Hyperventilation depletes the blood of carbon dioxide (hypocapnia), which causes respiratory alkylosis (increased pH), and causes a leftward shift in the oxygen–hemoglobin dissociation curve. This results in a lower venous partial pressure of oxygen, which worsens hypoxia. A normally ventilated breath-hold usually breaks (from CO2) with over 90% saturation which is far from hypoxia. Hypoxia produces a respiratory drive but not as strong as the hypercapnic respiratory drive. This has been studied in altitude medicine, where hypoxia occurs without hypercapnia due to the low ambient pressure. The balance between the hypercapnic and hypoxic respiratory drives has genetic variability and can be modified by hypoxic training. These variations imply that predictive risk cannot be reliably estimated, but pre-dive hyperventilation carries definite risks.