  Waste gas products from technological civilizations may accumulate in an
exoplanet atmosphere to detectable levels. We propose nitrogen trifluoride
(NF3) and sulfur hexafluoride (SF6) as ideal technosignature gases. Earth life
avoids producing or using any N-F or S-F bond-containing molecules and makes no
fully fluorinated molecules with any element. NF3 and SF6 may be universal
technosignatures owing to their special industrial properties, which unlike
biosignature gases, are not species-dependent. Other key relevant qualities of
NF3 and SF6 are: their extremely low water solubility, unique spectral
features, and long atmospheric lifetimes. NF3 has no non-human sources and was
absent from Earth's pre-industrial atmosphere. SF6 is released in only tiny
amounts from fluorine-containing minerals, and is likely produced in only
trivial amounts by volcanic eruptions. We propose a strategy to rule out SF6's
abiotic source by simultaneous observations of SiF4, which is released by
volcanoes in an order of magnitude higher abundance than SF6. Other fully
fluorinated human-made molecules are of interest, but their chemical and
spectral properties are unavailable. We summarize why life on Earth-and perhaps
life elsewhere-avoids using F. We caution, however, that we cannot definitively
disentangle an alien biochemistry byproduct from a technosignature gas.
