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Death and Kinetics

In the chemistry laboratory, we can use our understanding of kinetics to both measure and manipulate the timeline of death. Specifically measuring the time of death and preserving evidence after death.

There are two general methods by which investigators determine the time of death:

  1. The Rate Method � in this method the time of death is estimated by evaluating the presence/absence of an indicator in a deceased in conjunction with the known behavior of such indicators.
  2. The Concurrence Method: � in this method the time of death is estimated by evaluating events which happen at or near the time of death, or offer information suggesting a time period for the death event.

You will note that in each type of determination the word «estimated» is used to describe the method. This is because despite all of the advancements made in science, time of death is still one of the least reliable methodologies we have. There are just too many variables that can skew results for it to be an exact science.

The first steps in determining time of death are 1) collection of the body, 2) collection of any evidence around the body and 3) collection of information about the deceased.

Let’s go through each of the methods shown in the table above and discuss how they are used to determine time of death and then in the last section we will discuss in more detail the chemistry involved so you can impress your friends with your knowledge the next time you watch CSI or Bones.

Drying Blood

Blood is a complex mixture of many types of cells, nutrients and cellular waste but predominantly it is water. As an aqueous fluid, blood will dry at a predictable rate if we know the environmental parameters: temperature, humidity, airflow, surface area etc.

Rigor Mortis

Immediately after death all of the muscles in the body relax. Slowly over the next 24 to 48 hours the body starts to stiffen (not contract but just lock in place) due to a buildup of acid in the muscle tissues. This stiffening process, called Rigor Mortis, has a roughly known time of occurrence and can therefore be used to estimate time of death. In general:

  • If the body feels warm and no rigor is present, death occurred under 3 hours before.
  • If the body feels warm and stiff, death occurred 3-8 hours earlier.
  • If the body feels cold and stiff, death occurred 8-36 hours earlier.
  • If the body is cold and not stiff, death occurred more than 36 hours earlier.

The use of Rigor Mortis as a time of death indicator is less than ideal because of the large spans of time it encompasses. The windows can vary by as much as 24 hours. There are also several factors that can severely impact the onset and timeline of Rigor:

  • temperature
  • illness
  • activity before death
  • physical conditions where the body are found
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Because the reason for Rigor Mortis is a chemical reaction, the kinetics of the reaction can be affected by the same factors as any other chemical reaction. The temperature around the body will either increase the rate of rigor (hot) or slow it down (cold). Illnesses that already cause the body to have increased amounts of acid in the muscle tissue (higher concentration) will increase the rate of rigor. Strenuous activity right before death will also increase the rate of rigor since activity also increases the amount (concentration) of acid in the muscles. Other environmental aspects can cause a change in the rigor timeline, most specifically movement of the body once rigor has set in. If the body is manipulated during the process of rigor, forced to move or bend despite being stiff, the rigor is broken and will not reform back to normal standards. This makes setting time of death for bodies that have been manipulated or dumped using the Rigor Mortis timelines harder or impossible.

Rate of Rigor Mortis

Livor Mortis

Livor Mortis (Lividity) is the settling of blood in body due to gravity. Livor Mortis starts to develop 2-4 hours after death, becomes non-fixed or blanchable up to 8-12 hours after death and fixed or non-blanchable after 8-12 hours from the time of death. In addition to the pooling of blood, small vessels breakdown throughout the body producing what is called petechial hemorrhages or Tardieu Spots.

Blanching is what occurs when you press your finger on your skin and you see a white spot for a few seconds. The lightening of the skin comes from the pressure of your finger pushing the blood away from that area for a few seconds. Lack of blood in the area means lack of color until the blood rushes back once the pressure is removed. Investigators will press their finger in an area of pooled blood to see if the area is fixed or not to further determine the time of death. This process is generally done at the crime scene.


The pooling of blood is a physical process based on the loss of blood pressure when the heart stops beating and will therefore occur at the same rate whether the temperature is cold or not, so it is less susceptible to atmosphere than rigor. But as you can see from the timeline below, Livor does not have very precise measurement of the time of death after 12 hours so it is also less helpful on bodies found days or weeks after death.

Rate of Livor Mortis

Algor Mortis

Algor Mortis is the cooling of the body after death. Normal body temperature is maintained by blood circulation. When the heart stops, circulation ceases and the body begins to cool. Normal body temperatures vary but are generally thought to have an average of 98.6 o F (37 o C). Of the methods shown to determine time of death, body temperature is probably the most common mentioned on television and in books but is by far the least reliable due to the number of external factors that can effect it:

  • Body dimensions
  • Posture
  • Clothing
  • Ambient Temperature
  • Air movement
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The general equation used to measure time of death based on body temperature is the Glaister Equation:

This means that the body drops approximately 1.5 degrees for each hour after death.

