Path of Exile 1 Gameplay Help and Discussion - Poison DMG VS Chaos DMG - Forum

Poison damage is a specific type of Chaos Damage Over Time.

Chaos Damage Over Time is a specific type of Chaos Damage.

Chaos damage is not necessarily Poison damage. Chaos Damage Over Time is not necessarily Poison damage, either. Poison damage is very specifically the damage caused by the Poison ailment, and nothing else. Some skill gems cause the Poison ailment, but your attack, spell, or Caustic Arrow damage will never be increased by increases to Poison Damage.