Dr. John Graves, MD at the Mayo Clinic describes a recent case where a 78-year-old man with hypertension and diabetes suffered acute kidney failure and hyperkalemia (abnormally high blood level of potassium) shortly after starting to drink large quantities (3 to 4 liters/day) of water, which had been treated in a potassium-based water softener. Water softeners are used to soften hard water (water containing relatively large amounts of calcium and magnesium). Hard water is generally healthy to drink, but can cause difficulties in laundry and dishwashers. Water softeners work by exchanging the calcium and magnesium ions in the water with sodium or potassium ions. Depending on the hardness of the inflow water the treated water can end up containing significant amounts of sodium or potassium. Although softened water was not originally meant for human consumption many people now use water softeners to clean up their water and then use it for both drinking and cleaning purposes. One of the reasons why the patient was drinking this water was that he thought it would help his hypertension. Dr. Graves estimates that the man consumed about 1000 mg (21-28 millimol) of potassium a day from water alone. Dr. Graves concluded that this extra intake of potassium was a major cause of the renal failure and suggests that patients with kidney disease, diabetes or hypertension be warned against drinking water treated in potassium-based water softeners.
Graves, John W. Hyperkalemia due to a potassium-based water softener. New England Journal of Medicine, Vol. 339, December 10, 1998, pp. 1790-91 (letter to the editor)