The Collaborative International Dictionary of English v.0.48:

Harmful \Harm"ful\ (h[aum]rm"f[.u]l), a.
   Full of harm; injurious; hurtful; mischievous. " Most harmful
   hazards." --Strype. --Harm"ful*ly, adv. -- Harm"ful*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

harmfulness
    n 1: destructiveness that causes harm or injury [syn:
         harmfulness, injuriousness]
    2: the quality of being noxious [syn: harmfulness,
       noisomeness, noxiousness]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "harmfulness":
   air, badness, balefulness, banefulness, bitchiness, contamination,
   cussedness, deadliness, detrimentalness, devilment, devilry,
   deviltry, evil intent, hatefulness, health hazard, hurtfulness,
   iniquitousness, injuriousness, insalubriousness, insalubrity,
   invidiousness, lethality, maleficence, malice, malice aforethought,
   malice prepense, maliciousness, malignance, malignancy, malignity,
   meanness, menace to health, mischievousness, nastiness,
   noise pollution, noisomeness, noxiousness, ominousness, orneriness,
   pathenogenicity, perniciousness, poisonousness, pollution,
   toxicity, unhealthfulness, unhealthiness, unsalutariness,
   unwholesomeness, venomousness, viciousness, virulence, water,
   wickedness