Also occurring around the time of Algor is the filming over of the eyes:

Investigators at the scene can look at the eyes to make an approximate time of death based on the cloudiness seen.


The stages of decomposition are well known and can be used as a rough guide for the time of death especially in those bodies not found for weeks or even months. While Rigor, Livor and Aldor Mortis are all listed as stages of decomposition, they all occur within 1-48 hours after death. Later stages of decomposition must be used as estimates after the 48 hour window.

There are two main ways the body decomposes: Autolysis and Putrefaction. Both of these processes take place by chemical reaction so both are subject to the typical kinetic controls of a chemical reaction.

Autolysis is the process by which digestive enzymes within the body cells break down carbohydrates and proteins.

Putrefaction is the predominant cause of tissue degradation and is due to bacterial activity. Putrefaction starts 4 to 10 days after death. Most of the appearance of a dead body over time is due to putrefaction:

  1. Bloating
  2. Green discoloration of abdomen
  3. Marbling along blood vessels-a brown black discoloration in blood vessels caused by hydrogen sulfide gas
  4. Blisters and skin slippage
  5. Loss of hair and nails

Putrefaction occurs rapidly when there is excess heat, an illness present such as peritonitis where excess bacteria were already present or in an environment where external bacteria are high like a sewer. Cooling of the body can slow putrefaction and freezing can stop it entirely.

There are four general stages of putrefaction:

  1. Putrefaction (4-10 days after death) � Autolysis occurs and gases (odor) and discoloration starts.
  2. Black putrefaction (10-20 days after death) � exposed skin turns black, bloating collapses and fluids are released from the body.
  3. Butyric fermentation (20-50 days after death) � the remaining flesh is removed, butyric acid is formed «fermenting» the remains and the body begins to mold if in contact with the ground.
  4. Dry Decay (50-365 days after death) � decay is very slow now due to lack of fluids, hair and fingernails fall out.

The degree of putrefaction allows investigators to roughly estimate the time of death based on this timeline but again you should note the broad the ranges of time. This is not an exact science which will make prosecutors very unhappy.

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Flora and Fauna

Plants and insects are often used to approximate time of death. Through observation at a crime scene it is often possible to know how long a body has been there. For example, grass that is covered by the body will slowly wilt and die. The rates by which specific species of grass die when covered is actually known and taking a sample of the grass to a botanist can give you a time of death if the person was killed at that scene or a time when the body was dumped if they were killed elsewhere.

Insects, flies specifically, rapidly infest an unprotected body and maggot formation (while gross) has well understood timeline that can be used to determine time of death. Taking samples of the maggots present to an entomologist (bug doctor) will give you time of death fairly accurately assuming the flies have access to the body immediately after death.

Stomach Contents

The food found in a victim’s stomach can give approximate time of death based on the degree of digestion that has taken place:

In Death, Our Body Feasts on Itself

A pretty morbid question perhaps, but why is it that our body does not decompose while we are alive? Most of the post-mortem changes that affect our body are the result of things we already carry inside of us, so how is it that these potent destructors are kept in check before we die?

Blood is a big part of the answer. Blood carries oxygen and glucose to our cells, and it transports waste products and carbon dioxide out. This ongoing exchange system keeps our body happy, but when we die our heart stops pumping blood around. Our cells are no longer receiving the oxygen they need to break down sugar into a usable energy source, and more importantly, the carbon dioxide produced by the cell has nowhere to go. Carbon dioxide is an acidic molecule, so the pH inside the cell starts to drop.

The inside of our cells is a little bit like Jurassic Park. Important areas are enclosed behind molecular fences called membranes. When the pH drops, some of these membranes rupture and release their content. As the famous 1993 movie taught us, when the electrified fences stop working, the dinosaurs escape. One particularly potent area inside the cell is called the lysosome. A small sac filled with dozens upon dozens of enzymes that thrive in its acidic environment, the lysosome is the stomach of the cell, and each cell has hundreds of them. When its membrane gets breached after death, its enzymes are free to roam around and start breaking down every part of the cell, digesting it from the inside out. This important component of decomposition is known as autolysis and it can be particularly swift in organs that are rich in lysosomes, like the pancreas, stomach and liver.

