The process of a human body catching fire as a result of heat generated by internal chemical action.
Many people have felt as if they were about to explode, or that their bellies were on fire, but so far there are no accounts of anyone suddenly bursting into flames due to a case of spontaneous combustion originating within the body. It is true that many people have suddenly exploded or suddenly burst into flames, but these cases are all traceable to unfriendly fire from without. The very idea of a living animal, human or otherwise, igniting from spontaneous combustion in the belly or anywhere else in the body is ludicrous and preposterous. Perhaps that explains why stories of spontaneous human combustion abound in books about "mysteries."
Skeptics do not think there are any good documented cases of SHC, though we admit that there are stories which claim that at least a couple hundred human beings or human corpses have spontaneously combusted. Many of the stories have been related by police investigators of corpses who have been perplexed by partially ignited bodies near unburnt rugs or furniture. "What else could it be?" they ask. Indeed, what else? Well, how about self-ignition due to dropping a lit cigarette, or ignition due to another person putting the match to a person or place. Many of the allegedly spontaneously combusted corpses are of elderly or overweight people who may have ignited themselves accidentally.
The physical possibilities of spontaneous human combustion are remote. Not only is the body mostly water, but aside from fat tissue and methane gas, there isn't much that burns readily in a human body. To cremate a human body requires enormous amounts of heat over a long period of time. To get a chemical reaction in a human body which would lead to ignition would require some doing. The ignition point of human fat might be low, but to get the fire going would probably require an external source. If the deceased had recently eaten an enormous amount of hay that was infested with bacteria, enough heat might be generated to ignite the hay, but not much besides the gut and intestines would probably burn. Or, if the deceased had been eating the newspaper and drunk some oil, and was left to rot for a couple of weeks in a well-heated room, his gut might ignite.
I imagine that the reason some of these burnt bodies appear so strange has more to do with the difficulty of burning a human corpse than with spontaneous combustion. That a fire should burn some things and not others, that it should appear to have started from inside the body, or that there be no other evidence of foul play besides a smoldering corpse, doesn't imply that the most reasonable explanation is spontaneous human combustion.
There may be many things we can't explain because we don't have enough information. In such cases, it does not seem beneficial to speculate unless there is some substance to our speculation. Has anyone tried to get a corpse to spontaneously combust? Are there any coroners out there who have seen this happen? If so, where are their reports? Is there a universal conspiracy of coroners to hide from the public this vital information!! You'd think that if human corpses spontaneously combusted at least one coroner somewhere on the planet would have seen one.
Why do corpses only spontaneously combust for police officers, especially British bobbys?
There are a few other curious things about spontaneous human combustion that should be considered. Fire burns at over 200 degrees F. The human body, when alive, is usually under 100 degrees F. A corpse would tend to cool off to room temperature. If a living being ever spontaneously combusted, the warning signs would be phenomenal: a 212 degree F. burning sensation! If a corpse self-ignited it would be hard to keep it burning unless the room were very, very hot. In fact, the room would have to be nearly on fire itself to keep the corpse ignited. Once a fire has started, it will be self-supporting only if the temperature created by the combustion of the burning substance is as high or higher than its ignition point. A cool body in a cool room would be unlikely to do much more than smolder a little bit if it did self-ignite.