Many of the allegedly spontaneously combusted corpses are of elderly people who may have ignited themselves accidentally. Some may have been murdered. Many are elderly women who may have had osteoporosis, making their bones burn at a lower temperature than needed for healthy bones. Self-ignition due to dropping a lit cigarette or ignition due to another person are ruled out by some investigators as unlikely because they think the whole room should have gone up in flames in such cases. Even when candles or fireplaces present a plausible explanation for the cause of a fire, some investigators favor an explanation that requires belief in an event whose likelihood is extremely implausible.
Physical possibility of SHC
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 a temperature of 1600 degrees Fahrenheit for about two hours. To get a chemical reaction in a human body that would lead to ignition would require some doing. 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. And in each of these ludicrous scenarios additional oxygen would have to be introduced. These possibilities are so farfetched that I have no reason to believe they, or anything like them, has ever occurred.
...A more economical and reasonable theory of how human bodies burn in rooms without having the entire room engulfed in flames is the idea of the
wick effect. The ignition point of human fat is low and to get the fire going would require an external source. Once ignited, however, a "wick effect" from the body's fat would burn hot enough in certain places to destroy even bones. To prove that a human being might burn like a candle, Dr. John de Haan of the California Criminalistic Institute wrapped a dead pig in a blanket, poured a small amount of gasoline on the blanket, and ignited it. Even the bones were destroyed after five hours of continuous burning. The fat content of a pig is very similar to the fat content of a human being. The damage to the pig, according to Dr. De Haan "is exactly the same as that from supposed spontaneous human combustion." A National Geographic special on SHC showed a failed attempt to duplicate the burning pig experiment. However, it is obvious that the failure was due to leaving the door to the room open to the outside, which created a draft and led to the flames igniting everything in the room. Had the room been closed up, as are the rooms in which many of the elderly persons have died in fires attributed to SHC, it is likely that the pig would have smoldered for several hours without the rest of the room becoming engulfed in flames.
In their investigation of a number of SHC cases, Dr. Joe Nickell and Dr. John Fisher found that when the destruction of the body was minimal, the only significant fuel source was the individual's clothes, but where the destruction was considerable, additional fuel sources increased the combustion. Materials under the body help retain melted fat that flows from the body and serves to keep it burning. The reason some bodies are totally consumed except for the legs or feet probably has to do with the fact that these victims were seated when they caught fire and flames move upward.
Some alleged cases of SHC are cases of spontaneous combustion but they are explicable by natural means. For example, a chemical reaction on or in a person's clothing can result in spontaneous combustion. The National Geographic special, mentioned above, investigated a case of a woman whose clothes suddenly caught fire and burned the skin on her thigh. The most likely explanation is that she put a shell in her pocket that was covered in sodium from a fireworks show that had taken place on the beach where she had retrieved the shell. Later, she stuck a wet handkerchief in her pocket with the shell. The sodium may have reacted with the water, releasing hydrogen that self-ignited, causing her burns. In any case, she did not burn from the inside, as is claimed happens to SHC victims.
http://www.skepdic.com/shc.html
