Stale air is defined as recycled air combined with humidity and certain chemicals. It produces an unpleasant smell as contaminants build, and oxygen decreases. While stale air won’t immediately cause you harm, the air that contains certain toxins, chemicals, and bacteria can be harmful over a long time. When indoor air smells stale or feels stuffy, carbon-based molecules that occur from breathing, form together to produce the musty smell known as stale air.