For MAY 2000 issue

SCIENCE IN CRIME DETECTION

HOW DO WE RECOGNIZE BADLY BURNT PEOPLE IN FIRE

‑Dr. Anil Aggrawal

Several times we read in newspapers that a fire broke out somewhere and several people died in that fire. After such terrible fires, usually the victims are so badly burnt, that for an ordinary person it may be almost impossible to identify them. But not so for a forensic pathologist. He can employ several means to identify badly burnt people. One such method is by examination of teeth, and in this installment I am going to tell you about this.

Let us first get an idea of how much temperatures are reached at various fires. I will first take the example of crematoria. At crematoria, human remains are incinerated in ovens, usually fired by gas or oil at 9800 C, for a period of 1 and a half to 2 hours. Complete incineration of all but dental gold and cobalt steel appliances results. Some distortion of the metals is usually observed. This purposeful cremation provides a base line for comparative purposes.

(Editor: Please reproduce figure 001.jpg here. Caption – Cremated remains of teeth)

Any fire requires three components to support combustion: (i) fuel or a combustible material in the form of gas, liquid or solid; (ii) oxygen to support combustion, viz. - air contains 21% oxygen, but only 16% is required for effective support of combustion, and (iii) heat present in sufficient amount and intensity to vaporize and ignite the fuel. This may be in the form of the sun, sparks, friction, open flame, electrical or chemical heat or compressed gases.

Investigators have even set buildings on fire just to study the effect of fire on various objects. In one such experimental fire at Mimico, Ontario, a two storey house was allowed to burn unimpeded. It took 42 minutes to reach a maximum temperature of 12740 C on the main floor. During this time the second floor temperature did not rise above 2320 C. When the main floor ceiling collapsed, the second floor temperature peaked at 10040 C but 3 minutes later had dropped to 8700 C.

(Editor: Please reproduce figure 002.jpg here. Caption – Scene of an intense fire)

What does this experiment show? It shows that when a fire breaks out, generally it is the main floor which is at higher temperature than the floor above. When the ceiling collapses, then the temperature in the top floor may rise.

Today, it appears that one of our most frequent fires is that caused by careless smoking, wherein a chesterfield or bed may smoulder for an hour or more, until an extra force of air, brought on perhaps by a door opening in another part of the dwelling, ignites a fire of sudden high heat intensity. These temperatures are significantly higher than those of the crematorium and unless extinguished quickly, tend to result in complete or nearly complete incineration. Personal experience with fires of these types, and where the fire has been quickly extinguished, show that the dentition remained essentially intact with charring of the anterior incisors (technical name for the front teeth), but no observed fire damage to the teeth on the back portion of the jaws. This indicates that the time factor has much to do with the cremation and also that the thick facial muscles and the tongue - moistened by saliva - serves as a formidable barrier to dental destruction.

(Editor: Please reproduce figure 003.jpg and 004.jpg here. Caption – Effects of intense heat on teeth)

Let us now talk of another experiment. The National Automobile Theft Bureau of Santiago, California has conducted tests to ascertain the efficiency of the fire wall between motor and cab. It has also studied the results of careless smoking and arson within the cab. In the case of an engine fire where gasoline was poured on the manifold, the fire was self limiting, burning only the insulation around the various electrical wires. The firewall was indeed effective in protecting the cab. Temperatures within the cab did not exceed 2270 C. The effect on a victim within the cab would be minimal as far as the dentition is concerned. Time factor variable would be the only other consideration. When 1.137 Liters (or about 2 quarts) of gasoline was poured on the upholstery in the cab and ignited, a rapid intense fire was created, attaining a temperature of 8220 C at the dome light. The fire was of short duration and the fabrics were shown to be relatively poor combustants. Undoubtedly the rapid use of the available oxygen reduced the time interval.

