Hardness in Drinking-water by World Health Organization
= Hardness in Drinking-water by World Health Organization
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Institutional chemical fact sheet. Cited passage: Chemical Fact Sheets, hardness, printed p. 377, naturally low mineral content of rainwater and the soap-reaction description. Used to corroborate chemistry and qualify the historical generalization, not to supply a numerical health threshold.
https://www.who.int/docs/default-source/wash-documents/wash-chemicals/hardness-chemical-fact-sheet.pdf