If the engine failure is due to a bird ingestion, or an oil seal in the compressor section, the remains of the bird or the vaporized oil will come off in the bleed air and go in the packs/cabin.
This (oil seal) happened to an RJ last year on short final at Denver. Smoke was so bad, the crew couldn't see the instruments. They broke out at 100 ft and managed to land without further incident. Very lucky; weather was at minimums and they were inside the marker when it blew.
People in back suffered a lot of vapor inhalation.
If the FAs in this JB scenario called and said "We can't breathe!!", the crew could drop the masks with a switch.