Oxygen and the Gases That Hurt You: Hypoxia, Hyperventilation and CO

Hypoxia is simply not enough oxygen reaching the body, and it is dangerous precisely because its early symptoms — euphoria, poor judgment, tingling, and bluish fingernails and lips (cyanosis) — are subtle and can go unnoticed. The most common form in flight is hypoxic hypoxia from reduced pressure at altitude, but there are others: hypemic (the blood cannot carry oxygen, as in carbon monoxide poisoning), stagnant (poor circulation), and histotoxic (cells cannot use oxygen, as with alcohol). Hyperventilation is a close cousin in symptoms but the opposite cause: breathing too fast under stress blows off carbon dioxide, producing lightheadedness and tingling. Because the symptoms mimic hypoxia, the safe rule is to slow the breathing but assume hypoxia and use oxygen if any doubt remains. Carbon monoxide is the silent third threat — a colorless, odorless exhaust gas that binds to hemoglobin far more eagerly than oxygen and usually leaks in through a faulty exhaust or cabin heater. Headache and drowsiness are warning signs; the response is to shut off the heater, open the fresh-air vents, and use oxygen. The supplemental-oxygen rule (14 CFR 91.211) is tested with exact numbers keyed to CABIN PRESSURE altitude: above 12,500 feet MSL up to and including 14,000 feet, the required flight crew must use oxygen for any portion of the flight at those altitudes longer than 30 minutes; above 14,000 feet the required crew must use it for the entire time at those altitudes; and above 15,000 feet every occupant must be PROVIDED with oxygen. Keep the trio straight — 12,500/30 minutes, 14,000/continuous for crew, 15,000/provided to passengers — and remember the passenger rule is provide, not use.