The “Metallic Blood” Compound

Researchers have identified a specific chemical called trans-4,5-epoxy-(E)-2-decenal that gives mammalian blood its distinctive metallic odor. This single compound is so powerful that it can trigger the same behavioral responses in predators as real blood.

Studies with captive large carnivores—including Asian wild dogs, African wild dogs, and even Siberian tigers—showed remarkable results. When presented with wooden logs containing either real blood odor or just this single chemical compound, the animals displayed intense interest, sniffing, licking, and even guarding the scented objects as if they were protecting actual prey.^[1,2]

One chemical can be as effective as real blood for triggering predator responses

The Clock is Ticking: Chemical Signatures of Blood Change Over Time (Aging and Decomposition)

The First 48 Hours: A 2025 study by Whaley et al. profiled blood scent in detail during this critical window. A “fresh” profile, from 1 to 4 hours old, is dominated by aromatics and hydrocarbons. However, as decomposition begins, this signature evolves. An “aged” profile starts to emerge around 32 to 40 hours, characterized by a different set of chemicals, primarily aldehydes and ketones.

Research indicates multiple, distinct odor profiles or stages of decomposition based on VOC analysis: Fresh, Intermediate (or transitional mixture), and Aged

The Following Weeks: Research from Rendine et al. (2019) shows that the profile continues to change even after this initial period. In later stages of decomposition, around day 7 and beyond, sulfur compounds like dimethyl disulfide and carbon disulfide begin to appear, adding yet another facet to the scent picture.

This chemical transformation has profound, practical implications for training. The Whaley et al. study included canine trials that produced a startling finding:

Canines trained on older blood had difficulty detecting the fresh blood (1–2 h old) and had the greatest detection rate for the aged blood (34–36 h old).

This is a critical lesson for every handler. A dog trained exclusively on aged tracks may not recognize the chemical signature of a fresh one. To be truly effective, a detection dog must be trained on scents of various ages to learn the full spectrum of the target odor’s evolution.
This could explain some phenomenon about how some K9’s have little interest in some tracks and or blood. Also worth noting, if a deer is still alive, the blood is now in that fresh range.

(Investigation of Distinct Odor Profiles of Blood over Time Using Chemometrics and Detection Canine Response
Fantasia Whaley 1 , Valerie Albizu 1, Jordi Cruz 2 , Rushali Dargan 1 and Lauryn DeGreeff 1,*)

Chemical Stability (VOCs): Human blood, whether fresh or decaying, is a source of VOCs

These volatile molecules are released into the environment because of their physicochemical properties, such as low molecular weight and high vapor pressure. The decomposition process of blood involves a continuous continuum of chemical changes, generating a detectable scent signature rather than simply disappearing.

Survival Against Cleaning: Even if a criminal attempts to wash away biological material, trace evidence that appears invisible to the naked eye may still be detectable. In a study using cotton swatches, presumptive chemical tests like luminol remained effective in detecting blood washed up to five times with standard household detergent. Luminol’s effectiveness demonstrates that blood components persist even through aggressive cleaning cycles

(Investigating the detection limits of scent-detection dogs to residual blood odour on clothing
https://doi.org/10.1016/j.forc.2018.05.002 2468-1709/
2018 Elsevier B.V. All rights reserved.)

(Decomposing Human Blood: Canine Detection Odor Signature and Volatile Organic Compounds*
Marcello Rendine,1 D.B.A.; Carmela Fiore,2 M.D.; Giuseppe Bertozzi,1 M.D.; Dania De Carlo,1 M.D.; Vera Filetti,3 M.Sc.; Palmira Fortarezza,2 M.L.T.; and Irene Riezzo,1 M.D., Ph.D.)

Detection of Extremely Small Quantities

Research has experimentally demonstrated a canine’s capability to locate minute quantities of human blood. In one study, a detection canine successfully located extremely small quantities of aged blood—specifically, approximately one mg (two blood drops) deposited on carpet squares and aged for weeks—with no false alerts (D57 Use of Canines to Detect Dried Human Blood and Instrumental Methods for the Determination of Odor Profilesm Lauryn DeGreeff, PhD)

Another study confirmed dogs’ potential as screening tools, detecting blood volumes as small as 0.1 mL up to 32 hours post-deposition (A.G. Skalleberg, M.M. Bouzga, Detecting and collecting traces of semen and blood from outdoor crime scenes using crime scene dogs and presumptive tests, Forensic Sci. Int. 264 (2016) 146–152)

The Power of Blood’s Vapor Pressure (The Chemical Signal)

Blood, particularly as it degrades, continuously releases Volatile Organic Compounds (VOCs). These VOCs are the molecules that travel through the air to form the detectable scent.

