May 11 2009

Spontaneous Human Combustion

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On a recent episode of Fringe (a rather mediocre, in my opinion, series on the supernatural), the investigators come across apparent cases of spontaneous human combustion (SHC).  Popular Mechanics decided to write an article about the science behind SHC (it turns out there isn’t any) and luckily contacted me to give them the skinny. You can read the article here.

SHC is a fun pseudoscience in that there is nothing concrete at stake – no health claims, no products, no concerns about squandering limited research funds. It’s purely a scientific question, one highly amenable to skeptical analysis.

In order to understand SHC imagine the following scene: An elderly woman who lives alone is found dead in her apartment. She is the victim of fire; her body is mostly reduced to ash, and only the ends of her arms and legs remain. The ashen outline of her head lies upon the hearth of her fireplace, the iron grill of which has been knocked to the side. There are signs that a fire recently was burning in the fireplace. A brown greasy substance coats the walls and ceiling near the body, but otherwise the room is unharmed.

Now set aside all common sense and reason, and you’ll have a typical case of spontaneous human combustion.

The idea that people can suddenly and spontaneously burst into flames is quite old, existing in folklore for centuries and first chronicled by Jonas Dupont in 1763 (in De Incendiis Corporis Humani Spontaneis ). SHC was also made famous by Charles Dickens, who did away with a despicable alcoholic character named Krook in the novel Bleak House by having him combust spontaneously. This episode in fiction also furthered the notion that heavy alcohol use can lead to an inflammatory demise.

Believers in SHC commit the usual flaws in scientific thinking. Stories of alleged SHC are essentially cases of mystery mongering and anomaly hunting – events with curious or unusual details not easily explained by the lay person. Believers then couple the unusual with a logical fallacy known as the argument from ignorance: “We can’t explain how these people burned the way that they did, or what the source of the fire was, therefore my explanation is correct.” Finally, SHC supporters lack a little thing that scientists like to call “plausibility.” In other words, bodies don’t just burst into flames.

The alleged cases of SHC range form the peculiar to the laughable. Many cases involve obvious external sources of fire, matches and cigarettes being the most common culprits. At other times candles, fireplaces, pipes or lanterns are involved. But considering all the fires that are investigated it would be surprising if occasionally the circumstances of the fire were not mysterious. SHC believers proceed from the premise that we should be able to fully explain all fires, and therefore the inability to do means we have to introduce a new physical phenomenon – no matter how implausible. This is the argument from ignorance.

SHC believers often cite as evidence the fact that a body has been completely reduced to ash, except for the ends of the arms and legs and sometimes the head. But there is a good explanation for this phenomenon. It is called the wick effect. The clothing of victims can act as a wick, while their body fat serves as a source of fuel (like an inside-out candle). The burning of the clothes is maintained by liquefied fat wicked from the body of the victim, causing a slow burn that can nearly consume the victim and resulting in the greasy brown substance often coating nearby walls.

This has actually been experimentally confirmed using the bodies of pigs. Further, in a closed room a fire will quickly use up the oxygen, reducing the flames to slow burning embers. This is why firefighters do not simply burst into closed rooms in a fire – the sudden rush of oxygen can cause these embers to flare. But if left undisturbed for hours, these fires can consume a body without the flames spreading.

This leads to another bit of “evidence” often cited – the surrounding room is sometimes left unscathed by the fire. But this is not unusual at all. Fires burn up because hot air rises over cooler air. The temperature below or even a short distance to the side of a fire can be very low- low enough not to cause any fire damage. The upward direction of burning is also why the ends of the arms and legs are often unburned, just as the ends of a log may be left behind after the middle has been consumed.

In cases of “spontaneous” combustion, the victims are often overweight, and therefore have plenty of fuel for the fire. Many are old or infirm, and therefore might be unable to stop a fire once it started. Many are alcoholics or taking sedatives, and might not awaken after catching on fire. And many are careless smokers. Forensic biologist Mark Benecke thoroughly reviewed available cases of alleged SHC and concluded that “[t]he pictures and reports published on SHC up to now can be explained by well-known and understood mechanisms that are regularly found at the sites of burning.” In other words, there are no cases that constitute convincing evidence of SHC.

Besides, there is no mechanism that could possibly explain the energy required to ignite a living person. So SHC supporters resort to one of three strategies. They sometimes reply, with a quizzical look, that the mechanism is “a mystery.” Or they may invoke their favorite paranormal explanation, like psychic energy or something equally meaningless.

Some, however, have taken a third approach, trying to explain SHC with a scientific-sounding physical mechanism. Larry Arnold, in his 1996 book Ablaze!, speculates that SHC is caused by a previously unknown subatomic particle that he dubs the “pyrotron” and which he claims can spontaneously undergo nuclear fusion, releasing tremendous energy. But there’s no evidence for this notion, and Arnold appears totally ignorant of how nuclear fusion works.

Further, if there were a biological process by which large amounts of heat energy could be suddenly released, causing combustion, wouldn’t we expect to see this process in action – at least occasionally. At some point this process should occur but fall short of combustion. Why aren’t there cases of people presenting with ridiculously high fevers? Why aren’t there cases of people with high fevers bursting into flames before the eyes of the medical experts treating them? I guess SHC could be a rapid all-or-nothing process, but this is also post-hoc special pleading for the lack of supporting evidence.

SHC remains an argument from ignorance, without any positive evidence, based entirely on anomaly hunting from the many cases of people dying in fires. It is also not particularly interesting anomaly hunting – most alleged SHC cases involve the equivalent of old ladies found with their heads in the fireplace.

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I also blog on Mondays over at SkepticBlog – today I wrote about Simon Singh’s libel suit.

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