Source : Webster's Revised Unabridged Dictionary (1913)

Harmful \Harm"ful\, a.
   Full of harm; injurious; hurtful; mischievous. `` Most
   harmful hazards.'' --Strype. --{Harm"ful*ly}, adv. --
   {Harm"ful*ness}, n.

Source : WordNet®

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