Hard water, characterised by a higher concentration of dissolved minerals like calcium and magnesium, can leave deposits on hair, according to Desmond Tobin, professor of dermatological science at University College Dublin. These minerals are naturally present in water that has passed through soil and rocks, similar to the limescale found in kettles.
While some believe these mineral deposits can make hair appear duller and feel rougher, the evidence directly linking them to damage is less clear. However, removing this buildup can pose problems, particularly for individuals with dry or brittle hair.
Professor Tobin notes that shampoos designed to remove mineral buildup often contain ingredients like sulfates, which can be harsh on certain hair types. He describes this as a "balancing act between removing the buildup and leaving enough oil in the system to condition the hair."