While we are onboard airline flights of almost any distance--from puddle jumpers to transcontinental excursions--our oxygen blood level decreases significantly, according to new research from Hospital Sao Joao in Porto, Portugal. On long-distance flights, blood oxygen levels drop on average 10 percent, while on short-distance jaunts they fall on average 3 percent. The type of aircraft is not a factor.

"These changes seem to be well-tolerated in healthy individuals but may be dangerous for patients with respiratory disease," researcher Dr. Joao Carlos Winck told the European Respiratory Society 12th Annual Congress. Reuters reports that the team used continuous pulse oxymetry analysis in an evaluation of 20 healthy individuals on a series of transatlantic and short-distance European commercial flights. Oddly, the dip in blood oxygen levels varied in different parts of the aircraft; the lowest levels were recorded in the bathrooms. "Real flight effects on oxygenation need further clarifications," Winck said. "Similar studies are needed to assure the effects of real flights in patients with respiratory failure."

Lack of oxygen in varying altitudes above sea level becomes rather critical above 10,000 feet for all but the mountain goats. But, experienced mountaineers have climbed most of Mt. Everest without supplemental oxygen.
My doctor commented that he has to take our altitude in account (4,500 feet) when monitoring blood/oxygen levels in patients.
I get light-headed when DRIVING up Trail Ridge Road in Colorado's Rocky Mountain National Park.(12,000 feet)
My wife says that I'm light-headed at any altitude.