Post-mortem interval (PMI) is the time that has elapsed since a person has died. If the time in question is not known, a number of medical/scientific techniques are used to determine it. This also can refer to the stage of decomposition of the body.
Many types of changes to a body occur after death. Some of those that can be used to determine the post mortem interval are:
Algor mortis - body cooling;
Livor mortis - settling of blood in the lowest-placed parts of the body;
Rigor mortis - stiffening of limbs;
Forensic entomology - insect (especially blowfly) activity on the corpse;
Vitreous humour changes - eye chemistry;
State of decomposition - autolysis (process of self digestion) and putrefaction (process caused by bacteria found within the body).
A person who judges the time of death by the means of decomposition can refer to a simple five-stage process:
Stage 1: Initial Decay - Bacteria located mainly in the lower intestine begin decomposition, giving a greenish color to the lower abdomen.
Stage 2: Putrefaction - Bacteria grow throughout the body, releasing gases, including cadaverine, which in turn bloat the body and cause unpleasant odor.
Stage 3: Black Putrefaction - This stage brings further discoloration to the body. The gases from bacterial decay begin to escape, causing strong odor.
Stage 4: Butyric Fermentation - The internal organs liquefy and the body begins to dry out.
Stage 5: Mummification - This is the slowest of the five stages. In a hot, dry climate the body may dehydrate, inhibiting bacterial decay; the skin dries to a dark leathery appearance.
More advanced methods include DNA quantification, infrared spectroscopy. and for buried individuals changes in soils such as the levels of methane, phosphates and nitrates, ninhydrin-reactive nitrogen, volatile organic compounds and water conductivity.
