Sulfate is a common divalent anion found in a number of minerals in the environment. Sulfate salts of sodium ion, potassium ion, or magnesium ion are soluble in water, whereas salts of calcium ion, barium ion and other heavy metals are poorly soluble or insoluble. Humans are exposed to sulfate in drinking water, ambient air, and food. Drinking water is the predominant source of sulfate exposure. The levels of sulfate in drinking water range from 0 to 1,100 mg/L. Concentrations of sulfate in ambient air range from 0.5 to 228.4 micro grams/cu m. The U.S. Army has established a standard for military personnel of 100 or 300 mg sulfate/L based upon anticipated water consumption by military personnel of either 15 liters per day (in arid climates) or 5 liters per day (under less strenuous conditions), respectively. The World Health Organization has suggested an upper limit of 400 mg sulfate/L in drinking water.