Food is something you partake of at specific moments in time, and prayers setting apart this time are therefore appropriate. Air is something you breathe constantly, which would make 'air prayers' at specific times kinda weird. We pray repeatedly throughout the day different prayers asking for heath, mercy, life, etc. I think 'air' would come under that category. There is of course the Apostle's command to 'pray unceasingly' and to remember God 'more often that you breathe', which is why we are all encouraged to practice the Jesus Prayer.