Shallow water blackout can occur when a diver is not using scuba equipment, that is, freediving or snorkeling. It is the opposite of hypercapnia. Excessive hyperventilation (more than five times of deep inhaling and exhaling) before breath-hold diving may lead to shallow water blackout. Three to four deep full breaths is enough for most people.
When a diver hyperventilates excessively before a breath-hold dive, carbon dioxide levels will become extremely low. Because the carbon dioxide level stimulates breathing (and not low oxygen levels), this stimulus will not be present even when oxygen levels become too low. At depth, the increased oxygen partial pressure in the lungs of the diver allows the respiratory system to receive oxygen. However, when the diver makes an ascent (or goes to shallow depths), the partial oxygen pressure in his lungs will drop. As a result, the lungs no longer get oxygen from the air, leading to blacking out and possibly drowning. If you feel light-headed, nauseated, or even a headache you should stop and rest. Get your breathing back to normal before deciding to continue.