For a layperson, the significance of this powerful vapor release can be summarized in three points based on the physical properties of the compounds:
1. Volatility: The Ability to Travel: VOCs are volatile because they have physicochemical properties such as low molecular weight, high vapor pressure, and low boiling point.
◦ High Vapor Pressure means these molecules readily turn into gas and escape into the surrounding environment, allowing the scent to propagate away from the source (the bloodstain).
◦ This constant release of VOCs means that the scent of blood is a persistent trail rather than a fixed presence, enabling a dog to detect it even when far from the source.

2. Constant Emission During Decomposition: Blood does not simply dry up and become odorless; rather, the process of decomposition is dynamic and continuous. The breaking down of the blood’s components (proteins, lipids, etc.) by processes like autolysis and microbial action generates a continually evolving stream of VOCs.
◦ In later stages of decomposition (e.g., Day 7 onward), highly potent compounds like sulfur compounds (Dimethyl Disulfide and Carbon Disulfide) appear. These specific decomposition markers, which are associated with enhanced canine detection performance, dramatically amplify the power and distinctiveness of the vapor signature.
3. Detectability Despite Dilution (The Metallic Smell): Even fresh blood has a powerful, chemically distinct signature. For humans, the characteristic “metallic, blood-like” odor is strongly associated with the volatile compound trans-4,5-epoxy-(E)-2-decenal.

Humans are extremely sensitive to this odorant, with an extraordinarily low detection threshold for it, measured in parts per trillion (ppt). The existence of such a potent, low-threshold compound indicates that the chemical signal from blood is intrinsically strong and detectable even when highly diluted in the air.

Visualizing Detection Limits (The Olympic Pool Analogy) – PPM conversion for 0.1 mL of blood vaporized in an Olympic pool, we can use the reported parts-per-trillion (ppt) sensitivity of the key blood odorant (trans-4,5-epoxy-(E)-2-decenal) to illustrate the incredible scale of detection.

A typical Olympic swimming pool holds approximately 2.5 million liters of water.

The most potent blood odor compound, trans-4,5-epoxy-(E)-2-decenal, can be detected by humans at concentrations as low as 0.078 ppt. This means that the chemical signal is effective at concentrations involving only a fraction of a microliter spread throughout the entire volume of an Olympic swimming pool.

Since the canine olfactory system is 10,000 to 100,000 times more sensitive than the human nose, the minimal concentration detectable by a dog would be far lower than 0.078 ppt.

If a dog can reliably detect a 0.1 mL volume of blood (100 $\mu L$) on a surface, the actual concentration of VOCs it is responding to in the surrounding air is likely exponentially lower than 1 ppt, demonstrating the power of the vapor pressure and the canine’s ability to locate a signal that is diluted to an almost unimaginably faint chemical signature.

Decomposing Human Blood: Canine Detection Odor Signature and Volatile Organic Compounds* Marcello Rendine,1 D.B.A.; Carmela Fiore,2 M.D.; Giuseppe Bertozzi,1 M.D.; Dania De Carlo,1 M.D.; Vera Filetti,3 M.Sc.; Palmira Fortarezza,2 M.L.T.; and Irene Riezzo,1 M.D., Ph.D.
Investigation of Distinct Odor Profiles of Blood over Time Using Chemometrics and Detection Canine Response Fantasia Whaley 1 , Valerie Albizu 1, Jordi Cruz 2 , Rushali Dargan 1 and Lauryn DeGreeff 1,*

Following a Feeling: Your Dog Can Literally Smell Fear

We have long suspected that dogs are attuned to our emotions, but science now demonstrates they can detect our feelings through their noses. A groundbreaking 2024 study by Kiiroja et al. confirmed that trained dogs can discriminate between breath samples taken from people in a calm state versus a stressed state with approximately 90% accuracy.

