Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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