What water hardness affects at home
Hardness is not a health risk, but it changes how water behaves in appliances, with soap and on skin. Each point below names its source and how strong the evidence is.
- strong evidence A 336-child randomised trial in England (homes >= 200 mg/L CaCO3) found ion-exchange water softeners gave no additional benefit over usual eczema care at 12 weeks (SASSAD difference 0.66, 95% CI -1.37 to 2.69). (Source)
- strong evidence Hard water causes scaling in hot water systems, kettles, electric irons and domestic appliances; scale is produced from about 200 mg/L as CaCO3. (Source)
- strong evidence Scale on heat-exchange surfaces reduces heat-exchanger efficiency (qualitative; WHO gives no percentage). (Source)
- strong evidence Do not make up infant formula with artificially softened water; softeners can add significantly to sodium and may breach the sodium standard. Keep an unsoftened kitchen tap. (Source)
- moderate evidence In 80 adults washing with sodium lauryl sulfate, hard water left more surfactant on skin, increasing transepidermal water loss and irritation, especially with FLG mutations; ion-exchange softening reduced the effect. (Source)
- moderate evidence Manufacturers and WHO agree hard water raises soap/detergent consumption; Bosch UK: more detergent in hard water, less in soft (35 ml liquid for normal soil in soft-medium water in a front loader). (Source)
- moderate evidence Dishwasher makers state that hard water leaves limescale deposits on dishes and in the machine (Miele) and that too much detergent in soft water can etch glassware (Frigidaire), which is why softener salt and dose-by-hardness are specified. (Source)
- moderate evidence Soft water (about 100 mg/L CaCO3 or less) is more corrosive to pipework; unstabilised soft water has a greater corrosion tendency, and excessively hard water can also corrode. (Source)
- moderate evidence Miele states mains water above 5 °dH (0.9 mmol/l) must be softened and the salt indicator can be switched off below that; Beko DIN15210 says above 6 °dH must be softened and no salt needed below 8 °dF. (Source)
- weak or mixed evidence Pooled observational data (7 studies, 385,901 children) show higher odds of eczema in harder-water areas (OR 1.28, 95% CI 1.09-1.50) but certainty is very low; two RCTs show no benefit of softeners on established eczema (GRADE moderate). (Source)
- weak or mixed evidence In a DOE-commissioned test of 12 water heaters, the electric unit with heavy element scaling lost 8% energy factor versus about 4% for a comparably aged unit; authors say causation was not confirmed and recommend further study (single-unit observation). (Source)
- weak or mixed evidence Ecological studies often show an inverse link between water hardness (magnesium) and cardiovascular mortality, but WHO says the evidence does not prove causality and proposes no guideline value for hardness minerals. (Source)
Dishwasher water hardness settings by brand → · Water hardness, boilers and water heaters →