The voice of a person who has inhaled helium temporarily sounds high-pitched, resembling those of the cartoon characters Alvin and the Chipmunks (although their voices were produced by shifting the pitch of normal voices). This is because the speed of sound in helium is nearly three times that in air. As a result, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. The higher perceived pitch is only due to a different frequency shaping of the voice, the fundamental frequency of the vocal cords remains more or less the same.
Although the vocal effect of inhaling helium may be amusing, it can be dangerous if done to excess. The reason is not due to toxicity or any property of helium but simply due to it displacing oxygen needed for normal respiration. One must be aware that in mammals (with the notable exception of seals) the breathing reflex is not triggered by insufficient oxygen but rather excess of carbon dioxide. Unconsciousness, brain damage and even asphyxiation followed by death may result in extreme cases. Also, if helium is inhaled directly from pressurized cylinders the high flow rate can fatally rupture lung tissue.
