Algor mortis, a Latin term meaning “coldness of death,” is the process of a body cooling after death until it reaches the temperature of its surroundings. This phenomenon is one of the three primary physical changes that occur post-mortem, alongside livor mortis (discoloration from blood pooling) and rigor mortis (muscle stiffening). The body constantly maintains a stable core temperature, generally around 98.6°F (37.0°C). When life ceases, internal heat generation ends, causing the temperature to gradually drop toward equilibrium with the environment. This variable temperature change is a fundamental principle in post-mortem science.
The Physiology of Post-Mortem Heat Loss
The body’s ability to maintain its stable temperature relies on a balance between heat production and heat loss, a process known as homeostasis. Throughout life, metabolic processes within cells generate a steady supply of internal heat. Once circulatory and respiratory functions stop, the body’s metabolism quickly ceases, eliminating the source of this internal heat production. Without the brain’s thermoregulatory control, the body becomes a passive object subject to the laws of thermodynamics.
Heat moves from the warmer body to the cooler surroundings through three main mechanisms. Conduction is the direct transfer of heat through physical contact, such as when a body lies on a cold floor or metal surface. Convection involves heat transfer through the movement of air or water molecules, accelerated by drafts or wind passing over the surface.
Radiation is the emission of infrared electromagnetic waves, accounting for a substantial portion of total heat loss. Evaporation of moisture from the skin surface and mucous membranes also contributes to cooling, particularly in dry environments. This heat loss continues until the body temperature matches the ambient temperature, a point known as thermal equilibrium.
Variables That Affect Cooling Rate
The rate at which a body cools is influenced by a complex interplay of internal and external factors. The most significant external factor is the ambient temperature, as a larger temperature difference between the body and the environment leads to faster heat transfer. Air movement, such as wind or drafts, increases the cooling rate by enhancing convective heat loss across the skin’s surface. The type of surface the body rests on impacts cooling; surfaces with high thermal conductivity, like concrete or water, draw heat away more quickly than an insulating material, such as a thick blanket.
Internal factors related to the individual’s physical characteristics play a role in regulating the rate of cooling. A person’s body mass-to-surface area ratio is important; smaller individuals, such as infants, cool faster than larger adults due to their greater relative surface area. Subcutaneous fat acts as a layer of natural insulation, slowing the rate of conductive heat loss from the core to the skin. Clothing functions similarly, trapping warm air near the body and minimizing heat loss through convection and radiation.
The individual’s temperature at the moment of death provides the starting point for the cooling curve. A pre-mortem fever (hyperthermia) means the body must lose more heat to reach the ambient temperature, lengthening the total cooling time. Conversely, pre-mortem hypothermia reduces the initial temperature and accelerates the overall process. Because of these numerous variables, a single, universal cooling rate is not scientifically reliable.
Estimating Time Since Death
Algor mortis is a useful tool in forensic science for establishing the Post-Mortem Interval (PMI), the estimated time elapsed since death. To determine the core temperature, investigators typically use a long thermometer probe to measure rectally or directly within the liver, as these internal locations are less susceptible to immediate environmental changes. This measured temperature is then compared against the ambient temperature to calculate the rate of heat loss.
In the hours immediately following death, the body’s cooling often follows an S-shaped (sigmoid) curve. This curve begins with a brief initial plateau phase where the temperature drops slowly, followed by a period of more linear, steady cooling. A general guideline suggests a cooling rate of approximately 1.5°F (0.83°C) per hour during the first 12 hours, though this is a crude approximation. Modern methods use complex formulas, such as the Henssge Nomogram, which incorporate body weight, clothing, and environmental temperature for a more nuanced estimate.
Algor mortis provides an estimate, not a precise moment, of death. The utility of the temperature measurement diminishes after the body has been cooling for 18 to 24 hours, as it approaches thermal equilibrium with the surroundings. Forensic professionals do not rely on Algor mortis alone, but combine the data with observations of livor mortis and rigor mortis, and other investigative evidence, to bracket the estimated time of death.