If you’ve ever climbed out of a swimming pool and noticed your blonde hair gleaming with a faint green shimmer, you’re not imagining it. This strange transformation has been surprising (and annoying) some swimmers for years. The easy culprit to blame is chlorine, the harsh-smelling chemical we all associate with pools.

But the real story is a little weirder, and a lot more metallic.

The Copper Connection

Back in the 1970s, scientists started testing what actually makes hair turn green after repeated swims. A 1978 study dunked hair samples into water with chlorine, with copper, and with both. The results were clear: copper was the guilty party.

Hair exposed to copper ions—the tiny charged atoms of copper—took on a green color. Chlorine alone didn’t cause the shift. But chlorine did weaken hair, making it easier for copper to bind and intensify the effect.

Where does all this copper come from? Pools are often treated with copper sulfate to keep algae at bay. Only a small amount (around 0.5mg per liter or 0.5 parts per million) of copper sulfate is needed to prevent algal growth. On top of that, copper can leach in from corroded pipes, meaning some pools may carry higher-than-expected concentrations.

Copper sulfate itself is greenish-blue. When those ions stick to hair, especially hair that’s already been chemically treated or sun-damaged, the green stain sets in.

What’s Happening on a Molecular Level

The chemical structure of hair also plays a role.

Hair is mostly made of keratin, a tough protein that gives it structure. Within keratin are chemical groups like carboxyl, amino, and disulfide groups that copper ions love to latch onto.

A 2020 study revealed that copper especially prefers broken disulfide bonds, which are more common in bleached or heat-damaged hair. That’s why natural blondes may get only a slight shimmer, while bleached blondes sometimes walk away from the pool looking like they’ve been auditioning for a mermaid role.

Interestingly, this copper-binding trick is the same property that allows forensic scientists to analyze hair and detect exposure to metals like zinc, lead, or mercury. Our hair becomes a chemical archive of what it’s been soaking in.

Fighting Back Against Pool Hair

So, what’s a swimmer to do? Some people might even joke that the fix is just to coat your pool with poured rubber. In reality, the low-tech fix is simple: wear a swim cap. A chemical workaround is to adjust your hair’s pH. Washing with an alkaline shampoo or even adding a sprinkle of baking soda to your regular shampoo can block copper ions from binding.

If your hair is already green, chelating shampoos with EDTA can pull the copper back out. “These products contain a chemical called EDTA—it can bind to metal ions (such as copper) and thus will remove copper from the hair,” according to Magdalena Wajrak, a chemist at Edith Cowan University.

And what about the internet-famous ketchup remedy? The idea is that red pigments cancel out green tones. It’s creative, but scientists note there’s no real evidence it works.

The next time someone insists chlorine alone is the villain, you’ll know better. The truth is stranger: we willingly pour metal salts into our water to keep it safe, and then those salts sneak into our bodies’ most visible fibers. It’s a reminder that even everyday chemistry can surprise us—whether in a research lab, or while trying to explain why your summer selfies look like you’ve been swimming in radioactive goo.