"Be the air" is an idiom that refers to the act of being present or prevalent in a particular environment or situation, much like the air we breathe. Antonyms for this expression might include "absent," "invisible," or "unnoticed." Alternatively, one might use the opposites of the qualities associated with air itself, such as "heavy," "opaque," or "stagnant." Depending on the context in which "be the air" is being used, other suitable antonyms could include words like "disappear," "vanish," or "flee." Ultimately, the choice of antonym will depend on the specific meaning and tone of the original phrase.