|MadSci Network: Chemistry|
I have never heard of any documented case of anyone spontaneously combusting. Perhaps the best way to answer this question is to think about the process of combustion and how it usually occurs. Strictly speaking, combustion is any chemical reaction that is accompanied by the evolution of heat and light, although most cases of combustion involved substances burning in air (78% nitrogen and 21% oxygen). Before combustion can occur, the substances involved must be heated to the kindling temperature, which is the temperature at which the reaction is fast enough to proceed without further addition of heat from the outside. The kindling temperature for many substances varies and often depends on how finely divided the substance is. When the heat released during a slow reaction remains and raises the temperature of the reacting substances to the kindling temperature, spontaneous combustion may occur. A common example of such a reaction that has caused many tragic fires in peoples' homes is when rags containing paints, paint thinners, or lacquers are stored without enough circulation of air to carry off the heat produced by the slow oxidation of the solvents by oxygen in the existing air: the heat of reaction accumulates and raises the temperature of the system above the kindling temperature, and spontaneous combustion occurs.
Now that we know how spontaneous combustion usually occurs, you can ask yourself how likely it is for a human to spontaneously combust. What conditions would have to exist? First, the "substance," a human being (70% water), would have to be heated to the kindling temperature of a human body, and second, since we live in air, a slow oxidation reaction would have to occur in a closed environment and generate enough heat over time to raise the temperature of the body and its surrounding above the kindling temperature of a human body. I don't know the kindling temperature of a human body, but I would estimate it as follows. If someone cooked a chicken for you, that piece of meat, which is of approximately the same chemical composition as a human body, doesn't not burn like a fire (combust) but just gets very hot. But if one were to put a live chicken (or human) in an oven of the same temperature it would die very fast, so the kindling temperature of a human must be far above the temperature at which one could stay alive, which would put it at least above 110 degrees F. One could even go further and note that people who die in fires from burns do not "burn up," but are just burned on the outside, so the kindling temperature of a human must be higher than the fire that burns down a house.
The second condition for spontaneous human combustion is that a slow oxidation reaction would have to occur in a closed environment and generate enough heat over time to raise the temperature of the body and its surroundings above the kindling temperature of a human body. Since you leave a turkey in the oven at 400 degrees F for 6 hours on Thanskgiving and it does not burn up, this means that a human body would have to be in a closed environment for longer times and/or at higher temperatures in order to even have a chance at combustion, and it should be very clear to you that you would be very dead long before then, probably from lack of air but certainly from the heat. And of course once you die your body stops running chemical reactions, including all oxidations that would be needed to produce the heat for spontaneous combustion.
In summary, it is very hard to heat a body to its kindling temperature, and if you tried to do it by a slow accumulation of heat in a closed system generated by a normal chemical reaction of the body it would take so long that you would have died from lack of oxygen. And once you are dead your body doesn't rum many more chemical reactions!
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