  Detecting biosignatures, such as molecular oxygen in combination with a
reducing gas, in the atmospheres of transiting exoplanets has been a major
focus in the search for alien life. We point out that in addition to these
generic indicators, anthropogenic pollution could be used as a novel
biosignature for intelligent life. To this end, we identify pollutants in the
Earth's atmosphere that have significant absorption features in the spectral
range covered by the James Webb Space Telescope (JWST). We focus on
tetrafluoromethane CF4 and trichlorofluoromethane (CCl3F), which are the
easiest to detect chlorofluorocarbons (CFCs) produced by anthropogenic
activity. We estimate that ~1.2 days (~1.7 days) of total integration time will
be sufficient to detect or constrain the concentration of CCl3F (CF4) to ~10
times current terrestrial level.
