Actually, out-gassing inside a building tends to travel upward. It's not unusual for the higher floors to have higher levels of gases than the lower levels. For

Actually, out-gassing inside a building tends to travel upward. It's not unusual for the higher floors to have higher levels of gases than the lower levels. For gases to sink, they have to be heavier than air. Radon gas for example is 7.5 times heavier than air. Carbon monoxide is slightly lighter than air which is why detectors for it are better placed higher up in a room than on the floor.