By the time hypoxia is detected, it’s often too late, and the higher the cabin altitude; the less time pilots have to realize that they need to don oxygen masks.

Researchers have discovered a method that allows detection of hypoxia symptoms before the onset of “incapacitating physical symptoms,” according to the Mayo Clinic. In the study, published in the October issue of Aviation, Space, and Environmental Medicine, the team used the King-Devick (K-D) neurocognitive performance test on 25 participants “to assess cognitive function under conditions of low oxygen-simulating altitude.” The test involves reading aloud a series of numbers on three test cards.

“This study provides an objective indication of hypoxia that is involuntary, reliable and repeatable,” said Jan Stepanek, aerospace medicine program director and codirector of the Aerospace Medicine & Vestibular Research Laboratory. “This means that people can be tested for cognitive declines before having symptoms, because often people won’t have symptoms until it is too late.”

The study also noted, “The K-D test may also be used to afford a reassessment of traditional measures used to determine hypoxic reserve time.”