Spontaneous human combustion (SHC) is the alleged burning of a person’s
body without a readily apparent, identifiable external source of
ignition. The combustion may result in simple burns and blisters to the
skin, smoking, or a complete incineration of the body. The latter is
the form most often ‘recognized’ as SHC. There is much speculation and
controversy over SHC. It is not a proven natural occurrence, but many
theories have attempted to explain SHC’s existence and how it may
occur. The two most common explanations offered to account for apparent
SHC are the non-spontaneous "wick effect" fire, and the rare discharge
called static flash fires. Although mathematically it can be shown that
the human body contains enough energy stored in the form of fat and
other tissues to consume it completely, in normal circumstances bodies
will not sustain a flame on their own.

Theories about Spontaneous Human Combustion

- Alchoholism - many Spontaneous Human Combustion vicitms have been alcoholics.  But experiments in the 19th century
demonstrated that flesh impregnated with alcohol will not burn with the
intense heat associated with Spontaneous Human Combustion.

- Deposits of flammable body fat - Many victims have been overweight - yet others have been skinny.

- Devine Intervention - Centuries ago people felt that the explosion was a sign from God of devine punishment.

- Build-up of static electricity - no known form of electrostatic discharge could cause a human to burst into flames.

- An explosive combination of chemicals can form in the digestive system - due to poor diet.

- Electrical fields that exist within the human body
might be capable of ’short circuiting’ somehow, that some sort of atomic chain reaction could generate tremendous internal heat.

No satisfactory explanation of Spontaneous Human Combustion has ever been given.  It is still an unsolved mystery.

Types of Spontaneous Human Combustion

Some events of Spontaneous Human Combustion are witnessed but some are not.

All reported cases have occurred indoors.

The victims were always alone for a long period of time.

Witnesses who were nearby (in adjacent rooms) report never hearing any sounds, such as cries of pain or calls for assistance.

In the witnessed combustions - people are actually seen by witnesses to
explode into flame; most commonly. Here the witnesses agree that there
was no possible source of ignition and/or that the flames were seen to
erupt directly from the victim’s skin. Unfortunately, most of the known
cases of this type are poorly documented and basically unconfirmed.
Sometimes there are no flames seen by the witness.

Non-fatal cases - Unfortunately, the victims of these events
generally have no better idea of what happened to them than do the
investigators; but the advantage to this grouping is that a survivor
can confirm if an event had a simple explaination or not. Thus, there
are far fewer cases of Spontaneous Human Combustion with survivors that
can be explained away by skeptics without a second look.

Sometimes victims develop burns on their bodies that have no
known external cause. These strange wounds commonly start as small
discomforts that slowly grow into large, painful marks.

Sometimes the victim will exhibit a mysterious smoke from the
body. In these odd and rare occurences smoke is seen to emanate from a
person, with no associated fire or source of smoke other than the
person’s body.

Link and more information: http://www.crystalinks.com/shc.html