Question: Why can we breath in the world's atmosphere but not in the atmosphere outside of the earth?

Iain Bethune answered on 23 Jun 2015:

To be able to breath, we need an atmosphere that is at the right pressure (or concentration), and has the right chemicals in it – in our case we need oxygen for our bodies to function. If we climb to the top of very high mountains like Mount Everest, the atmospheric pressure drops low enough that it is hard to breathe. If we go even higher (into space), then the pressure drops essentially to zero, and there is no air for us to breathe. Other planets like Venus have an atmosphere, but it is 97% carbon dioxide, so it is no use to us (plus it’s 460C at the surface – far too hot for us to live!)…