Welcome or not, dying is a natural part of the circle of life. Death initiates a complex process by which the human body gradually reverts back to dust, as it were. In the language of forensics, decomposition transforms our biological structures into simple organic and inorganic building blocks that plants and animals can use.

Four main factors affect the pace and completeness of decay. The most important is temperature: the rate of chemical reactions in a cadaver doubles with each 10 degree Celsius rise. Humidity or water from the environment buffers those reactions, slowing their effects. Extreme acidity or alkalinity hastens how quickly enzymes degrade biological molecules—although again, the presence of ample water can mediate the effects. Finally, anything that blocks exposure to oxygen, such as burial, submersion or high altitude, will slow decomposition. Depending on the interplay of these four factors, the body can turn into a skeleton as rapidly as two weeks or take more than two years.

Forensic scientists use their knowledge of the biology and chemistry of decomposition, together with the variables that affect the speed of decay, to estimate a person’s time of death and to help investigators discover clandestine graves. Medical experts and ethicists may not agree on how to define the moment of death [see “When Does Life Belong to the Living?” by Robin Marantz Henig], but they know in great detail the stages through which a body gradually decomposes. The stages are described below. The timescales noted are approximate and refer to a body that is lying open to the air. Being buried unshrouded in soil or in a casket could extend the intervals significantly.

Stage 1: FRESH - Days 1 to 6
In the first stage, soft tissue begins to decompose in a chain of events that starts with autolysis, or self-digestion. When breathing and circulation cease, cells are left without a supply of oxygen. The cells survive for a few minutes to a few days, but they can no longer pass wastes into the bloodstream. Carbon dioxide, one of the by-products of metabolism, is acidic, and as it accumulates, the acidity inside a cell increases, causing cell membranes to rupture. Single membranes surrounding organelles called lysosomes tend to dissolve first. The sacs contain digestive enzymes normally used by cells to break down organic molecules such as proteins. As these enzymes spill out, they begin digesting the cell from the inside out, eventually creating small blisters in and on internal tissues and organs and on the skin. The blister fluid, consisting of digested cell innards, is rich in nutrients.

As blisters rupture, the fluids give the surface of the corpse a moisture-laden sheen. Deep skin cells begin to slough off, resulting in skin slippage, one of the first visually revolting signs of decomp­osition.

Within a few hours after death, several other phenomena also begin. Muscles stiffen (rigor mortis), starting in the eyelids, jaw and neck, when cells no longer pump out calcium ions; such pumping keeps muscles supple. For a time, muscle cells continue to convert nutrients into energy, but without oxygen the process produces lactic acid, which also causes muscles to contract. Gelling of the cell’s innards, resulting from increased acidity, contributes to the stiffening. Rigor mortis peaks in 24 hours but then relaxes as cells succumb to autolysis.

The body also starts to cool (algor mortis) to ambient temperature, generally at approximately 0.8 degree C per hour. Algor mortis can of course be influenced by the body’s location and size, clothing and weather conditions.

Within an hour or two of death, the pull of gravity makes red and white blood cells settle (livor mortis), gradually giving a purplish-red hue to the epidermis, except in areas that are being compressed, such as skin in contact with the ground. Maximum congealing takes place at six to 12 hours. Marbling occurs after several days as blood and proteins begin to decompose and liberate sulfur-rich compounds, giving the corpse one of its offensive odors.

Stage 2: BLOAT - Days 7 to 23
After about a week, the release of those nutrient-rich fluids begins to fuel an army of microbes that further liquefy the body’s soft tissue. Bacteria, fungi and protozoa (from the corpse and from the environment) attack the tissue, producing numerous gases, including carbon dioxide, methane, hydrogen sulfide, ammonia, and a variety of so-called volatile organic compounds such as benzene. Because the greatest concentration of microbes in the body is in the intestinal tract, the most obvious bloating, or distension, occurs there. Trapped gases can eventually erupt from the rectum or even rip apart the abdominal wall.

Stage 3: ACTIVE DECAY - Days 24 to 50
During this stage, insects (primarily maggots and beetles) and sometimes carnivores join microorganisms in removing the remaining traces of tissue. Much of the body’s muscle and fat has been reduced to a foul-smelling, liquidy pastelike substance. If the tissue has been open to air (aerobic conditions), it will have a pH greater than 9.0, highly basic (7.0 is neutral). If the corpse has been buried so that anaerobic (oxygen-free) conditions prevail, the body will be acidic (less than 7.0). The more extreme the pH, the quicker the decomposition.

If conditions are basic and also warm and moist, lipids (primarily triglycerides) will go through a chemical reaction called saponification that creates adipocere, also known as grave wax. (The reaction is the basis for how commercial soap is made from animal fat.) Adipocere can range in color from whitish to dark yellow, with the occasional brown chunk here and there. It can also have a variety of consistencies, from hard and crumbly if decomposition has progressed rapidly to soft and pasty for slower decay. If grave wax covers decomposing tissue, it will create an anaerobic environment and shield the tissue from its surroundings, retarding the process and potentially delaying complete liquefaction at that site for years.

Stage 4: DRY - Days 51 to 64
In the dry stage, the last traces of tissue are removed, leaving the human skeleton. Odors and disfigurement are largely gone. Bones then go through their own decomposition process, called diagenesis, which can last years to decades. Bone has two components: protein (collagen) and a mineral, hydroxyapatite. Protein degrades first, which leaves the remaining skeletal material susceptible to cracking and flaking. Once the protein is gone, freezing and thawing, moisture, carnivores and erosion will break it down into dust. But if the bones lie in soil that is very dry and contains certain minerals, the minerals can fill in the cracks and voids, bonding the hydroxyapatite and allowing the combination to fossilize and survive the ravages of time.

Time of Death?
Forensic scientists like myself study decomposing bodies to improve our methods for accurately determining how long someone has been dead and for finding clandestine graves. We have identified more than 400 chemicals released during decomposition that give us clues for both tasks. My laboratory has also created an electronic handheld instrument (called Labrador) that can detect many of these compounds. About 30 of the chemicals, when identified together, provide very good evidence that hidden human remains have been found. Among them:

Freons. These molecules are similar to the coolant in your refrigerator or air conditioner and accumulate (in ­inert form) in tissue and bone matrixes during a lifetime of ingesting fluoridated water or products such as toothpaste.

Aromatic Hydrocarbons. Human decomposition has a unique, sickly sweet odor, largely created by aromatics such as benzene, an important component of gasoline.

Sulfur compounds. The same dimethyl disulfides and hydrogen sulfides released
by decaying vegetation in swamps and bogs contribute a rotten-egg smell.

Carbon Tetrachloride. Created by bacteria during decomposition, this nasty compound was once used in fire extinguishers, as a dry-cleaning solvent and in making chlorofluoro­carbons (which partially destroyed the ozone layer). It is now banned from most applications because it is highly toxic and can even cause cancer. How ironic that after a lifetime of health-conscious living, we revert to this and other known carcinogens.