During what body condition does shallow water blackout most likely occur?

Shallow water blackout is primarily associated with excessive hyperventilation prior to a dive. This condition occurs when a diver hyperventilates to lower carbon dioxide levels in the blood, which can create a misleading sense of security about their oxygen levels. By intentionally lowering the carbon dioxide levels, the diver can extend the duration of breath-holding. However, this leads to an increased risk of blacking out due to hypoxia, as the body’s natural urge to breathe diminishes when carbon dioxide levels are artificially suppressed. In this state, a diver can lose consciousness without experiencing the usual warning signs, which is particularly dangerous in shallow waters where they might descend unexpectedly.

The other conditions mentioned may impact a diver's overall safety but do not specifically lead to shallow water blackout in the same manner. Intense workouts can cause fatigue, prolonged deep breathing might be a calming technique but doesn't lead to the same risk, and calm, controlled breathing ensures that oxygen and carbon dioxide levels stay balanced, reducing the likelihood of a blackout.