See details from WHO at https://www.who.int/water_sanitation_health/dwq/chemicals/hardness.pdf and https://www.who.int/water_sanitation_health/water-quality/guidelines/chemicals/hardness/en/ with the full guidelines at https://www.who.int/water_sanitation_health/publications/drinking-water-quality-guidelines-4-including-1st-addendum/en/