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Autolysis means everything inside the cell gets broken down into small and smaller molecules by these native enzymes. Proteins, the purposefully shaped chains of amino acids our body depends on, are cut down into proteoses, peptones and polypeptides before these get digested into amino acids, which are further cut down into smaller products like putrescine and cadaverine. Our fats and carbohydrates are similarly digested, and our own DNA and RNA molecules get chopped down into their individual building blocks: sugars, phosphates, and nitrogenous bases.

Another fascinating result of our body no longer pushing blood around is the temporary phenomenon known as rigor mortis. When we are alive, our muscles can contract and relax. These actions take place because of calcium moving in and out of muscle cells, a pumping action that requires energy. When we die, this energy is no longer being produced. Calcium accumulates inside muscle cells. The two filaments that bind to and release each other inside our muscles become stuck in the bound position, as they too lack the energy required to let go of each other. This is rigor mortis or a stiffening of the muscles after death, and it only goes away when the free-roaming enzymes inside muscle cells get to these muscle filaments and start munching on them.

And then there’s the issue of our bacteria. Our body has roughly as many bacteria as it has cells, and many of these bacteria live in our large intestine where they further digest our food and are known to produce flatus. When we are alive, our immune system keeps them in check. But when this system stops functioning after death, bacteria begin to feast on the products of autolysis and, unopposed, they start to move around. This is a process known as putrefaction. As they feed on our tissues, these bacteria expel gases like methane and ammonia that create the bloating frequently seen in the abdomen after death. Over the course of hours, our bacteria spread to our spleen, liver, heart and brain.

All of these destructive processes arise simply because our body is a tightly regulated ecosystem that depends on the continuous cycling of oxygen, carbon dioxide, nutrients, and waste products. When the old ticker stops, the cycle ends and the whole system falls out of balance. It also exposes the body to the action of scavengers. Some of these critters are quite small, like insects, while others are the ones we purposefully brought into our homes. On a macabre note, a book on what happens to the body after death notes that “beloved, but hungry pets will bite the hand and face that no longer feeds them.” Something to keep in mind next time Fido or Kitty looks back and forth between you and their empty bowl.

Take-home message:
-Our body does not decompose while we are alive because blood flow keeps oxygen, carbon dioxide, nutrients, and waste products moving to where they need to go.
-When carbon dioxide accumulates inside a cell, it makes it more acidic, which leads to membranes breaking down, releasing enzymes that begin digesting the cell from the inside out.
-Our bacteria are normally kept in check by our immune system, but after death, they are free to roam around and digest our tissues, a process known as putrefaction.

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Overview — Gangrene

Gangrene can occur as a result of an injury, infection or a long-term condition that affects blood circulation.

Symptoms of gangrene include:

  • red, purple or black skin in the affected area, which may be harder to see on black or brown skin
  • swelling of the skin in the affected area
  • either a loss of sensation or severe pain in the affected area
  • sores or blisters in the affected area that bleed or produce a foul-smelling pus

You should see your GP immediately if you’re worried you may have gangrene.

Who’s affected

Anyone can develop gangrene, particularly after a serious injury, but there are certain groups of people who are more at risk.

These include people with long-term conditions that can affect the blood vessels, such as:

  • diabetes – a condition that causes a person’s blood sugar level to become too high
  • atherosclerosis – where the arteries become clogged up with a fatty substance called plaque, narrowing them and restricting blood flow
  • peripheral arterial disease – where a build-up of fatty deposits in the arteries restricts blood supply to leg muscles
  • Raynaud’s – where blood vessels in certain parts of the body, usually the fingers or toes, react abnormally to cold temperatures

How gangrene is treated

The earlier treatment for gangrene begins, the more successful it’s likely to be. The main treatments include surgery to remove damaged tissue, known as debridement, and antibiotics to treat any underlying infection.

In some cases, surgery may be needed to restore blood flow to the affected area.

In more severe cases, it may be necessary to remove an entire body part such as a toe, foot, or lower leg. This is known as amputation.

Preventing gangrene

Many cases of gangrene can be prevented.

If you have a condition that increases your risk of getting gangrene, such as diabetes, it’s important you have regular check-ups to assess the state of your feet. Report any problems to your GP as soon as possible.

Stopping smoking if you smoke and adopting a healthy lifestyle, with a low-fat diet and regular exercise, can also improve your circulation and reduce your risk of developing gangrene.

Page last reviewed: 29 September 2022
Next review due: 29 September 2025

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