[sherwingoff]

Time of death: How long after a person’s death does it take for a human body to begin to decompose (begin to corrupt)? What are the steps—the signs in this natural process after a person has died?

[Xrayman]

As soon as the heart stops and oxygen is no longer being used by muscle, decomposition starts.

[Fr_Chuck]

This web site describes it in fairly well detail.

** it is somewhat graphic
Decomposition: Stages of Decomposition

[Xrayman]

Here is an albeit long winded, but well described explanation.


also I like the link above^^ provided by Fr_Chuck!



The Indicators




When a body is discovered (as opposed to a set of bones), the first procedure is to identify it via such things as its gender, age, dental features, blood factors, scars, fingerprints and personal effects. Whether that's successful, the time of death must be established as soon as possible after discovery. It is one of the central factors in any murder case and can eliminate suspects, break alibis, or place victims clearly with a suspect. The sooner after death this is established, the more accurate it is.


A medical thermometer (AP)
At the site of the crime the coroner or medical examiner makes an informed guess at the approximate time when the individual expired, because it will then be subject to photographs, transportation, and other delays, and over that period there will be changes in the corpse. At the very least, the ME should take a temperature reading with a rectal thermometer and examine body coloration and eye fluids.

Time of death estimation is based on a variety of changes following death, because they generally proceed in a predictable order. None are wholly reliable, since all are affected by diverse factors, but taken together, they can provide a good estimate. These include:



Food digestion
As in the Lynne Harper case, an examination of her stomach contents, along with the knowledge of exactly when she had eaten her last meal, placed the time of death approximately two hours after eating. Although this was based on an assumption that the stomach digests food and empties into the intestines at a predictable rate, in fact many things can influence this process. The type of food, the body's metabolizing rate, the presence of drugs or medication, and the person's emotional condition prior to death may all have some effect on how fast food is processed. Even exercise right before death can slow it down, and the amount consumed. A light meal may remain about two hours, a heavy meal from four to six hours. Examination of the small intestine is also done to trace the path of the food.
Body temperature (algor mortis)
After death, the body is no longer taking in oxygen for maintaining its normal temperature of 98.6 degrees Fahrenheit, so it begins to cool at the rate of approximately one to one and one-half degrees per hour until it takes on the temperature of the surrounding medium (air, water, dirt). A rule-of-thumb formula that some forensic personnel follow is: 98.6 – rectal temperature, divided by 1.5 = approximate number of hours since death

However, several factors can affect this. If the weather is cold, the temperature may drop more quickly than on a warm day, but an obese person tends to cool less quickly than someone with little body fat. Heavy layers of clothing can act as insulation, and some drugs raise the body temperature prior to death.
Discoloration (livor mortis)
Also known as postmortem lividity or hypostasis, this refers to the dark purple color of the body that is found closest to the ground where it's lying. It appears about one to two hours after death and becomes fixed within eight to ten hours. Lividity is caused by blood darkening and settling into the lowest parts because the heart has stopped pumping it through the body. The white and red cells no longer mix and the red cells settle to the bottom. Only the parts of the body in contact with something that causes pressure will prevent the blood from settling there. A person found lying on her back will exhibit lividity on her back. If lividity instead appears on the front or one side, that means that the corpse has been moved since death. Skin that is discolored but blanches when touched indicates that lividity is not yet permanent, and that helps to indicate that the death was more than two hours earlier but probably not as long as ten.

Some deaths will have a different appearance. Carbon monoxide poisoning, for example, keeps the blood a bright red color, and bodies that have lost a lot of blood will not discolor.
Rigor mortis
Right after death, the body goes limp, but within fifteen minutes to fifteen hours, the muscles begin to stiffen from accumulating waste products. The average amount of time for this to begin is two to three hours after death. It first shows in the face, lower jaw and neck, and over the next twelve to eighteen hours spreads throughout the body. The rigid muscles contraction can last up to thirty-six hours. Then the stiffness disappears, beginning again in the head and neck area. It may take as long as ten hours from start to finish for the corpse to be entirely flexible again.

Factors that affect the onset and release include the presence of heat, which speeds it up, and differences in musculature. It is also the case that not all obese people develop this stiffness, and not all muscle rigidity is actually from rigor. "Cadaveric spasm," which is an immediate stiffening of the hands and arms, is the effect of extreme agitation or tension just before death. A suicide gripping a revolver, for example, is not necessarily in a state of rigor. It has happened that valuable trace evidence, such as a suspect's hair, was found in a victim's tightly-clenched hand.
Decay/decomposition
Immediately upon death, microorganisms that live in the body go to work to dissolve the internal organs. They produce gas, which bloats the body before it eventually escapes. The face darkens and liquids escape the nose and mouth. The tongue swells and the abdomen begins to turn a greenish-yellow color. Eventually the skin blisters and fills with fluid or gas. If the weather is warm and humid, putrefaction may set in within a day, but when left in a very cold area or storage space, may be retarded for several months.

If left in a place where flies can get to it, the body becomes host to maggots, and the larval stages can also help to determine time of death—although shifting climatic conditions can affect the developmental progress by several hours. (An elderly woman's death in England had initially been determined to have taken place on a certain day, but a study of how the heating controls in her home had extended the stages of larval development revised the date to an earlier time—coinciding with the presence of the suspect in her area. He had an alibi for the initial estimate, but not for the final one.) Different species of insects can help determine if a body has been moved from one location to another, and some lay eggs only during daylight hours.

Bodies found in a moist area may develop a waxy substance from the decomposition of body fat, known as adipocere. It makes the body float in water and can preserve the internal organs. (In Scotland, the bodies of two children found in a river nearly two years after their death were covered in adipocere; as a result the contents of their stomachs were still fresh enough to determine their last meal, which lead right to the killer---their father.)
Very dry air may produce mummification, in which the body tissues dry and harden.
Ocular
If the eyes remain open after death, a thin film forms on the surface. The potassium content from the breakdown of red blood cells enters the eyes and within two to three hours, they look cloudy. Eyes that are closed develop the same conditions, but it takes much longer; the cloudiness may not occur for an entire day. This process is not affected by the ambient temperature and some pathologists think that it's a more reliable measure of time of death than the other five.
Personal factors
Aside from indicators in the corpse, other factors during the investigation may also play a part in estimating time of death. If someone was last seen or heard from at a certain time, then obviously that person was not dead. If he or she had an appointment and failed to show up, that also helps. Lynne Harper was seen on a road at a certain area just after 7:00 p.m. and a boy coming along that same road shortly thereafter saw no one.

[J_9]

I am sorry I did not read all of Xrayman's response, it was just a little long winded for me in my state of exhaustion. But here in Tennesse we have what is called the Body Farm. It is a place where people donate their bodies to science and we can determine under what conditions bodies begin to decompose.

In short,

A body farm is used in the study of forensic anthropology: the study of human decomposition that occurs after death. Various forensic disciplines benefit from the studies done through a body farm, among them law enforcement, medical examiners and crime scene investigation. The research assists examiners in developing a better understanding and developing better methods of determining what the actual time of death was. The Federal Bureau of Investigation holds training courses at the University of Tennessee Body Farm, in order to expose agents to crime scene simulations where they have to dig up bodies. There are two such facilities in the United States.