After hyperventilating for sometime, holding the breath is dangerous, since:

In respiratoy alkalosis, there is an increase in extracellular fluid pH and a decrease in H+ concentration. The cause of the alkalosis is a decrease in plasma pCO2, caused by hyperventilation. The reduction in pCO2, then leads to a decrease in the rate of H+ secretion by the renal tubules. The decrease in H+ secretion reduces the amount of H+ in the renal tubular fluid. Consequently, there is not enough H+ to react with all the HCO3- that is filtered. Therefore, the HCO3- that cannot react with H+ is not reabsorbed and is exreted in the urine. This results in a decrease in plasma HCO3- concentration and correction of the alkalosis. Therefore, the compensatory response to a primary reduction in pCO2 in respiratory alkalosis is a reduction in plasma HCO3- concentration, caused by increased renal exretion of HCO3-  Changes in plasma hydrogen ion concentration can influence the degree of calcium binding to plasma proteins. With acidosis, less calcium is bound to the plasma proteins. Conversely, in alkalosis, a greater amount of calcium is bound to the plasma proteins. Therefore, patients with alkalosis are more susceptible to hypocalcemic tetany.