The hardness of water is determined by the amount of salts (calcium carbonate [CaCO3] and magnesium sulphate [MgSO4]) present in water. If these salts are present in excess, the water is considered to be hard, and may damage hair.
Hard water sometimes can leave a white-ish buildup on pipes and appliances like dishwasher and washing machines, cause soap to lather less than it would soft water, and leave a dull buildup on clothes after they have been washed. While hard water is not considered to be a health hazard, it can be annoying.
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Because there is limited research on the effect of hard water on hair, no definitive conclusions can be made. Still, there is compelling evidence that long-term exposure to hard water may be damaging to hair.
For example, a recent 2018 study found that 10 minutes of exposure to hard water every other day over the course of three months significantly decreased the tensile strength of 70 tested hair samples. When compared to hair samples that were exposed to de-ionized water and hair samples that were not exposed to any water, the hairs that had been exposed to hard water were more likely to break when exposed to tension.
Other research has indicated that hard water might not always damage hair if the exposure is short-term and if the water is less hard. In particular, one 2013 study demonstrated that the tensile strength of 15 hair samples was not affected when the hair samples were exposed to hard water for only 30 days. Besides including a shorter period of exposure, the water used in this study was less hard than the water used in the 2018 study. As noted by the authors of the 2018 study, it is possible that water may need to reach a certain level of hardness to actually damage hair.
Have questions about how hard water might be affecting your hair? Get in touch with one of our formulation techs while taking our hair quiz, and we'll see what we can do about it together.