Anyone with blonde and frequent hair usually has probably noticed, at least once, an unpleasant greenish shade in their hair after the bathroom. For years, chlorine has been considered the culprit number one. But the truth is more surprising: it is not chlorine that dyes green hair, but a well -known metal, copper.
Already in the 70s, researchers began to study what green coloring really caused the hair after frequent swimming. A 1978 study analyzed the effects of water containing chlorine, copper, and a combination of both on different hair samples. The verdict was clear: copper is the real culprit.
When the hair comes into contact with copper ions – small particles full of this metal – they acquire a green color. Chlorine alone is not enough to cause this reaction. However, it contributes indirectly: by weakening the structure of the hair, it promotes the link between the copper ions and the capillary fibers, amplifying the phenomenon.
Where does the copper come from in the swimming pools? The answer is twofold. First of all, copper is intentionally added in the form of copper sulphate, used to prevent algae growth. Just 0.5 mg per liter (equal to 0.5 parts per million) are enough to obtain this effect. Secondly, copper can infiltrate the water through corrods or old systems, leading to concentrations even higher than expected.
The copper sulphate, with its characteristic green-blue color, binds to the hair, especially if these are already chemically treated or damaged by the sun. This promotes the appearance of the typical “algae” green shade.
The link between the molecular structure of the hair and metal ions such as copper
To fully understand this phenomenon, it is useful to take a look at the hair chemistry.
The hair is mainly composed of keratin, a very resistant protein. Inside the keratin there are several chemical groups – such as carboxylic, amino and dysolphurous bridges – to which metal ions can easily bind.
An important 2020 study has shown that copper tends to preferably bind to the bonds disolfuro broken, a very common condition in bleached hair, treated with warm or chemically stressed. That’s why people with natural blonde hair can observe only a slight greenish reflection, while those who have decolved hair often come out of the pool with a hair that seems to have come out of a fantasy movie.
Curiously, precisely this copper ability to bind to keratin is also exploited in the forensic field, to analyze exposure to heavy metals – such as zinc, lead or mercury – through hair. In practice, our hair record and retain the chemicals with which they come into contact.
How to prevent (or solve) the green hair problem after the swimming pool
There are several solutions to prevent blond hair from being transformed into a little mermaid nuance. The simplest remedy is also the oldest: wearing a swimming cap. If you prefer to leave your hair free, you can intervene by adjusting the hair pH.
Use an alkaline shampoo or add a pinch of sodium bicarbonate to your usual shampoo can help block the adhesion of copper ions.
If the hair is already green, the best solution is a chelating shampoo, containing a substance called EDTA. This molecule is able to bind to metal ions such as copper and effectively remove them from the hair. Magdalena Wajrak, chemistry of Edith Cowan University, also confirms this, which explains how the EDTA is one of the most effective substances in counteracting the accumulation of metals on the hair.
And the home remedy with ketchup, so widespread online? It is based on the idea that red pigments neutralize the green. Even if the approach is creative, there is no scientific evidence of its effectiveness. More than a cure, it is a do-it-yourself attempt without guarantees.
The next time someone will attribute to chlorine the fault of green hair, you will know that the truth is much more subtle (and metallic). The reality is that we voluntarily introduce metal salts into the water to keep it clean, and these end up binding to our hair. A perfect example of how daily chemistry manages to surprise us even outside the laboratory – maybe just as we take a selfie in the pool with an unusual fluorescent green color.
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