Nowhere makes it harder to breathe (4,3)
I believe the answer is:
thin air
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for thin air that I've seen before include "impression of slimness" , "Nothingness into/from which things might magically (dis)appear" , "State of invisibility" , "Vanish into . . . . . . . - disappear completely" , "Vanish completely as into . . . . . . ., on high mountain?" .)