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Elayne Pope's group spends its time setting fire to corpses in a range of different circumstances, to work out exactly how the human body burns
The fire started with a match held under a cotton blanket close to the man's waist. Within 2 minutes, the flames had spread across the single bed he was lying on and were consuming his cotton sweatshirt and trousers.

Around a dozen onlookers were at the scene - including police, fire investigators and death investigators - yet all they did was watch. That was, after all, their job. The "victim" had in fact died some time ago, having previously donated his remains to medical research.

His body had reached a unique team led by Elayne Pope, a forensic scientist at the University of West Florida in Pensacola. Her group spends its time setting fire to corpses in a range of different circumstances, to work out exactly how the human body burns. They seem to be the only group carrying out such systematic studies in this area, and are certainly the only ones publishing their work.

Until now, scientific knowledge about burned remains has been limited. Anything that wasn't based on speculation has come either from post-hoc examination of burnt corpses - where the exact circumstances of the fire are usually unclear - or from the deliberate burning of pig corpses, which have key differences to humans. "There wasn't much literature," says Pope. "The science is young."

Pope formerly worked at a medical examiner's office - similar to a UK coroner's - where she saw how often burned bodies were surrounded by unanswered questions. Many people die in house fires, car fires, or as a result of arson, and sometimes the bodies of murder victims are burned to destroy evidence. "Fire deaths by their nature are very confusing to look at," says Pope (see image).

The million-dollar question is whether a fire death was accidental or deliberate. As with other areas of forensic science, some textbook dogmas have never been tested. That can lead to a fire being labelled as accidental when it has a more sinister cause. Worse, it can lead to innocent people being convicted on the basis of unsound evidence (New Scientist, 28 February, p 6).

Eight years ago, a medical institute in Memphis, Tennessee, agreed to provide Pope with some of its donated bodies and she began her unusual mission. To date she has made use of about 30 whole corpses and various additional body parts.

Sometimes the team burns a single body set up in a commonly encountered situation, such as sitting in a car (see image). Depending on her resources and the questions she needs to answer, she may also burn bodies in batches, in several variations on a theme or in a range of different situations.

So what happens after they light the fire? "A human limb burns a little like a tree branch," says John DeHaan, a fire investigator at Fire-Ex Forensics in Vallejo, California, who works with Pope. First, he says, the thin outer layers of skin fry and begin to peel off as the flames dance across their surface. Then, after around 5 minutes, the thicker dermal layer of skin shrinks and begins to split, allowing the underlying yellow fat to leak out.

"That's when the fire gets most interesting," says DeHaan. Body fat can make a good fuel source, but it needs material such as clothing or charred wood to act as a wick. Like that in a wax candle, a wick absorbs the fat and pulls it into the flame, where it is vapourised, so enabling it to burn.

Assuming there is sufficient wick material, the body can sustain its own fire for around 7 hours. During this time, the heat causes muscles to dry out and contract, making the limbs move and sometimes adopt characteristic postures. Bone takes longer to burn, so by the end the skeleton is usually laid bare like a charred anatomical model, coated in the greasy residue of burned flesh.

That is unless someone agitates the bones (which have become brittle though heating) to break them up, which helps them continue burning. Still, there is usually something left - often teeth or fragments of bone - that gives the game away. "In most cases something survives," says DeHaan.

One thing DeHaan can't describe is the odour, because he has no sense of smell. For this reason, he pays particular attention to other people's descriptions: "There's a complete range, from 'it smells just like barbequing pork ribs', to 'it is the most revolting odour and it stays with you forever'." DeHaan suspects that it is decaying bodies that smell worse when they burn, although he hasn't tested this theory.

"Each environment is unique and produces very distinct burn patterns on the body," says Pope. If a body burns in the front seat of a car, for example, it will remain suspended on the wire frame of the seat, where it is exposed to flames from the upholstery. A body on the back seat, by contrast, will be somewhat protected by the metal bench that lies beneath the upholstery (see image).

A body can be completely destroyed if it is put in the trunk of a car, especially if it lies on a rubber tyre. While the rubber burns, the body is suspended on the metal rim and so exposed to the intense heat. "I've been able to get complete bone destruction after a body has been in the trunk for 4 to 5 hours," says Pope. "It's like a mini-crematorium."

Exploding skulls

Some of the group's studies tested common beliefs about how bodies burn. For example, many textbooks state that if a skull is initially intact, the brains will boil and cause the skull to explode into small fragments. Investigators may therefore see the lack of such an explosion as a sign of foul play. Pope's team tested this theory by systematically burning 40 human heads - some injured, others intact. They found that skulls do not explode, burning in broadly the same way regardless of trauma (see image). The team also showed that a skull may look like it has exploded if debris falls on it once the heat has made it brittle (Journal of Forensic Sciences, vol 49, p 431).

The group has refuted several other beliefs about how bodies burn. It is also establishing new axioms to help guide fire investigators in future. The question of foul play is most difficult to establish if a body is so badly burned that only a charred skeleton remains. In the same paper, Pope's team showed that if someone is shot or stabbed, the wound opens up early during the fire, exposing that part of the skeleton to more heat and leaving a permanent record in the bones. "What I look for is whether there are heavily burned-out injuries," says Pope.

