Humans breath Air which is a mixture of about 78% Nitrogen, 21% Oxygen and trace other gases including 0.03% Carbon Dioxide. Much higher levels of Oxygen would make it easy for things to catch fire. More than 2.0% or 3.0% Carbon Dioxide causes headaches and higher levels can cause death. Plants can tolerate a higher level because they use the Carbon Dioxide. and water to make sugars and cells.