Oxygen at 15 percent is not immediately fatal but produces measurable cognitive and physical effects: fatigue, impaired judgment, slower reaction times, reduced coordination. People living at high altitude experience similar conditions and their bodies adapt over time; the same process was occurring, involuntarily, for the team at Nern Nom Sao.
The cause of the oxygen depletion was the bacteria in the cave environment. Microorganisms in soil and cave sediment consume oxygen as part of normal metabolic processes. In a confined space with thirteen breathing humans, the combination of human respiration and microbial oxygen consumption was gradually reducing the available oxygen. The situation was not acute — it was not moving toward zero at a rate that would have been immediately life-threatening — but it was an additional physiological burden on people who were already malnourished and cold.
The rescue teams addressed this by pumping supplemental oxygen into the cave through a hose as part of the supply operations. This brought the oxygen level back toward normal and made the conditions inside the chamber more sustainable. The boys’ physical improvement in the days between discovery and extraction was partly attributable to better nutrition and better oxygen levels working together.
This detail is significant for the extraction decision. One of the factors weighing against delay was the continuing deterioration of conditions inside the cave, including the oxygen situation. Even with supplemental oxygen being pumped in, the cave environment was not safe for extended habitation. The window was not simply a matter of monsoon water levels — it was also a matter of how long the team’s physical condition could be maintained in the conditions they were living in.