Furthermore, the inhalation of any gas that is capable of displacing oxygen in the lungs (especially gases heavier than oxygen) carries the risk of hypoxia as a result of the very mechanism by which breathing is triggered. Since reflexive breathing is prompted by elevated carbon dioxide levels (rather than diminished blood oxygen levels), breathing a concentrated, relatively inert gas (such as computer-duster tetrafluoroethane or nitrous oxide) that removes carbon dioxide from the blood without replacing it with oxygen will produce no outward signs of suffocation even when the brain is experiencing hypoxia. Once full symptoms of hypoxia appear, it may be too late to breathe without assistance, especially if the gas is heavy enough to lodge in the lungs for extended periods. Even completely inert gases, such as argon, can have this effect if oxygen is largely excluded.