The Collaborative International Dictionary of English v.0.48:

Toxicity \Tox*ic"i*ty\, n.
   The quality or state of being toxic or poisonous;
   poisonousness.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

toxicity
    n 1: the degree to which something is poisonous
    2: grave harmfulness or deadliness [syn: perniciousness,
       toxicity]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "toxicity":
   balefulness, banefulness, communicability, contagiousness,
   deadliness, destructiveness, detrimentalness, harmfulness,
   hurtfulness, infectiousness, infectivity, injuriousness, lethality,
   malignance, malignancy, malignity, mischievousness, noisomeness,
   noxiousness, ominousness, perniciousness, poison, poisonousness,
   unhealthiness, venom, venomousness, viciousness, virulence,
   virulency