Unmasking the Culprits: The Science of Smell at Crime and Biohazard Scenes
Crime and biohazard scenes often come with a set of distinctive odors that are hard to forget. When a death, illness, or accident leads to the release of blood or other bodily fluids, or when a body starts to decompose, it triggers an elaborate chain of interrelated chemical processes, and the byproducts are responsible for a variety of odors.
These odors are not just unpleasant; they're often the result of complex chemical reactions and the decomposition process, making them difficult to fully eliminate without the right knowledge and tools. Moreover, they can pose significant health risks if not handled correctly. This is why cleanup of the scene is generally not a do-it-yourself job and is best left to technicians with appropriate training and equipment.
While the odors discussed in this post can be unsettling, especially to those who haven’t experienced them before, it’s worth remembering that they are a normal and inevitable part of life and death. Let's delve into just a few of the specific compounds responsible for these odors and understand their origins and impacts.
Blood and Bodily Fluids
Blood, while almost odorless when fresh, can develop a metallic smell as it ages due to the oxidization of the iron in hemoglobin [1]. In some medical emergencies, the interaction of blood with other body systems has a significant effect on the odors produced. Gastrointestinal bleeding, for example, can produce dark, tar-like stools known as melena, which has a very strong, sickly foul odor caused by the digestion of hemoglobin as it moves through the GI tract [2].
Other bodily fluids, such as sweat, urine, and vomit, release their own distinct smells due to various organic compounds:
Ammonia (NH₃) and trimethylamine (N(CH₃)₃): Nitrogen-containing compounds found in urine, these substances are released as urea breaks down. Ammonia is sharp and pungent, while trimethylamine has a ‘fishy’ odor [3, 4, 5].
Skatole (C₉H₉N) and indole (C₃H₇N): Produced in the intestine during digestion, these compounds give feces its characteristic smell. (Surprisingly, at lower concentrations, skatole and indole have a floral aroma and are sometimes used in perfumes.) [6, 7, 8]
Butyric acid (CH₃CH₂CH₂CO₂H): Though it contributes to the flavor profile of butter and cheeses (“butyric” comes from the Greek word for “butter”), this fatty acid also largely responsible for the odor of vomit [3, 9].
Decomposition
Decomposition is a more complex process that involves a multitude of chemicals, which can vary depending on environmental conditions and the stage of decomposition. These chemicals are produced by the action of bacteria and enzymes on the body's soft tissues and have a profound impact on the odor profile at crime and biohazard scenes [6].
The decomposition process itself is broadly divided into four phases: the “fresh” phase, early decay, active decay, and advanced decay [10].
“Fresh” Stage
The “fresh” phase begins within minutes after death and lasts for 1–3 days. During this stage, blood flow ceases and the body cools to ambient temperature. Gravity causes the blood to settle in lower areas of the body in a process called livor mortis (also known to healthcare workers as ‘dependent lividity’). Critically, this is when the process of autolysis (cellular breakdown by enzymes) begins. Aside from pallor of the skin, there is generally no pronounced discoloration and very little odor during this stage [10].
Early Decay
Early decay (i.e., the “putrefaction” or “bloat” stage) begins about three days after death and lasts until day 7–10. During this stage, degraded cells release nutrients that feed bacteria and other microbes. As a result of increased microbial activity, soft tissues break down into gases, liquids, and simple organic compounds. Typical observations include bloating, discoloration, sloughing of skin, and the emergence of strong odors. This stage ends with the deflation of the body [10].
Some of the most noticeable odors at this stage are caused by sulfur-containing compounds, including:
Hydrogen sulfide (H₂S): Associated with a rotten egg smell, this gas is released early in the decomposition process [6, 11].
Methanethiol (CH₃SH): Also known as methyl mercaptan, this is the same chemical additive that gives natural gas (which is normally odorless) its distinctive ‘rotten cabbage’ odor [6, 12].
Dimethyl disulfide (CH₃SSCH₃) and dimethyl trisulfide (C₂H6S₃): These compounds contribute strong garlic-like odors [6, 13, 14].
Active Decay
Active decay begins during day 7–10 and lasts until about day 20. Extensive breakdown of proteins and fats into simpler compounds leads to liquefaction of the body’s soft tissues, and this cadaveric fluid leaks out into the surrounding environment. Observations during this stage include increased insect activity, as well as continued presence of strong odors, including:
Skatole and indole: Already mentioned, these are the compounds prominent in feces [10].
Cadaverine (C₅H₁₄N₂) and putrescine (C₄H₁₂N₂): These nitrogen-containing compounds are produced during the breakdown of amino acids. As the names suggest, cadaverine has a “foul, rotting flesh” odor, while putrescine’s odor is described as “putrid and nauseating” [6, 10, 15, 16].
