Hardness is the amount of dissolved minerals, mainly calcium and magnesium, in water. It's not a health risk but can cause scale buildup in pipes and appliances.

Total hardness, measured as calcium carbonate (CaCO3), is a natural characteristic of water that develops as it moves through soil and rock. It primarily comes from limestone and gypsum deposits. While not harmful to health, hard water can lead to mineral buildup in plumbing, reduced soap efficiency, and dry skin or hair. Water softeners are often used to reduce hardness. The concept of water hardness dates back to the 19th century when soap use became widespread, as hard water interferes with soap lathering.