An inert gas, or inert element, is a type of gas that does not react with other chemicals. Such elements in this group include helium, neon, argon, krypton, xenon, and radon.
The atmosphere of Talos IV was heavy in inert elements. However, it was within safety limits for Humans. (TOS: "The Cage", "The Menagerie, Part I")
In 2366, Data proposed that a force field containing xenon or another inert gas could be placed over an incision in Ishara Yar to remove her implanted proximity detector, which was rigged to explode when it came into contact with air. (TNG: "Legacy")