I am not a meteorologist but I believe that it would be more accurate to say that there is less air pressure up there at 10,000 ft.
I found this:-
The atmosphere extends above the Earth's surface 100,000 ft, and generally remains consistent, with a 78% nitrogen/21% oxygen mixture to about 70,000 ft. Above that, it becomes more a mixture of helium and other gases. This being the case, and because the highest elevation we find on Earth is about 30,000 ft, we can't really say that oxygen decreases with height.
However, what does decrease with height is air pressure. At sea level, the pressure of the atmosphere is 14.7 pounds per square inch (psi). At 18,000 feet the pressure is 7.34 psi. By 34,000 feet, the pressure is reduced to one-half the value at the 18,000 foot level (3.62 psi). It is this reduction in pressure (or in other words, the less dense air) that causes hypoxia, a condition during which the pressure in the blood is not sufficient enough to deliver oxygen to the brain.
The barometric pressure at 10,000 ft is ~670 mb. Meteorologists often use the following scale when describing specific altitudes. For instance, they will say, "The freezing level is at 850 mb."
1000 mb ~ 360 feet (110 m)
850 mb ~ 5000 feet (1500 m)
700 mb ~ 10,000 feet (3000 m)
500 mb ~ 18,000 feet (5400 m)
250 mb ~ 34,000 feet (10,200 m)
E & OE
best
nio