Do you spent a lot of time flying unpressurized aircraft at high altitudes? If so, it's important to monitor your oxygen levels. Alex Gertsen, NBAA's director of airports and ground infrastructure, is also a pilot, and said he uses supplemental oxygen in flight and counts on a pulse oximeter to measure his oxygen levels. "It's your health, your life on the line when you're at those high-altitude levels," Gertsen said. Dr. Brent Blue, a senior aviation medical examiner, agreed, "It's not about the altitude, it's about how you feel. You can't use too much oxygen."