The common symptoms of hypoxia include increased breathing rate, dizziness, headache, sweating, reduced peripheral vision, and fatigue, but the most insidious symptom is a feeling of euphoria. Pilots suffering from hypoxia often experience a false sense of security rather than a sense of the danger inherent to this condition.
Hypoxia also impairs night vision. Because the rod cells in the eye, which give us night vision, require a lot of oxygen, a lack of oxygen causes visual impairment.
For pilots, hypoxia's adverse effects are described in terms of time of useful consciousness (TUC) and effective performance time (EPT).
TUC is a measure of your ability to function in a meaningful way. In other words, it's a kind of threshold on the pathway to becoming, first, something like a drooling fool, and second, unconscious and certifiably out of it.
EPT is defined as the time from the loss of significant oxygen to the time when you are no longer able to perform tasks in a safe and efficient manner. This is a dangerous condition, because hypoxia's onset is subtle. Pilots may think they're doing just fine — and in fact, may well have things under control — even though their EPT is dwindling away, and the countdown clock to unconsciousness is surely running. This false sense of well-being is, in itself, a symptom of hypoxia. But usually, at this point the pilot doesn't care.
People are not the same. Even though we've just been talking in terms of EPT and TUC guidelines, it's time for a reminder: Not all pilots have the same EPT or TUC. If you're a smoker, under a great deal of stress, or don’t exercise regularly to increase your heart rate, your EPT and TUC will be considerably shorter than the published guidelines. A pack-a-day cigarette smoker is physiologically hypoxic at sea level. The smoker's lungs are so damaged that they're incapable of absorbing as much oxygen as those of a nonsmoker, so at sea level, the smoker's blood-oxygen concentrations are already at the 7,000-foot level.
For this reason, smokers and those with more sedentary lifestyles lose consciousness faster at altitude than the smoke-free and fit, and they should use begin using oxygen at altitudes lower than required by the regulations. Other day-to-day factors such as nutrition, alcohol use, and quality and amount of sleep can also affect your oxygen requirements. There's even evidence that poor air quality can lower your blood oxygen saturation level. Maybe that's why "oxygen bars" are seen in high pollution metropolitan areas like Los Angeles, Mexico City, Tokyo.
Doctors and hospital staff want to see your blood oxygen saturation level at 96 to 98 percent. That's considered normal. You can measure O2 saturation with a relatively inexpensive pulse oximeter that clips over your finger tip. A 100-percent level is as good as it gets, and 95 percent is considered a minimum. An oxygen saturation level below 90 percent is a warning sign. That's when patients — and pilots — begin to experience hypoxia.