Xenon

Xenon is a chemical element (atomic number 54) which at normal temperatures and pressures is an odorless gas. It does not form compounds with other elements except in extreme conditions. It is heavier than air and occurs naturally in the atmosphere in trace amounts. It can be separated from other gases in the air by its molecular weight. 
In medicine, Xenon, in a mixture with 20% oxygen, can be used as an anesthetic although the expense makes it prohibitive. However, it is being studied as a viable option as new advances allow it to be recycled from patient to patient and it's non-reactive nature makes it non-toxic with fewer side effects. In this application, xenon also can protect the heart and nerves from damage during surgery. Radioactive xenon can be used in a PET scan and has the advantage of allowing the patient to inhale it rather than ingest it or have it injected. Hyperpolarized xexon can be used as a contrast in an MRI scan with fewer side effects than other contrast agents. Xenon lasers have wavelength frequencies that make them useful in dermatology procedures. 
Unfortunately, xenon can encourage the production of erythropoietin and athletes can abuse it for this purpose. The practice is now banned.
Xenon at Wikipedia