Water Hardness
Cities measured
87
Detected in
87 / 87
Highest
34.8 °dH — Rome
Overview
Hardness measures dissolved calcium and magnesium. Stockholm water is soft (4–8 °dH), reflecting its granitic bedrock source. Hard water leaves scale in kettles and pipes; soft water is more corrosive to metal infrastructure.
Health Relevance
Hard water is associated with cardiovascular protection in epidemiological studies — higher magnesium via drinking water correlates with reduced cardiac mortality. Soft water is more corrosive to lead and copper pipes, potentially increasing metal ingestion.
Regulatory Limits
Drinking Water Directive 2020/2184
No regulatory limit (indicator parameter only).
Controversy & Contested Science
The WHO 2006 report found 'suggestive but inconclusive evidence' linking hard water to lower cardiovascular mortality. Ion-exchange water softeners replace Ca²⁺/Mg²⁺ with Na⁺ — raising sodium intake by up to 400 mg/day for a household drinking 2L of softened water at 200 mg/L sodium. Cardiologists raise concern; softener manufacturers argue most users don't drink softened water. Several EU countries require bypassing softeners for drinking taps. The infrastructure trade-off — softer water extends appliance life but worsens cardiovascular risk — remains an unresolved regulatory grey area.
Note
The term 'hard' water came from the observation that soap produces a 'hard' scum (calcium stearate) rather than lather in mineral-rich water.