Estimating time of death
Estimating the time of death
One of the responsibilities of the forensic pathologist is to estimate the time of death. An accurate assessment is of great importance to police in narrowing down the list of suspects. It can allow police to pinpoint the time during which they need to find out what the suspects were doing and allows them to eliminate people who have an alibi for that period from there enquiries.
There are three main factors which can be used by the forensic pathologist to estimate the time of death. These are; body temperature, rigor mortis and lividity.
Following death, the body begins to cool at a rate which depends on a range of factors. A naked body will cool faster than a clothed one; a large adult will cool slower than that of an infant; a body in a prone spread-eagled position will cool faster than a body slumped in the corner; a body exposed to air currents will cool faster than one in a protected area.
The environmental temperature is also a crucial factor. In 1978, a case occurred in a remote part of New South Wales which illustrates this rather dramatically. The local telephone operator suspected foul play when she could not connect a call to a remote farmhouse after a number of attempts. Eventually a young child answered. After some questioning the child explained that she did not ‘…….like Mummy any more because she is turning black’. When the police arrived they found two bodies in quite different states of decay even though they had been killed at the same time. The father had been shot on the kitchen and lay dead on the cool linoleum floor. The mother was found shot dead in bed and was in a far more advanced state of decomposition because the electric blanket had bee left on. Because the body had been kept warm the bacteria found naturally in the human body had multiplied rapidly and begun the decay process. Flies had laid eggs in the body and maggots had hatched and grown. The state of the woman’s corpse had deteriorated rapidly, while very little change occurred in the male victim.
As a general rule a body starting at 37°C will drop on average by 1°C per hour. This rate is then adjusted according to particular circumstances. To determine rate of cooling more accurately a pathologist arriving at the scene of a crime will take body temperature readings at timed intervals. These results are plotted on a graph of temperature vs. time and the graph is extrapolated back to give an estimate of the time of death.
The term ‘stiff’ is often used to refer to a dead body as a result of the stiffness that occurs in body joints after death. This condition, known as rigor mortis, occurs because of the failure of enzymatic processes and the build up of wastes in the body. One such waste which rises in concentration is lactic acid. As this builds up in muscle tissues it causes the muscle fibres to shorten and stiffen. Small muscles, such as the jawbone, are usually affected first. The condition later spreads to the larger muscles in the limbs. Rigor mortis develops during the first 12 to 24 hours, but then decreases again until 36 to 48 hours later it has completely disappeared. This is because the muscles begin to degenerate as decomposition sets in.
Lividity or ‘liver mortis’ is also known as hypostasis. It is a term used to describe the draining of the blood to lower portions of the body due to the influence of gravity. The body develops a patchy discolouration within 1-2 hours of death and the process is complete within 6 to 12 hours.
On its own, lividity is of limited use in establishing the time of death, but can be used in conjunction with other signs and can provide other useful information. For example, areas of the body which experience continual pressure do not usually show lividity. Consequently, if the body has been moved, there may be parts of the body which show no lividity even though it would be expected there. In addition, the colouring of the skin can indicate certain types of death. Carbon monoxide poisoning causes the skin to have a cherry-pink colour and cyanide poisoning creates a bright red colour.
The table below summarizes the key changes which take place within 48 hours of death:
Time since death: …. Change observed
1-2 hours: ………Early signs of lividity.
2-5 hours: ………Clear signs of lividity throughout body.
5-7 hours: ………Rigor mortis begins in face.
8-12 hours: …….Rigor mortis established throughout the body, extending to arms and legs
12 hours: ……….Body has cooled to about 25°C internally.
20-24 hours: …..Body has cooled to surrounding temperature.
24 hours: ……….Rigor mortis begins to disappear from the body in roughly the same order as it appeared.
36 hours: ……….Rigor mortis has completely disappeared.
48 hours: ……….Body discolouration shows that decomposition is beginning.