Calcium carbonate latter is especially evident in day-to-day life of people who have hard water. Water in aquifers underground can be exposed to levels of CO much higher than atmospheric. As such water percolates through calcium carbonate rock, the CaCO dissolves according to the second trend. When that same water then emerges from the tap, in time it comes into equilibrium with CO levels in the air by outgassing its excess CO. The calcium carbonate becomes less soluble as a result and the excess precipitates as lime scale. This same process is responsible for the formation of stalactites and stalagmites in limestone
Calcium supplement is rare, and difficult to achieve without administering calcium intravenously. For example, the oral median lethal dose (LD) for rats for calcium carbonate and calcium chloride are 6.45 and 1.4 g/kg, respectively. Calcium supplements by mouth diminish the absorption of thyroxine when taken within four to six hours of each other. Thus, people taking both calcium and thyroxine run the risk of inadequate thyroid hormone replacement and thence hypothyroidism if they take them simultaneously or near-simultaneously. The intravenous formulations of calcium include calcium chloride and calcium gluconate. The forms that are taken by mouth include calcium acetate, calcium carbonate, calcium