration; pale blonde hair provides much less color to mask it. That makes the same residue easier to see, particularly on lightened hair, whose outer layers may be more porous.
Chlorine gets blamed because it is strongly associated with pools, but it is not the green coloring agent. Copper is the key ingredient in this particular stain. Rinsing hair with clean water after swimming can limit how much pool water dries on it, while repeated discoloration may be a clue to check the pool water or household plumbing for copper. The surprising part is that a nearly invisible amount of metal in the water can leave a very visible trace on pale hair.
