What are the atmospheric conditions (by percentages) that human's can breath on say an alien planet? I know that Earth's atmosphere contains 21% oxygen and 78% nitrogen and 1% other gaseous molecules. I would like to know if we could breath on a planet that has say 64% nitrogen 28% oxygen, 6% carbon dioxide and 2% water vapor and other gaseous molecules. And if so what would an increased amount of oxygen do to us as humans? (Science fiction question really).
What would the increased amount of carbon dioxide do to us as well?