RevTraveler

a period of around 300 years, most of the alleged cases are characterized by the lack of a thorough investigation, or rely heavily on hearsay and oral testimony. In many of the more recent cases, where photographic evidence is available, it is alleged that there was an external source of heat present (often cigarettes), and nothing occurred "spontaneously."

• Paranormal explanations (e.g., a ghost or divine intervention)
• Natural explanations based on an unknown and otherwise unobserved phenomenon (e.g., production of abnormally concentrated gas or raised levels of blood alcohol cause spontaneous ignition)
• Natural explanations that involve an external source of ignition (e.g., the victim dropped a cigarette)

Unverified natural phenomena

• Since every human body contains varying strengths of electrical field and the human body also contains flammable gases (mainly methane in the intestines), an electrical discharge could ignite these gases.
• SHC victims are sometimes described as lonely people who fall into a trance immediately before their incineration. Heymer suggests that a psychosomatic process in such emotionally-distressed people can trigger off a chain reaction by reacting nitrogen within the body and setting off a chain reaction of mitochondrial explosions. This hypothesis has been criticized on the basis that Heymer "...seems to be under the illusion that nitrogen exist as gases in the blood and are thus vulnerable to ignition, which is, in fact, not the case." (Mitochondria are organelles found within cells.) The hypothesis also fails to take into account the fact that nitrogen is an inert, non-flammable gas.
• Another hypothesis suggests high-energy particles or gamma rays coupled with susceptibilities in the potential victim (e.g., increased alcohol in the blood) triggers the initial reaction. This process may use no external oxygen to spread throughout the body, since it may not be an oxidation-reduction reaction. However, no reaction mechanism has been proposed, nor has a source for the high-energy particles.
• The victim is an alcoholic and has been smoking while drinking or shortly after drinking a strong spirit. There are claims that this raises the blood alcohol level to a point where it ignites; however, this theory is considered implausible. However, this does introduce the probability that the victim falls asleep while holding a lit cigarette.
• Another hypothesis is that both clothing and the person are ignited by a static electric discharge. A person walking across a carpet can build up sufficient charge and voltage to create a spark. It is unlikely that this could start a clothing fire, as although the voltage can be high (several thousand volts), the stored energy is very low (typically less than a joule). Proponents of this hypothesis say that records show there has never been a recorded case of a naked SHC victim.
• The controversial phenomenon of ball lightning has also been proposed as a cause of spontaneous combustion.
  
• One hypothesis for SHC takes into account mitochondria, the "internal combustion engine" of higher living organisms. Mitochondria are the powerhouses of the body, and sites of highly active energy packet production that uses an oxidative phosphorylation mechanism. Uncoupling of the metabolic processes in mitochondria from its energy production results in generation of large amounts of heat that is called thermogenesis. (This heat production is different from fever, which is an inflammatory response to infection.) Mitochondria are especially abundant in the skeletal muscle cells that require high energy output for their function. Since skeletal muscles constitute a major portion of the body, they harbor an enormous number of energy producing mitochondria. Under stress and certain extreme physiological conditions, the hyperactivity of energy producing mechanism of the body may exceed the conservation or utilization. Available body fat may also serve as additional fuel for combustion. This hypothesis still requires rigorous laboratory testing