The Collaborative International Dictionary of English v.0.48:

Effluvium \Ef*flu"vi*um\, n.; pl. Effluvia. [L., a flowing
   out, fr. effluere to flow out. See Effluent, a.]
   Subtile or invisible emanation; exhalation perceived by the
   sense of smell; especially, noisome or noxious exhalation;
   as, the effluvium from diseased or putrefying bodies, or from
   ill drainage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

effluvium
    n 1: a foul-smelling outflow or vapor (especially a gaseous
         waste)

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "effluvium":
   afterdamp, aroma, aura, blackdamp, breath, chokedamp, cloud,
   coal gas, damp, definite odor, detectable odor, ectoplasm,
   ectoplasy, emanation, essence, exhalation, exhaust, exhaust gas,
   exteriorized protoplasm, fetid air, firedamp, flatus, flavor,
   fluid, fragrance, fume, malaria, mephitis, miasma, odor,
   puff of smoke, redolence, reek, savor, scent, smell, smoke, smudge,
   spoor, steam, stench, subtle odor, trace, trail, vapor, volatile,
   water vapor, whiff