Chemistry for Liberal Studies - Forensic Academy / Dr. Stephanie R. Dillon

Chemical Decomposition

By definition, death is the termination of all biological functions that sustain a living organism. In a human being, the signs of death are:

  1. No breathing
  2. No heartbeat
  3. No pupillary response to light
  4. No response to pain stimuli

We often hear the term "brain dead" being used for those persons whose breathing and circulation is being maintained by machines. Since the brain is responsible for sending the impulses to our lungs and heart to function, if it is damaged or "dead" a person is essentially dead as well. The function of the lungs and heart (circulation) simply staves off the normal decomposition that would start after complete stoppage of those systems.

So why does the stoppage of respiration and blood flow start the process of decomposition? The answer is all chemistry.

5 stages of decomposition


Immediately after the heart stops, blood stops flowing through the veins and arteries and begins to pool and spread to the lowest lying areas of the body. This gives the skin a "bruised" appearance. Investigators can use this post mortem (after death) "bruising" to make determinations about the position of the body at the time of death. This process of blood seeping into the peripheral tissues of the body is called livor mortis.

Between 3 to 6 hours after the heart stops, a chemical in the body called ATP (Adenosine Triphosphate) runs out.


ATP requires the intake of oxygen to be produced so its production ceases with death. This is an important chemical since it is responsible for the flexing of muscle:

When ATP is no longer available, the chemical myosin becomes irreversibly locked onto the muscle tissue and the muscle "locks" into place. This is called rigor mortis.

Breakdown of ATP and Cross Bridge Movement during Muscle Contraction
Biology / Medicine Animations HD (YouTube)

Time Event Appearance
After 2 hours Rigor begins eyelids, jaws stiffen
2-6 hours Rigor continues then center of body stiffens
12 hours Complete Rigor Entire body rigid
15-36 Slow loss of rigor, small muscles first Lost in head and neck, last is bigger leg muscles
36-48 Rigor disappears, muscles become relaxed Entire body limp

Also from the time the heart stops, the body starts to cool. This process is also a consequence of the lack of ATP production. ATP is the energy molecule the body uses to maintain temperature. Once the body runs out of ATP, those processes also stop and the temperature of the body begins to cool. Normally about 1.5 degrees per hour until it reaches the environmental temperature. This is called algor mortis. This rate of cooling makes several assumptions. 1) It assumes that the external temperature is below 98.6°F. In Florida and other warm temperature locations, if the environmental temperature is higher, this can mean that the loss of body heat is minimal or that the body may even rise in temperature until it equilibrates with the surroundings. 2) It assumes the body is not laying on a surface that is hyper-cooled or hyper-heated in comparison to the environment. Any of these changes can affect the rate of cooling since as we know reactions are temperature dependent. Because of these issues, body temperature, although often shown on TV as setting the time of death is really an unreliable method.

10 Things That Happen After You DIE
Alltime10s (YouTube)

Other Reactions Taking Place

As soon as blood stops flowing, the various cells of the body stop getting the nutrients they need to survive. Oxygen is no longer being taken in by the lungs so the oxygen that is in the blood at the time of death is rapidly consumed and used up by the cells throughout the body.

Normal Respiration

Normal Cellular Respiration Process


Without oxygen, the carbon dioxide (CO2) and Water (H2O) produced by normal cellular respiration accumulate and react to form carbonic acid (H2CO3) acid. As the acid in our blood increases and the higher acid content drives the pH down and acidosis occurs. Acids have the ability to destroy tissue and therefore the buildup of acid starts to break down the cellular tissues of the body. As the tissues break down (autolysis), various bacteria found in the body normally held in check by the immune system begin to grow and spread devouring the body from the inside.

Carbonic Acid Production

Chemical Production of Carbonic Acid

Certain types of flies can detect a dead body from miles away and quickly lay eggs in the soft tissue areas (mouth, eyes, open wounds, etc.) Although gross, the quick acting flies can give investigators a better time of death than many other methods if the body is several days or weeks old because the flies gestation stages are well known. The area of investigation that uses bugs to make determinations about both the time and often location of a death is called forensic entomology.

Green Bottle Fly and its complete Metamorphosis


As bacteria inside the body continue to multiply and consume tissue, the product gases of their digestion: hydrogen sulfide, carbon dioxide and methane (H2S, CO2, CH4) cause two very well-known features of death: The smell and the swelling.

The smell is predominantly caused by Putrescine, or tetramethylenediamine, and Cadaverine or 1,5-pentanediaminethe. These chemicals are produced when amino acids in the body breakdown as part of autolysis.

For a comparative for the odor think about day old fish. The amine groups in the compounds give the distinctive and rotten odor. The presence of hydrogen sulfide (H2S) just adds to the odor. For a comparison you can understand think about the smell of boiled eggs. It is the sulfur in the eggs that gives them the "rotten" smell as well but you can multiply that smell by 1000 when comparing it to the smell of a dead body. Dead fish + boiled eggs = Peeuww!

The bloating or swelling of the body is due to the pressure (CO2, H2S) build up as gases are released from the autolysis of cells as well as the bacterial digestion of tissues. The gases will continue to expand the body literally until it ruptures.

Active Decay

Once the body cavities are opened to the environment, the loss of mass is quite rapid. As bugs and bacteria continue to digest tissue it appears as if the body is "liquefying". Fluids seep away from the body and are evaporated over time.

Advanced Decay

Decomposition is severely retarded during this phase due to the lack of bodily material. Insect activity is decreased. The environment surrounding the body will show increase in soil nutrients such as phosphorous, potassium, calcium, magnesium, and nitrogen.

Dry Remains

All that remains of the body at this stage are dry skin, cartilage, and bones. This may dry out and bleach if exposed to nature.

Time line of Decomposition


One final word about death. How does an investigator preserve tissue and bodies for evidence if the processes of death are so destructive? Again the answer is in the chemistry and the kinetics of the reactions we have just discussed.

The simplest and most efficient way to stop decomposition is freezing, called cryopreservation. All bodies are kept at low temperatures before autopsy but if needed to be kept for a very long time they are kept at freezing temperatures. Tissues and organs can also be kept frozen but if the freezing process will cause too much damage and chemical analysis of the tissues is no longer needed they can be preserved in a solvent like formalin:


A saturated water solution, of about 40% formaldehyde by volume or 37% by mass, is called "100% formalin". A small amount of stabilizer, such as methanol, is usually added to suppress oxidation (reactions with oxygen in the air) and polymerization (unwanted linkages forming between molecules). Formalin preserves tissues by creating stabilizing crosslinks in proteins in the tissue:

The crosslinks prevent autolysis from occurring and therefore stop decomposition before it starts. The process of preservation is irreversible and will interfere with further chemical tests on the tissue so it should only be used after those processes are complete.