The term "fresh air" signifies clean and pure air, but there are various antonyms for this phrase that denote unclean, contaminated, or stale air. The first antonym could be polluted air that is found in industrial areas or cities. This type of air is full of harmful chemicals and can cause respiratory problems. Another opposite meaning of fresh air could be foul or musty air found in damp and congested areas like basements or storage rooms. The third antonym for fresh air would be stagnant air, commonly found in enclosed places with poor ventilation. This type of air is unhealthy and can cause headaches, nausea, or feelings of fatigue.