Definition from Wiktionary, the free dictionary
Alternative forms 
- harmfull (archaic)
Usage notes 
- Nouns to which "harmful" is often applied: effect, consequence, impact, influence, emission, chemical, ingredient, substance, gas, agent, additive, drug, radiation, dust, organism, plant, animal, insect, action, act, behavior, component, content, activity, interference, use.
likely to be damaging