A body begins to cool immediately after death, but the time it takes to become completely cold is highly variable. This natural post-mortem change is known as Algor Mortis, a Latin term meaning “coldness of death.” The rate at which the body temperature drops is not constant, which makes estimating the time of death challenging for investigators. Understanding the physics and biology behind this phenomenon helps explain why there is no single, simple answer to this common question.
The Science of Body Cooling
The human body is homeothermic, maintaining a stable core temperature of around 98.6°F (37°C) through continuous metabolic processes. This temperature is maintained by balancing heat generation and heat loss. Upon the cessation of life, metabolic activities that produce heat, such as cellular respiration, stop almost completely.
With heat production halted, the body loses stored heat to the surroundings through three primary physical mechanisms: conduction, convection, and radiation. Conduction is the direct transfer of heat to surfaces in contact with the body. Convection is the transfer of heat to moving air or water, and radiation is the emission of infrared energy. This heat transfer continues until the body’s internal temperature reaches equilibrium with the ambient temperature.
Standard Timeline for Temperature Drop
Forensic science uses a standard rate for post-mortem cooling under typical circumstances. Following an initial plateau phase, where the core temperature remains steady for up to a few hours, the body begins a steady decline. This decline is often approximated as a linear process for simplicity in early estimations.
The average rate of cooling is commonly cited as a drop of approximately 1.5°F (0.83°C) per hour. In the first few hours after death, the drop may be slightly faster, sometimes closer to 2°C (3.6°F) per hour, before slowing down. Under standard conditions, a body is expected to take between 18 and 24 hours to cool completely and reach the temperature of the surrounding environment.
Variables That Speed Up or Slow Down Cooling
The rate of cooling is influenced by environmental and intrinsic factors that cause deviations from the standard timeline.
Environmental Factors
Ambient temperature is the most significant external factor. A colder environment increases the temperature difference, accelerating heat loss. Conversely, a body in a warm environment cools slower, or may gain temperature if the surroundings are hotter than the body itself.
Insulation and Air Movement
Insulation, such as clothing or blankets, slows the transfer of heat from the body. Thick clothing creates a thermal barrier that traps heat, lengthening the cooling process. A nude body loses heat faster due to the increased exposed surface area. Air movement, such as wind or a fan, accelerates heat loss through convection, causing the body to cool more rapidly.
Intrinsic Factors
The physical characteristics of the deceased person also play a role in heat retention. Larger, heavier bodies tend to cool more slowly than smaller bodies. This is because a larger body has a smaller surface-area-to-volume ratio, retaining more internal mass relative to the surface area for heat escape. The body’s temperature at the time of death is also a factor. A person who died with a high fever starts from a higher initial temperature, lengthening the cooling time, while a person who died from hypothermia reaches equilibrium faster.
How Cooling Relates to Determining Time Since Death
Algor Mortis is a primary tool used by forensic investigators to estimate the Post-Mortem Interval (PMI). The process involves taking a core body temperature reading, typically from the rectum or the liver, to measure internal temperature. This measured temperature is then compared against the ambient temperature and the standard body temperature at the time of death.
To translate the temperature drop into a time estimate, specialists use mathematical formulas. While the Glaister equation is an older, simpler formula, modern methods rely on sophisticated models, like the Henssge Nomogram. These models incorporate variables such as body weight, ambient temperature, and clothing to improve accuracy.
The PMI estimation is most reliable within the first 12 to 18 hours after death, while the body is actively cooling. Algor Mortis is seldom used alone to determine the precise time of death. Instead, it is used in conjunction with other post-mortem changes, such as Livor Mortis (blood pooling) and Rigor Mortis (muscle stiffening), to establish a more accurate timeline.