In a super highway accident involving 18 vehicles, a fire of high intensity ensued, augmented by lumber on a truck. Twelve people died in the accident. The bodies were severely charred, and no identification was possible by ordinary means. Yet as many as nine persons could be identified by teeth only, as teeth usually survive severe fires. In the centre of the fire was a Volkswagen, carrying a husband and wife. Both were almost totally incinerated. Premortem records showed that the woman had had two fixed bridges replacing upper second bicuspids, plus five other gold crowns. All were melted into a mass of gold. The husband, however, had an upper lateral incisor replaced by a porcelaine fused to gold restoration with the lateral cantilevered to the central incisor. Although the porcelaine had melted, this gold bridge had retained its form. Since dental golds melt at 10240 C and the special gold used in porcelaine fused to gold melts at 12210 C, one might rightfully assume that the maximum temperature of the fire remained below 12210 C yet in excess of 10240 C. Even at this high range of temperatures, a multiple surface amalgam restoration with pin retention survived these temperatures, and provided another facet of postmortem identification when compared with premortem radiographs and documented records.

Silver amalgam consists of silver 72%, tin 26-29%, copper 3-6% and zinc 0-2%. When triturated with mercury, complexes of silver and tin and of Copper and Zinc occur, being luted together by the mercury. Thus when heated, the mercury is released as beads at 650 C, evaporates at 1000 C and boils at 3560 C. With the loss of mercury the alloy usually, but not always, begins to powder into the Silver-tin and Copper-zinc complexes. The Silver-tin melts at 5000 C but since Silver alone melts at 9600 C and tin at 2310 C, the complex breaks apart and forms silver-oxide, a black powder. One might say that between 5000-10000C the amalgam loses form, colour and integrity.

In practice, investigators have discovered an amalgam profile still intact in form, even though the tooth enamel has "popped off". Comparison with pre and postmortem radiographs can aid in identification in these cases.

Dental prostheses are composed of acrylics, gold or chrome steel alloys. Teeth can be of porcelaine or acrylic. Since acrylic melts and ignites at 2000-2500 C its stability in hot fires is poor. However, the other metals endure temperatures to 11000 C. For identification purposes, prostheses indicate both missing and clasped teeth. Investigators have found them to be highly durable even in high temperature fires - this again attests to the protection offered by the soft tissues of the oral cavity.

In the sustained hot fire, another identifiable tissue exists, the dental pulp. Dentinal tubules (minute structures within the teeth) contain 8-10% of water. When this becomes heated to boiling, the enamel cap explodes with the dentine losing its water and undergoing shrinkage. The dental pulp with its two blood vessels and nerve complex reacts similarly. The shrinkage of the pulp and tooth is consistent with the original tooth and pulpal form, and therefore offers identifiable potential by means of radiographs or tracings. Residues of amalgam fillings may remain as a stain in the dentine. In a recent case involving a motor accident and fire of high intensity, the victim had the condition of dentinogenesis imperfecta revealed by his premortem radiographs. The enlarged pulpal morphology offered points of concordance when compared to radiographs of the badly burned and shrunken molars. Variables of shrinkage and radiographic distortion did not appear to be a deterrent.

(Editor: Please reproduce figure 005.jpg here. Caption – How forensic experts chart the teeth in case of badly burnt victims)

In summary, the collation of all information involving the victims of serious fires, where visual identification is impossible, is still paramount in importance. The necessity for police and fire officials to garner all specimens to aid in identification is most vital. In many cases it would be preferable to ask the odontologist to conduct his own search. The removal of the jaws is essential for accurate charting, radiographs and photographs. Whenever we encounter badly burnt bodies, and there is a problem in identification, we make it a point to remove the jaw, so that the teeth can be studied later at leisure. Often they give away the identity of the person, as outlined above.

As is the case with bite marks identifications, so it is with victims of fire - the gathering of facts pertinent to cause, degree of burning, possible homicide, arson, temperature estimates, time and intensity of fires - all offer statistics which could be accumulated by odontologists and blended into an extensive report. The collation of these facts and statistics might well offer a viable extension into the world of forensics.

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(Dr. Anil Aggrawal, M.D. is a Professor of Forensic Medicine at the Maulana Azad Medical College, New Delhi. His E-mail is dr_anil@hotmail.com, and his website is http://members.tripod.com/~Prof_Anil_Aggrawal/index.html)