We can breathe in the Troposphere.
The troposphere is the lowest layer of Earth's atmosphere, and it's where we live and where most weather occurs. This layer contains the air composition that supports human respiration. It holds the air that plants need for photosynthesis and animals, including humans, need to breathe. The troposphere is ideal for breathing as it contains the necessary concentration of oxygen and a tolerable mixture of other gases.
Essentially, it is the only atmosphere naturally suitable for human breathing. Other layers of the atmosphere have different compositions, temperatures, and pressures that are not conducive to human life without specialized equipment.