Advanced Decay
Advanced decay begins roughly one month after death and can continue for weeks or months. This stage includes development of the cadaver decomposition island (CDI), an area of increased soil fertility that supports plant and animal activity. Further decomposition of soft tissues leaves only the skeleton, skin, and hair behind. The advanced decay stage eventually transitions into “skeletonization,” wherein the last remnants of soft tissue decompose, leaving only the skeleton exposed [10].
Addressing the Challenge of Biohazard Cleanup
Crime and biohazard scenes frequently contain significant health hazards that are invisible to the naked eye. Depending on the manner of illness or death, infectious agents such as hepatitis B and C, HIV, and MRSA (Methicillin-resistant Staphylococcus aureus) may have been present for some time. And because of the many types of organisms involved in decomposition itself, the health risks at such scenes can be difficult to quantify.
It is important to note that some of the chemicals emitted during decomposition are toxic or can act as irritants. Hydrogen sulfide, for instance, can irritate the eyes or respiratory system even at low concentrations [11]. Therefore, crime scene and biohazard cleanup professionals use personal protective equipment (PPE), including masks or respirators, eye protection, and protective clothing, to safeguard against potential hazards.
Proper biohazard cleanup achieves not just the removal of hazardous biological materials but also the eradication of the odors associated with them. This is done through a systematic and detailed process of cleaning and disinfecting. A significant challenge can arise when blood, bodily fluids, and/or liquefied remains seep into porous surfaces such as upholstery, carpeting, wood furniture, and walls. In extreme cases, fluids may infiltrate wood framing and subfloors, or leak into lower floors or adjacent rooms through voids in walls and ceilings.
If cleanup efforts fail to identify and reach these hidden areas, noxious odors can persist and even worsen over time, even though the space appears to be clean. For these and other reasons, cleanup of biohazard scenes is best done by professionals with appropriate training and equipment.
Conclusion
The odors found at crime and biohazard scenes are a mixture of various chemicals released during the breakdown of blood, bodily fluids, and tissues. Understanding these compounds is extremely helpful for effective cleanup and remediation. Despite their pungent nature, these odors provide a window into the intricate chemistry at play between life and death—a grim but fascinating reminder of the biological processes that continue even after life has ceased.
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Citations
[1] Moran, J. K., Dietrich, D. R., Elbert, T., Pause, B. M., Kübler, L., & Weierstall, R. (2015). The scent of blood: A driver of human behavior?. PLOS One, 10(9), e0137777. https://doi.org/10.1371/journal.pone.0137777
[2] DiGregorio, A. M. (2023, June 5). Gastrointestinal bleeding. StatPearls - NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK537291/
[3] The chemistry of the smell of toilets & human waste. (2015, June 2). Compound Interest. https://www.compoundchem.com/2015/06/02/toilets/
[4] Wikipedia contributors. (2024, February 15). Ammonia. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Ammonia
[5] Wikipedia contributors. (2024, January 4). Trimethylamine. In Wikipedia, The Free Encyclopedia. Retrieved February 20, 2024, from https://en.wikipedia.org/wiki/Trimethylamine
[6] The chemistry of the odour of decomposition. (2021, October 31). Compound Interest. https://www.compoundchem.com/2014/10/30/decompositionodour/
[7] Wikipedia contributors. (2024, January 24). Skatole. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Skatole
[8] Wikipedia contributors. (2023, November 26). Indole. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Indole
[9]: Wikipedia contributors. (2024, February 9). Butyric acid. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Butyric_acid
[10] Cieśla, J., Skrobisz, J., Niciński, B., Kloc, M., Mazur, K., Pałasz, A., Javan, G. T., & Tomsia, M. (2023). The smell of death. State-of-the-art and future research directions. Frontiers in Microbiology, 14, 1260869. https://doi.org/10.3389/fmicb.2023.1260869
[11] Hydrogen sulfide. (n.d.). The National Institute for Occupational Safety and Health (NIOSH). Retrieved February 15, 2024, from https://www.cdc.gov/niosh/topics/hydrogensulfide/default.html
[12] Wikipedia contributors. (2023, October 8). Methanethiol. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Methanethiol
[13] Wikipedia contributors. (2024, January 5). Dimethyl disulfide. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Dimethyl_disulfide
[14] Wikipedia contributors. (2023, December 22). Dimethyl trisulfide. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Dimethyl_trisulfide
[15] Wikipedia contributors. (2023, December 25). Cadaverine. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Cadaverine
[16] Wikipedia contributors. (2023, December 25). Putrescine. In Wikipedia, The Free Encyclopedia. Retrieved February 19, 2024, from https://en.wikipedia.org/wiki/Putrescine