What is the opposite of helium?

A gas that has the opposite effect of helium on the human voice is xenon. Another gas that reverses the temporary high-pitched effect of inhaling helium is sulfur hexafluoride. Both gases are more dense than air and magnify deep tones in the voice.

Helium is the second-lightest chemical element and the second-most commonly found in the universe. It temporarily gives an odd, high-pitched quality to human voices because sound within helium moves at almost three times the speed of sound within air. When someone inhales helium, his vocal tract experiences a corresponding increase in frequency. The inhalation of excessive amounts of helium is dangerous. The gas crowds out the oxygen people need to breathe and can cause suffocation.