Dave Asprey talks about breathing in low oxygen air in his new book Head Strong. It is supposedly very helpful.

Also mentioned is the Wim Hof breathing method, which is supposed to saturate your body with oxygen and increase PH levels.

Wim Hof exercises - Wim Hof, The Iceman - Innerfire

I tried the WH breathing. It can be dangerous, so a person has to be smart where and when they do it.