What element makes your voice deeper. The opposite of helium.

Sulfur hexafluoride (SF6) is an inert gas which is six times heavier than the air we breathe, and helium is six times lighter, so sulfur hexafluoride has the opposite effect as helium in being able to make your voice deeper.

Voice pitch decreases tremendously when inhaling SF6 because the speed of sound in SF6 is much less than in air. Sound travels through SF6 at only around 120 m/s, whereas it travels through air at around 340 m/s. Sound travels through helium at around 900 m/s.

SF6 is a non-poisonous gas, but should only be inhaled in small quantities (if inhaled at all). It is suggested that one have a tank of pure oxygen gas on hand before demonstrating SF6 inhalation and breathe pure oxygen for about ten breaths before inhaling SF6. Persons should not repeatedly inhale SF6 over a short period of time and should beware of fainting or hypoxia.

It is not recommended that demonstration of sulfur hexafluoride inhalation be done by anyone without professional scientific experience (i.e., those who are not science teachers, medical professionals, etc.).

In a press release published on April 16th, 2012, the Environmental Protection Agency (EPA) reported that overall emissions of the six main greenhouse gases in the U.S. - one of the six of which is sulfur hexafluoride - grew by over 10 percent in the period from 1990 to 2010.