Often, the team is asked to help with specific investigations. In one, a man had been arrested on suspicion of murdering his wife, burning her body in a metal barrel and dumping the ashes over a cliff. The team tried to replicate the cremation to see if it was physically possible. "We wanted to see how much time, how much heat, how much fuel, how much agitation was needed," says Pope. "If you just start a fire and walk away it's going to go out [before the bones are destroyed]."

The researchers proved that in the suspect's window of opportunity, he could have rendered down the body completely by returning to agitate the burning remains. They do not know if the suspect was ultimately convicted.

A big problem is that forensic evidence is often lost in the life-threatening drama of the crime scene. "When firefighters get to the fire, the goal is always to get the victim out," says Steven Symes, a forensic anthropologist at Mercyhurst College in Erie, Pennsylvania, who is a former teacher of Pope's. Of course this is crucial if the victim still has a chance, but it sometimes happens even if they are obviously long dead.

Symes is overseeing a study investigating how much more information could be gleaned if burned bodies are examined in situ, instead of being moved to the medical examiner's office. He also hopes to start similar burning experiments to Pope's later this year.

As their work becomes more widely known, Pope hopes that more bodies will be made available so that she and her colleagues can carry out more ambitious reconstructions. One of her more macabre plans is to set fire to a bus with several corpses in different seats. Next month they plan to stage a body burning in a light aircraft.

As word spreads, Pope and DeHaan hope fire investigation will become more evidence-based. "We can't do it halfway and say 'this is what I was taught'," says Pope. "We need a more scientific approach and to actually get the facts. People's lives are at stake here."

Vanishing hands and other myths

MYTH: Hands and feet burn away, leaving the arms and legs as stumps

Fingers and hands curl and contract as they are heated, but once all of the supporting soft tissues burn away, many of the small bones in the fingers, hands and wrists simply fragment and fall off, according to studies by Elayne Pope at the University of West Florida in Pensacola, and colleagues. Because they are often buried in ash and debris, these bones may be overlooked, but hands are often used to fend off an attacker, so they may hold vital clues as to what happened to a victim.

MYTH: Fluid-filled blisters on the skin mean the person was alive at the time of burning

Pope's team has shown this is not true. Higher temperatures or a longer exposure to heat is needed to form blisters on dead skin - but it does happen. Post-mortem blisters are created from moisture drawn out of the tissue through heat exposure.

MYTH: Complete destruction of a body means flammable liquids were used

It is sometimes assumed that, outside of professional cremations, if a body or body parts have been completely destroyed by fire then a flammable liquid must have been used - potentially turning an accidental fire into a murder scene. In fact, a variety of everyday materials can generate extreme temperatures.

A related misconception is that flammable liquids alone are sufficient to destroy a body. "If you dump gasoline on someone, it will burn for about a minute," says Pope's colleague John DeHaan of Fire-Ex Forensics in Vallejo, California. "That's enough to cause localised burns, but not enough that the skin will split and the body fat gets involved." The body needs to be exposed to fire for about 5 minutes for that to happen.

MYTH: Bodies usually sit up as they burn

This is a myth prevalent in crematoria. Pope's team has found that limbs may flex and move during burning (see Body of evidence) but bodies don't actually sit upright.

Spontaneous human combustion

In 1998, a girl's burned body was found in the undergrowth of a forest in Oregon. Her head and torso were charred almost beyond recognition, suggesting an intense heat, yet her surroundings - and even her shoes - appeared untouched. How could she burn without igniting anything around her?

When John DeHaan of Fire-Ex Forensics in Vallejo, California, was called in, the local police suggested the unevenly burned remains bore the classic hallmarks of spontaneous human combustion.

With cases of SHC found indoors, there is often little or no sign of fire elsewhere in the room, aside from a greasy residue occasionally left on furniture and walls. The victim may seem to have just dropped where they stood, suggesting a sudden and intense conflagration.

Various non-supernatural explanations have been put forward for such cases, yet according to DeHaan, "some trained fire investigators still believe in SHC". So his team have carried out a series of experiments using pigs, and more recently a human body, which are helping to finally nail the myth.

The best explanation for the unevenly burned remains has long been the wick effect. Once the skin burns enough to split open, the underlying fat gets involved, with clothing or other charred materials absorbing the fat and holding it close to the flames so it heats enough to vapourise and burn. Since in people most fat is stored around the torso and thighs, it is hardly surprising that these are the areas most badly burned. "One argument is that it is unusual to be left with hands and feet - but that's what happens when bodies burn," says Elayne Pope of the University of West Florida in Pensacola.

Another misconception is that a human body cannot burn in a confined space without igniting its surroundings or running out of oxygen. DeHaan's team has shown that a fire fuelled solely by body fat typically releases just 40 to 80 kilowatts of heat - no more than a large wastebasket fire - which would not necessarily ignite nearby objects. "Most rooms are going to have enough oxygen," says DeHaan.

Another feature of many supposed cases of SHC is lack of an ignition source. In one recent case, a woman's charred body was found by her front door, while the rest of the room was untouched. But closer inspection revealed a tiny trail of burned plastic and clothing leading to the kitchen, where the stove was still on. DeHaan believes that the victim's food caught fire, she grabbed it and made for the front door, setting her blouse alight along the way. If she had inhaled the flames, this would have caused her to collapse and succumb.

"Because the scene was perfectly preserved, you could see the connection," says DeHaan. "But most scenes aren't that pristine, so you have an ignition source in one room, and the body somewhere else, and people don't understand it."