The Collaborative International Dictionary of English v.0.48:

Mephitic \Me*phit"ic\, Mephitical \Me*phit"ic*al\, a. [L.
   mephiticus, fr. mephitis mephitis: cf. F. m['e]phitique.]
   1. Tending to destroy life; poisonous; noxious; as, mephitic
      exhalations; mephitic regions.
      [1913 Webster]

   2. Offensive to the smell; as, mephitic odors.
      [1913 Webster]

   Mephitic air (Chem.), carbon dioxide; -- so called because
      of its deadly suffocating power. See Carbonic acid,
      under Carbonic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mephitic
    adj 1: of noxious stench from atmospheric pollution [syn:
           miasmic, mephitic]