Over on the GURPS forums, there's a thread on what happens to a fantasy character in light armor who gets dropped into a bonfire. The short answer is, not much in the short term, as his armor protects him for most of the direct damage from fire. People then pointed out additional consequences, and I wanted to go through all the rules and see what happens.
In this example, three delvers are dropped in a 15' deep pit filled with a perpetual fire. The delvers are Neil Knightly, a ST 14 HT 13 Knight in DR6 plate harness; Theo Thiefly, a ST 11 HT 11 Thief in DR2 leather armor; and Will Wizardly, a ST 10 HT 12 wizard in a DR 1 padded cloth robe. Will was hit by a magebane potion before being dropped in the pit and can't cast any spells. All three made Acrobatics rolls to reduce the distance fallen to effectively 0 yards and took no damage on landing: they're at full HP and FP but have no way of leaving the pit.
Fire, Heat, and SmokeTheir immediate problem is the flame of the fire pit. Fires of ordinary intensity do 1d-1 bu damage per second (Basic 433) as large-area injury. Armor DR protects normally. Neil's and Theo's armor have full body cover with no weak points, so the average of their torso DR and worst armor DR is 6 and 2 respectively, but Will is wearing a pointy hat instead of a full face helmet, so the average of his torso DR and the least protected hit location is 1/2, rounded down to 0. Neil's armor will completely protect him from the flame damage as the maximum damage of 5 is less than his armor DR; Theo will take 0-3 injury from the flame each second, averaging 1 point of damage every other second, and Will will take 1-5 points of injury from the flame because the minimum damage from a non-crushing attack is 1 and he has 0 effective DR. On average, he takes 8 points of injury every 3 seconds.
All of their clothing and armor is resistant to burning, and requires at least 10 points of burning damage in a single attack to ignite. Prolonged exposure has about a 1/6 chance of igniting something every 10 seconds, which isn't a concern since they're already surrounded by fire, but it might effect their gear eventually. Their flesh, being highly resistant to burning, will not ignite in this ordinary fire.
Their second problem is the heat (Basic 433). The temperature comfort zone for humans is 55 degrees wide and maxes out at 90 degrees F; any heat over the zone maximum plus 6 times the zone width is considered intense. In this case, that's 420 F, and the fire is burning at well over 800 F. Each delver's armor protects them against the effects of heat for 3 x DR seconds, and then they have to make HT rolls every second or lose 1 FP. They'll start to risk unconsciousness after losing FP equal to their HT, and FP loss of more than twice their HT is converted to HP loss. This isn't a concern for Will, who will be killed by the flames long before this is an issue, but Neil has to worry about it.
The final problem is that fires produce smoke, which GURPS treats as a poison (Basic 439) with a 10 second delay and a HT roll to resist. Failure causes coughing. The rules say that dense smoke causes damage but don't specify a mechanism: I'm going to rule that dense smoke has a secondary effect of choking, caused by failing a second HT roll to resist or failing the first HT by 5 or more. Each delver can avoid the effects of the smoke by holding their breath, which they can do for 2xHT seconds (see Basic 351: being on fire is mild exertion, but they were surprised and didn't have a chance to take deep breaths before falling into the pit). After that, they either breath smoke or lose 1 FP per second.
Slow Cooking a KnightWill is doomed, and quickly. The flames burn him for more than HP damage within 6-7 seconds, and then he falls unconscious within a few seconds more. He probably dies before he inhales enough smoke to worry about coughing, as he hits -1xHP and his first HT roll to resist death around 11 seconds and -5xHP and automatic death around 24 seconds.
Theo is not much better off. The flames do cumulative damage equal to his HP after 23 seconds, just as his breath gives out. He falls unconscious a few seconds later. He goes below -1xHP after 40 seconds, and dies sometime between then and 90 seconds, when he hits -5xHP.
Neil is in comparatively good shape. His armor completely protects him from the flames, protects him from the heat for 18 seconds, and he can hold his breath for 26 seconds. He doesn't have to make a single HT for 18 seconds, and even when he does, his high HT means he only fails 1 in 6. On average, Neil should be conscious long enough to watch Theo and Will die, though he passes out shortly after that. He doesn't take any HP damage for nearly 3 minutes, and he doesn't accumulate enough damage to risk death for nearly 6 minutes. He has good odds of surviving for over 10 minutes, but will inevitably die around the 12 minute mark.
How Real is It?This depressing article suggests that GURPS over estimates the lethality of fires: lethal fire damage takes 3-4 minutes, not less than half a minute as happened for unprotected Will. It also over estimates the effectiveness of armor protection: death comes from "first degree respiratory burns" which means your lungs burn out.
Fire damage in GURPS should probably be reduced to 1d-2 or 1d-3, but being in or on fire should also be treated as an respiratory agent with an onset time of 5 seconds that does 1 bu, resisted by HT, with a symptom of -1 HT at 1/3 HP damage and -2 HT at 2/3 HP damage. Fire respiratory damage would also be cyclic for every 5 seconds as long as the victim is in or on fire.
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