The normal oral temperature fluctuates between 35.90 C (96.70 F) and 37.20 C (990 F) while the rectal temperature is about 0.3-0.40 C higher. Since heat production ceases soon after death but loss of heat continues, the body cools.
A living human loses heat by radiation, convection, and evaporation with conduction loss not being very significant. After death, however, heat loss by conduction may be considerable if the body is lying on a cold surface. The fall in body temperature after death mainly depends upon a loss of heat through radiation and convection, but evaporation may be a significant factor if the body or clothing is wet. The cooling of a body is a predominantly physical process which, therefore, is predominantly determined by physical rules.
Newtons law of cooling states that the rate of cooling of a body is determined by the difference between the temperature of the body and that of its environment. Consequently, a plot of temperature against time gives a curve, which is exponential. However, Newtons law applies to small inorganic bodies and does not accurately describe cooling of the human body which has a large mass, irregular shape, and is composed of tissues of different physical properties. Practical observations indicate that the cooling of a human body is best represented by a sigmoid curve when temperature is plotted against time. Thus, there is an initial maintenance of body temperature which may last for some hours - the so-called temperature plateau - followed by a relatively linear rate of cooling which subsequently slows rapidly as the body approaches the environmental temperature. The post-mortem temperature plateau is physically determined and not a special feature of the dead human body. Any inert body, which has a low thermal conductivity, has such a plateau during its first cooling phase. It is this plateau which produces the sigmoid shape of the resultant cooling curve. The post-mortem temperature plateau generally lasts 1/2 to one hour but may persist as long as three hours and some authorities claim that it may persist as long as five hours.
The linear rate of post-mortem cooling is affected by environmental factors and cadaveric factors other than the environmental temperature and the body temperature at the time of death. These include:
The size of the body - The greater the surface area of the body relative to its mass, the more rapid will be its cooling. Consequently, the heavier the physique and the greater the obesity of the body, the slower will be the heat loss. Some authors claim that in obese individuals the fat acts as an insulator, but for practical purposes body mass, whether from muscle mass or adipose tissue, is the most important factor. Children lose heat more quickly than adults because their surface area/mass ratio is much greater. The exposed surface area of the body radiating heat to the environment will vary with the body position.
Clothing and coverings - These insulate the body from the environment and therefore cooling is slower.
Movement and humidity of the air - Air movement accelerates cooling by promoting convection and even the slightest sustained air movement is significant. Cooling is said to be more rapid in a humid rather than dry atmosphere because moist air is a better conductor of heat. The humidity of the atmosphere will affect cooling by evaporation where the body or its clothing is wet.
Immersion in water - A dead body cools more rapidly in water than in air because water is a far better conductor of heat. For a given environmental temperature, cooling in still water is about twice as fast as in air, and in flowing water, about three times as fast. Clearly the body will cool more rapidly in cold water than warm water.
It is important to note that this heat loss occurs only in cool and temperate climates since in tropical regions there may be a minimal fall in body temperature post-mortem and in some extreme climates, the body temperature may even rise after death.