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]