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Dregs were the mutated remains of those surviving humans who were left behind following a catastrophic war on Earth, now designated Orphan 55. Unlike their human ancestors, Dregs required carbon dioxide to breathe and exhaled oxygen as waste. (Orphan 55).
Ryan Sinclair’s worst nightmare was that he would travel with The Doctor for too long, and by the time he returned, the Earth would be destroyed and everybody living on it would be turned into Dregs. (Can You Hear Me?)