There is far more confidence about this online than there is evidence. The most-cited study washed hair in hard water for thirty days and found no meaningful difference in strength.
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Search this and you'll find a 100 confident answers. The evidence is much thinner than the confidence. The most cited study washed hair in hard water for thirty days and found no meaningful difference in strength. Minerals do build up, and hair can feel rougher and duller. That's a texture problem. Nobody has shown it takes hair out of your scalp.
Minerals do build up on the strand, and hair can feel rougher and look duller as a result. That is a texture problem, and a clarifying wash addresses it. Nobody has shown that hard water takes hair out of your scalp.