Algor mortis, a Latin term meaning “coldness of death,” refers to the postmortem reduction of body temperature. This phenomenon is one of the primary physical changes that occur after death, alongside livor mortis (discoloration from blood pooling) and rigor mortis (muscle stiffness). The process of algor mortis causes the internal temperature of the deceased to gradually fall until it matches the temperature of the surrounding environment. Observing and measuring this cooling process is used in forensic science to help estimate the time since death occurred.
The Immediate Start of Algor Mortis
The process of algor mortis begins immediately upon somatic death, which is the point at which the body as a whole ceases to function. For a living person, the body’s core temperature is maintained through a complex balance of heat production and heat loss. Heat is continuously generated by cellular metabolic processes throughout the body. The moment the heart stops beating and respiration ceases, the body’s internal heat-generating engine shuts down. The continuous metabolic activity stops abruptly, instantly initiating a thermal imbalance, and the body shifts from actively regulating a steady temperature to passively losing heat to its cooler surroundings.
The Mechanics of Postmortem Heat Loss
The deceased body loses its internal heat through three primary physical mechanisms of thermal transfer until it reaches thermal equilibrium with the environment.
Conduction
Conduction is the transfer of heat through direct physical contact. For example, a body lying on a cold concrete floor will lose heat quickly to that surface because the heat energy is passed directly from the warmer skin to the cooler floor.
Convection
Convection involves heat transfer through the movement of air or fluid over the body’s surface. A body exposed to a breeze or a fan will cool faster because the moving air constantly carries away the layer of warm air that would otherwise insulate the skin.
Radiation
The third process is radiation, which is the emission of infrared energy from the body to cooler surrounding objects or air without any direct contact. A body can radiate heat to a cold wall or a cold ceiling, even if it is not touching them, because heat naturally moves from a warmer object to a cooler one in the form of electromagnetic waves.
Variables That Influence Cooling Speed
The rate at which a body cools is highly variable and depends on a number of surrounding and internal factors.
The most significant external influence is the ambient temperature of the environment; a greater temperature difference between the body and the air will result in a faster cooling rate. Conversely, a body in a hot environment may not cool at all, or may even increase in temperature until it matches the surroundings.
Insulation also plays a large role in slowing heat loss, with clothing, blankets, or even a thick layer of body fat acting as barriers. A fully clothed individual cools much more slowly than an unclothed one because the layers trap the heat and reduce the effects of convection and radiation.
Body composition is another internal factor. Larger individuals with greater body mass and a thicker layer of subcutaneous fat possess more thermal insulation and cool at a reduced rate compared to thinner individuals. The ratio of body surface area to mass also affects cooling, where a body with a greater relative surface area loses heat more quickly.
The body’s temperature at the time of death is a factor. A person who died with a fever will have a higher starting temperature, which prolongs the total time required to reach ambient temperature. A person who was hypothermic at the time of death will cool much faster because the initial temperature differential is smaller.
Determining Time of Death
The measurement of algor mortis is a standard procedure in forensic pathology used to estimate the postmortem interval (PMI). This estimation typically involves measuring the core body temperature, most commonly taken from the rectum or the liver. The standard cooling rate often cited is a drop of approximately 1.5 degrees Fahrenheit (or about 0.83 degrees Celsius) per hour under average conditions.
Forensic investigators use mathematical models to convert the temperature reading into an estimated time of death. However, these calculations are based on the assumption of a normal starting temperature and a stable environment, which is rarely the case. The high number of variables makes algor mortis an imprecise tool.
The estimation is considered most reliable only within the first 12 to 18 hours after death. Once the body’s temperature approaches the ambient temperature, the rate of cooling slows significantly, and the temperature curve flattens out. After this period, the temperature difference becomes too small to provide an accurate estimate of the time of death.