The fatal secret behind inhaling helium is how quickly one may lose consciousness due to asphyxia. During the exchange of gases in the normal breathing process, the blood stream absorbs oxygen from air in the lungs, while carbon dioxide passes from the blood to the air. When you hold your breath, the exchange of gases slows, as “stale” air in the lungs is no longer replaced by “fresh” air. This process does not stop immediately, however. Time will pass before serious physical distress is experienced. For example, you would likely have time to pick up and put down an object, walk across a room, or find a chair and sit down before feeling compelled to breathe again.
However, when the lungs are filled with helium, a different process takes over. Oxygen is actually removed from the blood stream during the exchange of gases. Depending on how completely oxygen is replaced by helium, consciousness can be lost quickly and without warning—you may literally pass out while still standing. The usual result is an uncontrolled fall that can cause serious injury, even if normal breathing resumes.
Attempting to inhale helium from a commercial helium balloon filling system poses an even greater hazard than inhaling helium from a balloon. The pressure of gas entering the lungs is the agent that can kill instantly. The flow rate and pressure available from a helium balloon filling system can fatally over-pressurize human lungs in a fraction of a second.
For additional safety information on helium, or any other compressed gas, always refer to the product’s SDS.
Source: CGA (Compressed Gas Association)