The Postmortem Interval (PMI) is the time elapsed since an individual’s death until their remains are discovered and examined. Establishing this timeline is a foundational goal in forensic investigations, providing a temporal framework for all other evidence. The accuracy of a PMI estimate depends heavily on the condition of the remains and the environment, utilizing biological and chemical changes that occur after death.
Estimating Time Using Body Temperature
The cessation of metabolic processes after death causes the body to gradually cool, a phenomenon known as Algor Mortis. This cooling occurs as the body attempts to reach equilibrium with the ambient temperature. While the rate of cooling is highly variable, a rough estimate suggests a temperature drop of approximately 1.5 degrees Fahrenheit (0.83 degrees Celsius) per hour during the initial phase.
This method is considered most reliable only within the first 12 to 24 hours post-death, before the core temperature nears that of the surroundings. Multiple factors significantly influence the rate of heat loss, making simple formulas imprecise. For example, a large body mass or heavy clothing acts as insulation, slowing the cooling process.
Conversely, a small body size, a cold environment, or exposure to air movement will accelerate the rate of cooling. Investigators must also account for pre-death conditions, such as fever or hypothermia, that may have altered the body’s temperature. The temperature method provides a useful, early window for the PMI.
Assessing Postmortem Changes in Tissue
Beyond temperature, two physical changes in the body’s tissues provide measurable indicators of the time elapsed since death: Rigor Mortis and Livor Mortis. Rigor Mortis refers to the stiffening of the muscles, a process driven by the depletion of adenosine triphosphate (ATP) reserves in muscle cells. Without ATP, the protein filaments remain locked in a contracted state, causing rigidity.
This rigidity typically begins in smaller muscles, such as those of the face and neck, within one to two hours. Full stiffness usually develops across the entire body in about 12 hours. The rigidity remains for roughly 12 to 24 hours before naturally disappearing as muscle proteins degrade through decomposition.
Livor Mortis, or postmortem lividity, is the purplish-red discoloration that occurs as blood settles in the capillaries of the body’s lowest-lying areas due to gravity. This pooling can start as early as 30 minutes after death and becomes most pronounced within eight to twelve hours.
Initially, the lividity is considered unfixed, meaning the color will temporarily disappear when pressure is applied to the skin. After about eight to twelve hours, the blood components become fixed in the tissues, and the discoloration will no longer blanch. The pattern of fixed lividity can indicate whether a body was moved after the initial hours of death, as the blood pooling will not shift with a change in position.
The Biological Clock: Forensic Entomology and Decay Stages
For intermediate and late PMI estimates, when early physical changes have resolved, methods focusing on decomposition and insect activity become necessary. Forensic entomology offers a reliable minimum PMI estimate by studying the life cycles of necrophagous insects, such as blowflies, which are often the first organisms to colonize a body.
The time required for an insect to progress through its developmental stages—from egg to larva, pupa, and adult—is highly predictable and dependent on temperature. Scientists measure the size and stage of the largest insect larvae found on the remains and calculate the thermal energy required for them to reach that stage.
This calculation uses accumulated degree hours (ADH) or accumulated degree days (ADD), which quantifies the total heat energy absorbed by the insects over time. By comparing the ADH/ADD required for development to the recorded scene temperatures, the entomologist estimates the time when the first eggs were laid, providing the minimum time since death.
Entomological findings are integrated with the stages of decomposition, which describe the progressive breakdown of soft tissues. The process includes:
- The fresh stage.
- The bloat stage, characterized by swelling from gas production by bacteria.
- The active decay stage, as tissues begin to liquefy.
- Advanced decay and skeletonization.
The timeline for these decay stages is heavily influenced by environmental factors such as temperature and moisture. While general timelines exist, localized environmental data are necessary to provide an accurate estimate for long-term assessment of the PMI.