As others have noted, ...

As others have noted, hardness is directly linked to calcium and magnesium content of the water, but these do not in general impact on the safety of supplies, but rather on the potential for precipitation and scaling, particularly in hot water systems.  It is usually desirable to differentiate between temporary and permanent hardness, which are a function of the anions (CO3-, SO4) rather than the cations (Ca and Mg).  Temporary hardness causes scaling which can be more easily removed/dissolved than permanent hardness.