From Glossary of Meteorology
Extremely unreactive gas, formula SF6, which is of entirely anthropogenic origin.
It is used as a dielectric insulator or to provide a chemically inert atmosphere. Due to its inertness, SF6 has a very long atmospheric lifetime (>1000 years) and has been used to estimate the age of stratospheric air. It is also used as a chemical tracer in tropospheric field experiments.