Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. Don't inhale or ingest helium, it may prove deadly if done to an excess. The speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract . The fundamental frequency (sometimes called musical pitch) does not change, since this is produced by direct vibration of the vocal folds, which is unchanged. However, the higher resonant frequencies cause a change in timbre , resulting in a reedy, duck-like vocal quality. The opposite effect, lowering resonant