Elemental area damage is an area-level modifier that applies to all players residing(including those in air below specific minimum height) in it, usually adding elemental damage(such as poison/fire) as deterrent(esp. for weak chars) or additional difficulty emulating "natural hazard".
Swamp area in zone X, gives 2-3 poison damage each second.
Lava field, 3-10 fire damage per second.
Ice lake, 3-5 cold damage per second.