Caution: SF6 is described by Airgas, a company that sells it, as a substance that may cause damage to the upper respiratory tract. Not medical advice: The gas that makes the human voice sound much deeper is sulfur hexafluoride. It is a non-toxic, invisible gas with the chemical formula SF6. It is often considered the anti-helium gas because helium is about six times lighter than air and sulfur hexafluoride is about six times heavier. It also makes the human voice deeper while helium makes it higher. Those with a penchant for experimenting can use sulfur hexafluoride to float a boat.. To begin, pour sulfur hexafluoride into an aquarium or large beaker.