What are the effects of hard water on hair?

Tap water naturally contains minerals, including calcium and magnesium. When their concentrations are high, it is termed hard water or limescale-rich water. This limescale originates from the dissolution of the limestone rocks traversed by the water during its natural cycle. Water hardness varies significantly between regions and is measured in French degrees (°f). The higher the concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, the harder the water is said to be. While this mineral content is not problematic for health, it can interact with the surfactants in cleansing products and leave deposits on the skin or the hair fibre. It is in this context that the question of the link between limescale-rich water and hair arises, particularly when one observes changes in texture or shine after washing.

While it is often said that hard water damages hair, the underlying mechanisms deserve clarification. During washing, calcium and magnesium ions present in hard water can interact with the surfactants in shampoos and gradually deposit on the surface of the hair fibre. These mineral residues then alter the hair's electrical charge, affect its surface and can disrupt the cohesion of the cuticle scales. Over time, this accumulation is likely to increase roughness and reduce shine, hence the common belief that hard water renders dry hair.

To better understand the tangible impact of hard water on hair fibres, an experimental study compared hair strands washed for 30 days with either hard water or distilled water. Fifteen volunteers provided hair samples, which were divided into two batches: one exposed to water containing 212.5 ppm of CaCO₃, the other to lightly mineralised water (10 ppm).

Scanning electron microscopy analyses revealed that hair washed in hard water exhibited a more irregular, "rough" surface with markedly higher mineral deposits.

The mean calcium content was 0.804% in hair exposed to hard water compared to 0.26% in hair washed with distilled water, while the magnesium concentration was 0.34% versus 0.078%. Notably, the average fibre thickness following hard water treatment was lower than that of hair washed with distilled water, suggesting a possible structural alteration.

These results therefore support the idea that hard water and hair are not compatible, particularly because of the gradual build-up of mineral deposits on the fibre surface.

Building on these observations, another study turned its attention not to surface appearance but to the mechanical strength of hair exposed to hard water. Seventy-three volunteers living in an area with low water hardness provided hair samples, each split into two: one half was exposed for three months to particularly hard water, the other to deionised water. The researchers then measured tensile strength. The results show a significant decrease in the tensile strength of hair treated with hard water. In other words, repeated exposure to highly mineralised water appears to weaken the hair fibre and encourage breakage—without directly inducing hair loss.

Finally, a third study confirmed these findings by simultaneously comparing untreated hair, hair exposed to deionised water and hair subjected to hard water. 70 participants provided hair samples, which were divided into three groups (a control group, a group treated with deionised water and a group exposed to hard water), whilst tensile strength was evaluated. The results indicate that the average tensile strength of hair exposed to hard water was significantly lower than that of both the control and deionised water groups, whereas no difference was observed between the control and deionised water groups.

These data therefore reinforce the idea that repeated exposure to mineral-rich water can weaken the hair fibre and increase the risk of breakage, even though it does not necessarily mean hair loss in the strict sense due to hard water.

Beyond the hair fibre, hard water can also interact with the scalp. Studies on the epidermis suggest that repeated exposure to calcium-rich water can disrupt physiological calcium signalling, essential for proper keratinocyte renewal. In the skin, calcium indeed follows a precise gradient between the deeper and superficial layers. A disturbance of this balance could impair cell cohesion and weaken the skin barrier. When applied to the scalp, this mechanism could promote irregular flaking, heightened sensitivity and, in some individuals, scalp itching caused by hard water. Moreover, by reducing soap solubility, hard water encourages the formation of insoluble residues that may remain on the scalp surface, locally altering the pH and disrupting its comfort.

Regarding the dandruff in hair caused by hard water, some researchers suggest a possible indirect link: by weakening the skin barrier and leaving mineral deposits, hard water could create conditions favouring more visible flaking. However, it is important to recall that dandruff is primarily linked to the proliferation of yeasts of the genus Malassezia and individual factors, rather than to limescale alone. Similarly, some people mention oily hair due to hard water, implying that it stimulates the production of sebum. Nevertheless, to date, no scientific data indicate that hard water has an effect on the sebaceous glands. The sensation of heavier hair could instead be explained by the accumulation of mineral deposits that alter the tactile texture, without actually greasing the scalp.

There are straightforward solutions to protect your hair from limescale and limit the build-up of mineral deposits. The objective is twofold: to reduce contact with calcium and magnesium ions, and to counteract their effects on the hair fibre and scalp. Here are the main strategies you can adopt:

• Install a limescale filter on the shower : These devices lower the concentration of minerals during rinsing and offer a long-term solution against limescale.

• Use an anti-limescale shampoo or a hard-water shampoo occasionally : These formulations often contain chelating agents capable of capturing calcium and magnesium ions to remove limescale from hair.

• Carry out a mild acidic rinse (for example with a diluted apple cider vinegar solution) : This traditional method provides an effective treatment for limescale deposits on hair, helping to close the cuticles and reduce surface roughness.

• Apply an anti-limescale hair care treatment regularly (for example, a nourishing mask) : This type of product strengthens the hair fibre’s lipid barrier and compensates for the loss of softness when hard water renders hair dry.

• Adapting the routine for textured hair : The effects of hard water on curly or coily hair can be more pronounced, as these hair types are naturally drier. Regularly hydrating and protecting the hair fibres with masks and serums can therefore be particularly helpful.

In practice, we recommend combining prevention (filtration, appropriate rinsing) with repair (moisturising and protective treatments) to limit the effects of limescale on your hair.

• RANGACHARI S. C. & al. Effects of hard water on hair. International Journal of Trichology (2013).

• RANGACHARI S. C. & al. Scanning electron microscopy of hair treated in hard water. International Journal of Dermatology (2015).

• JAVAID U. & al. Effect of topical application of hard water in weakening of hair in men. Journal of the Pakistan Medical Association (2016).

• AYYAS M. & al. To evaluate and compare changes in baseline strength of hairs after treating them with deionized water and hard water and its role in hair breakage. International Journal of Trichology (2018).

• CORK M. & al. The effect of water hardness on surfactant deposition after washing and subsequent skin irritation in atopic dermatitis patients and healthy control subjects. Journal of Investigative Dermatology (2018).