Moving to a different city can change how hair behaves without any change of products. The variable is usually the water, and the mechanism is straightforward.
What hardness actually means
Hard water contains dissolved calcium and magnesium picked up as groundwater passes through certain rock types. The concentration varies enormously by region.
These minerals are invisible in the water itself and tasteless at typical levels. Their effects appear only when the water meets soap or is left to evaporate.
The same minerals cause limescale in kettles and the film on shower screens, which is why local hardness is easy to judge without testing.
Minerals react with cleansing agents
Traditional soap reacts with calcium and magnesium to form an insoluble compound, the residue known as soap scum. It does not rinse away cleanly.
On hair this residue coats the strand unevenly. The result is a dull, coated feeling that shampooing more frequently tends to worsen rather than resolve.
Modern shampoos use synthetic detergents that are far less reactive, which reduces the problem substantially but does not remove it entirely.
Deposits build up over time
Minerals also bind directly to hair, which carries a slight negative charge along its surface that attracts positively charged metal ions.
This accumulates with every wash. A single shower makes no perceptible difference, while months of washing produce a noticeable change in weight and texture.
Damaged and porous hair binds more, since a lifted cuticle presents more surface area and more attachment points than a smooth one.
Why colour-treated hair suffers most
Chemical processing lifts the cuticle and leaves the strand more porous, so mineral uptake is faster on hair that has been coloured or lightened.
Certain metals in water can also interact with dye molecules and with the peroxide used in lightening, which affects how tone develops and holds.
This is one reason a colour result achieved in a salon can look different after a few weeks of washing at home.
What actually removes the build-up
Chelating agents bind metal ions and hold them in solution so they rinse away. Clarifying shampoos containing them are the direct answer to mineral deposits.
Acidic rinses help modestly by dissolving some deposits and by smoothing the cuticle, though they do not remove bound minerals as effectively.
A shower filter reduces the input rather than treating the result, which is the more thorough approach where hardness is high and permanent.