Perhaps the most important function of a forensic pathologist is the establishment of the time of death of a victim. Time of death narrows the window of probability when considering factors leading to death. It provides investigators a direct bearing on legal questions of alibi and opportunity. If a suspect can prove they were somewhere else at the time of death then their innocence is implicit. If it is shown that the suspect was in the vicinity of the victim at the time of assault/death then it shows that they had ample opportunity to commit the crime. The science isn’t exactly, but here are the tricks of the trade when it comes to estimating the time of death.
1.) For many pathologists the golden standard for estimating time of death within 24 hours is body temperature (Algor Mortis). The body will retain a base temperature immediately after death and the cooling rate follows a sigmoidal curve (it cools at a steady pace). We assume that the deep rectal temperature of a human body at the time of death to be 37.2 degrees C. From there on out we use the Hennsge method in order to calculate the rate of cooling. The premise is that the body will cool to the same temperature as the environment it is in. As a result we have to take into account the size of the body; the bigger the body, the bigger the surface area and the more rapidly it will cool down. On the other hand the heavier the body the is the slower it will cool down. So we take the two figures and combine it into a surface area/mass ratio. We have to account for clothing/covering on the body, as this will also slow down cooling. The humidity and movement of air around the body can affect the rate of cooling; moist air is a better conductor of heat than dry air, convection carries heat away from the body. Finally we have to take into account if the body was immersed in water or not. It has been noted that bodies in water that contain a lot of organic matter tend to cool slower than in water without it. Here is a nifty online calculator for the Hennsge method and all of the little caveats for its use.
2.) At the moment of death and immediately after the body is floppy, followed by a stage of Rigor Mortis, or muscle stiffening. Rigor mortis initiates in all muscles simultaneously, but it’s obviously going to take less time in smaller muscles than larger muscles. Eventually the rigor dissipates; in temperate climates it can take between 36 to 48 hours after death to start to disappear. In warmer climates it can take less time, between 9 and 12 hours after death to disappear because bodies decompose MUCH faster in warm climates. Because rigor is sensitive to many variables it’s not entirely useful for determining time of death. For instance in cases of death by hanging or carbon monoxide poisoning it’s slow to set in. It develops quickly in death by infections. Some forensic pathologists say there is no value in using rigor as a metric except for around 2 days after death when the body is cool but hasn’t started to decay. Others use this simplistic rule of thumb: if the corpse is still warm and there isn’t any rigor present the death occurred within the past 3 hours. If rigor is progressing the death probably occurred within 2-9 hours. If a body is fully in rigor mortis the death occurred over 9 hours prior. That doesn’t seem very reliable though.
3.) Livor Mortis, or lividity is the discoloration of the body due to gravity pooling the blood. If someone dies on their back all of their blood will pool in the back and turn the skin reddish-purple. This process starts as soon as circulation stops, and after 30-60 minutes after death blood becomes permanently incoagulable (pre-existing clots will still be there). Lividity starts to show up 20-30 minutes after death. After 10-12 hours the lividity will be “fixed” in place which is a great way to determine if a body has been moved or not.
4.) Putrefaction or decomposition is the destruction of the soft tissues of the body by enzymes and bacteria. It results in the gradual dissolution of tissue into gasses, liquids and salts. Visually this can be seen by the change in color of tissue, the presence of gases and liquefaction. Like body cooling, putrefaction is dependent on temperature: the warmer it is the faster it decomposes. The first visual sign of putrefaction is a greenish discoloration of the skin over the anterior abdomen, then spreads all over the body. As this greenish discoloration progresses the veins in the skin turn a dark purple-brown which is why the two discolorations combined are referred to as “marbling.”
Shortly after the marbling the epidermis begins to easily shed with even light contact. Gas formation in the intestines cause the abdomen to be distended. The pressure causes the body to leak blood-stained fluid from the nose, mouth, vagina and rectum. The pressure from the gases will become so great that they’ll eventually find an escape and the body will “deflate.”
As I stated earlier there are a lot of variables that go into the onset of decomposition. A general rule of thumb in a temperate climate is that the greenish discoloration on the abdominal wall begins 36 and 72 hours after death. Gas formation occurs a week after death.
5.) Vitreous humor potassium levels in the eyes are measured because there is a linear correlation between potassium concentration and the time after death up to 120 hours.
There are whole textbooks dedicated to the art of determining ‘time of death’ and this guide isn’t exhaustive by any means, but it’s fascinating that we can use the signs and landmarks of the body to determine when it stopped being a living human being.