Higher than 400 km above the ground, where the International Space Station (ISS) is flying, there is almost no air. This state where no air exists is called a vacuum. Space is a vacuum.
Generally, altitudes of 100 km or above are considered outer space.
In higher elevations, the amount of air is significantly less. That is why the air becomes thinner as the higher you go up. Running on top of Mt. Fuji would leave you breathless.
Air that is closer to the ground is being squashed since the air supports the weight of the air above it.