For an object to combust spontaneously, three things need to happen. First, the body must be heated to its ignition temperature — the point at which it will catch fire without being exposed to an external flame or spark. If the heat building up inside the object cannot escape, and if it's being exposed to a steady flow of oxygen that isn't rapid enough to cool it down, the stage will be set for spontaneous ignition [source: South Australian Metropolitan Fire Service].
We know this can happen to certain inanimate objects that will be discussed later. But does the phenomenon occur in humans? The jury is still out. That said, a few explanations for how people could hypothetically experience spontaneous combustion have been forth over the last few centuries.
Charles Dickens blamed booze. In the 1850s, the writer ignited great interest in SHC by using it to kill off a character in his novel "Bleak House." The character, named Krook, was an alcoholic following the belief at the time that spontaneous human combustion was caused by excessive amounts of alcohol in the body. American prohibitionists helped spread this notion as they denounced the evils of alcoholism [source: Behr].
Other ideas are more popular today. One widespread belief says the fire is sparked when methane (a flammable gas produced by gut bacteria) builds up in the intestines and is ignited by enzymes (proteins in the body that act as catalysts to induce and speed up chemical reactions) [source: Reville].
Yet this begs the question of why there are no reported instances of spontaneous combustion in cows, which produce far more methane than people [source: Radford].
It's also been suggested that the fire begins because of static electricity building up inside the body or from an external geomagnetic force. A self-proclaimed expert on spontaneous human combustion, Larry Arnold, has suggested that the phenomenon is the work of a new subatomic particle called a pyroton, which he says interacts with cells to create a mini-explosion. But as of August 2018, there's no scientific evidence that proves the existence of this particle — or spontaneous human combustion itself [source: McCarthy].
If spontaneous human combustion isn't real, then what's the explanation for the stories of people who have seemingly burned from within? And what really occurred in the many pictures that depict the charred bodies of alleged victims?
A possible explanation is the wick effect. When lit by a cigarette, smoldering ember or other heat source, the human body acts much like an inside-out candle. A candle is composed of a wick on the inside surrounded by a wax made of flammable fatty acids. The wax ignites the wick and keeps it burning. In the human body, the body fat acts as the flammable substance, and the victim's clothing or hair acts as the wick. As the fat melts from the heat, it soaks into the clothing and acts as a wax-like substance to keep the wick burning slowly. Scientists say this would explain why victims' bodies are destroyed yet their surroundings are barely burned [source: McCarthy].
Forensic scientist John DeHaan once watched this gruesome spectacle unfold in real time — to a pig, anyway. In a 1998 experiment that was televised on the BBC, he wrapped a pig corpse in a blanket, then lit the garment ablaze with some petrol. As DeHaan looked on, the animal's body fat liquified, adding more fuel to the fire. By the time he put out the flames a few hours later, the slow, intense burn had converted a large percentage of the pig's flesh and bones into ash. (The rest of the room suffered minimal damage.) [source: Kelly].
Yet the dead pig's feet remained intact. This is consistent with reports of SHC leaving disembodied feet or hands behind. Extremities don't contain as much fat as the core of the body does, so they're less likely to go up in smoke when the wick effect occurs.
Now how does science account for the greasy stains left on walls and ceilings after a spontaneous combustion? Those could simply be the residue that was produced when the victims' fatty tissue burned [source: Nickell].
Once again, we must stress that no one has ever conclusively proven (or disproven) the existence of SHC. Most scientists say there are more likely explanations for what happened to those who died in the cases we've discussed. Many so-called victims of SHC were smokers who probably died by falling asleep with a lit cigarette, cigar or pipe. Several them were believed to have been under the influence of alcohol — or suffered from a movement-restricting disorder that prevented them from moving quickly enough to escape the fire [source: Benecke].