Rainwater is pure water, but when it falls into soil, lakes, and rivers, where the bedrock and surrounding environment consist of limestone and chalk deposits, these minerals filter into the water which becomes hard. So it is just in certain areas that have hard water.
The total water hardness is the sum of the molar concentrations of Calcium and Magnesium ions. It may be classified as Soft, Moderately hard, Hard and Very hard. Internationally it is measured in many different units, like degrees of General Hardness (dGH), German degreed (dH), parts per million (ppm, mg/l) etc...
In Sweden dH is used and internationally ppm can be used as a translator.
Which damage can hard water cause
WHO states, there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans.
Hard water forms deposits that clog plumbing. These deposits are called limescale. The resulting build-up of scale restricts the flow of water in pipes. In boilers, the deposits impair the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this overheating can lead to failure of the boiler.
Soap & detergent
With hard water, soap solutions form a white precipitate (soap scum) instead of producing lather. Hardness can thus be defined as the soap-consuming capacity, which means that it demands more soap and detergents to the same job.
The mineral water we drink is hard water. The rain has poured through different deposits in the ground which adds tasty minerals to the water.
The mineral composition of hard water gives it a ton of health benefits such as protecting your heart and bones, if you drink sufficiently.
What salt products should I have?
Salt tablets or Granular salt is 100% pure sodium chloride and does not give any residue in your softener.
See the following links to the product pages: