(CNN) A new survival challenge faces the Thai boys and their soccer coach: Oxygen levels in the cave air have plummeted dangerously to just 15%, Thai Navy Seal chief Rear Adm. Aphakorn Yoo-kongkaew said Friday.
He did not speculate how long the team could survive at that oxygen level in the cave at the Tham Luang Nang Non complex, where they have been trapped for 14 days.
According to the US Department of Labor's Occupational Safety and Health Administration, the "optimal range" of oxygen needed in the air a person breathes in order to maintain normal function is between 19.5% and 23.5%.
"We originally thought the young boys could stay safe inside the cave for quite a long time but circumstances have changed," Yoo-kongkaew said, adding that getting more oxygen to the boys and their coach had become a priority.
As oxygen drops below the lower threshold of the optimal level, the body begins to undergo changes and may face the serious risk of hypoxia, the condition that causes altitude sickness, according to OSHA.
Dr. Norman H. Edelman, senior science adviser to the American Lung Association, said the boys' ability to survive "all depends on the mechanics of the ventilation and how big the air space is."
Because the boys and their coach have been trapped for weeks, Edelman "hypothesized" that some ventilation is taking place, though he cautioned that he did not have detailed knowledge of the situation.
"The first thing that happens to the body as oxygen levels decline is, you have a signal to make you breathe more. It's totally analogous to climbing a mountain and going up to, say, 10,000 to 12,000 feet when the oxygen pressure declines," Edelman said.
"So the first thing you feel is a need to breathe more; you may feel a little lightheaded; you may feel a little dizzy. They may have trouble sleeping; they may have headaches while they sleep," he said.
The other problem the boys may experience with declining oxygen is increasing carbon dioxide, the gas we exhale with each breath, Edelman said.
"When carbon dioxide goes up, that's a strong stimulus to breathe, so they'll begin to feel short of breath," he said. "They may be dizzy, they may be confused, but if it happens slowly enough, they will adapt. It's just like climbing a mountain. They will eventually adapt to the low oxygen, unless it goes too low, of course."
If the oxygen level gets severely low, "they will slip into a coma," he said. "if you're comatose, you're not going to last very long."
Breathing air containing just 6% to 10% oxygen causes nausea, vomiting, lethargic movements and perhaps unconsciousness, according to OSHA; at oxygen levels less than 6%, the body experiences immediate convulsions, followed by cessation of breathing and then cardiac standstill.
Aside from such dire conditions, Edelman said that individual people react differently to inadequate oxygen and that although it is assumed there are different reactions for children, the process has not been studied.
Still, his comments offer hope. "You can sustain life at low oxygen levels," he said, adding that even at moderately low levels, "you can go indefinitely."