I think there should be a set amount of damage per round from being in fire/poison/electricity. (Separate from burning/poisoned effects, which take place at the beginning of your turn regardless.)
For every action that causes a character to spend or lose AP, they take a fractional amount of that round's damage at the beginning of that action.
If moving, it's applied at the beginning of each 1 AP worth of movement. If doing anything else, it's applied right before the action. Death will interrupt the movement or action. If the character is unable to act, they take the full round worth of damage.
Speed has the advantage of letting you run through fire faster, thus taking less damage compared to a slower character while crossing the same distance.
Ditto for chance to catch fire / become poisoned -- break it up fractionally according to AP usage.
