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.
It can lead to CO2 necrosis
Due to lack of stimulation by CO2 anoxia can go into dangerous levels
Decreased CO2 shift the oxygen dissociation curve to the left
Alkalosis can lead to tetany