The problem is not that burning and poisoning do too much damage, the problem is that we encounter these things too early in the game. Undeads at level 2 or 3 should be skeletons doing physical damage. Having to deal with poison and burning at level 2 or 3 when you don't already have the response to it is a problem. They are doing too much damage compared to low level health. But if you reduce the damage, then these conditions will barely scratch a high level player.
You can scale the damage to the player's level ( not very realistic ) or introduce conditions later in the game.
Also damage is high and duration is short. Reducing the damage and increasing the length would give players some time to react and would make removal spells more useful.
