Answer:
As altitude increases in Earth's atmosphere, both air pressure and the amount of oxygen decrease.
At sea level, the air pressure is about 1013.25 millibars (mb) and the concentration of oxygen is about 21%. As you go higher in the atmosphere, the air becomes thinner and less dense, so the air pressure decreases. At the same time, the concentration of oxygen also decreases because there is less air overall to contain oxygen molecules.
This is why it is harder to breathe at high altitudes, such as on a mountain. The air is less dense, so there are fewer oxygen molecules per breath and the body has to work harder to get enough oxygen.