Sources & corrections.

Core water-hardness references

• U.S. Geological Survey — Hardness of Water: definition, common classifications, regional context and limitations of national maps.
• Montana State University Extension — Hardness: hardness units, grains per gallon and water-quality interpretation.
• U.S. Environmental Protection Agency — Protect Your Home's Water: private-well testing considerations and examples of water-quality clues.
• USGS Water-Quality Information FAQs: context on water-hardness maps and the absence of a federal legal hardness limit.

Cleaning and appliance safety

• Centers for Disease Control and Prevention — Cleaning and Disinfecting with Bleach: do not mix bleach with other cleaners or disinfectants.
• Health Canada — Household Chemical Safety: safe use and storage of household cleaning products.
• For any particular tap, dishwasher, coffee maker, kettle or water heater, consult the current manual for the exact model. Product and surface instructions take precedence over general online tips.

Health evidence

For skin and eczema claims, we link to relevant published research and describe its limitations. For example, a systematic review on water hardness and atopic eczema reported uncertainty in observational findings and did not find significant improvement in objective eczema severity in two trials of household softeners. We do not present softeners as medical treatment.

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How sources are used

Article-level citations should support the nearby claim and should be read directly before publication. We aim to distinguish measured values from estimates, historical maps from current local data, and manufacturer claims from independent evidence. External sources can change, so important pages should be checked periodically.