The dogs are likely keying on the distinct VOCs produced by our two main stress-response systems:

• The SAM axis: This is the fast-acting “fight-or-flight” system that floods the body with adrenaline. In the study, one dog’s detection performance correlated specifically with the human donors’ self-reported feelings of fear.

• The HPA axis: This is a slower, more sustained system that releases glucocorticoids like cortisol. The other dog’s performance in the study correlated with the donors’ self-reported feelings of shame.

This finding is profoundly important for tracking and search-and-rescue work. A dog on the trail of a missing person or a fleeing suspect isn’t just following a generic human scent. They may be specifically locked onto the chemical signature of that person’s fear, distress, or shame. This emotional scent is an operationally relevant cue, providing a powerful and specific target for the dog to follow in high-stakes situations.

It is highly plausible to draw the conclusion that specialized canines could be trained to track wounded or dying deer by generalizing based on the hormonal or physiological indicators present in the blood and other scent traces left on the trail.

This plausibility rests on three primary points supported by the sources: the demonstrated ability of canines to detect volatile compounds related to stress hormones, the established link between hormones and scent secretions in deer, and the innate predatory instinct to follow the odor of blood.

1. Canine Capability to Detect Stress-Related Chemical Signatures
The ability of canines to detect human emotional states linked to hormones establishes a strong foundation for tracking wounded animals:
• Detection of Stress VOCs: Dogs can detect putative Volatile Organic Compounds (VOCs) associated with elevated human stress levels from sources like breath and sweat. This is linked to the endocrine stress response, specifically involving the Sympathetic-Adreno-Medullar (SAM) axis (adrenaline, noradrenaline) and the Hypothalamic-Pituitary-Adrenal (HPA) axis (glucocorticoids like cortisol).

• Generalization of Stress: Canines are specialized biological devices capable of olfactory generalization, meaning they spontaneously learn to respond to variations of a target odor by learning their common properties. Studies confirmed that dogs can generalize target stress volatiles across different individuals and different stressful events. This mechanism suggests a dog trained on the “scent of physiological distress” would be able to recognize it in a novel context, like a wounded deer.

• Blood as a Vehicle for Distress Signals: While stress hormones themselves (like cortisol) are complex analytes, the physiological state they induce results in volatile byproducts. Studies in rats showed that they avoid paths covered with spilled blood from a stressed rat, but not a non-stressed rat, demonstrating that stress-related chemosignals contained within blood influence behavior. A wounded or dying deer would be in a state of extreme stress, releasing these detectable chemosignals.

• Olfactory Sensitivity to Blood: Carnivores’ hunting behavior suggests they use the odor of blood to home in on wounded prey. The scent of mammalian blood, including its character impact compound (trans-4,5-epoxy-(E)-2-decenal), elicits significantly more interactions (sniffing, biting, toying) from large carnivores (including wild dogs and tigers) than non-prey odors.

2. Hormonal Influence on Deer Scent Trail
Deer naturally use scent profiles that are directly tied to their hormonal status and physiological health, making the idea of detecting a distress signal plausible:

• Scent Reflects Status: Deer rely on olfaction and deposited semiochemicals to facilitate communication among conspecifics. These secretions reflect hormonal activity, particularly dominance and reproductive status.

• Hormonal Control of Secretions: In white-tailed deer, the interdigital gland (which leaves the olfactory trail) secretes volatile compounds whose production is under hormonal control. For example, the presence of androgenic hormones (like testosterone) increases sebaceous gland size and sebum production, leading to differences in concentrations of volatile compounds between dominant and subordinate males.

• Scent of Wounding/Distress: Since deer scent profiles reflect normal hormonal fluctuations (e.g., testosterone-driven dominance cues), it follows that an extreme and sudden physiological change, such as severe wounding or imminent death, would drastically alter the volatile profile released in sweat, breath, and glandular secretions along the trail. This stressed profile could combine with the odor of fresh or decomposing blood.

Integration of Multi-Sensory Data

Canines combine blood traces, pheromones, and environmental signals to form a comprehensive scent profile, improving tracking accuracy.

Environmental Scent Markers

Tracking relies on cues like crushed vegetation, disturbed soil, and volatile organic compounds (VOCs), all of which dogs interpret to follow a trail.

Hormonal Indicators of Stress

Dogs detect human stress hormones like cortisol, which closely resemble those found in wild animals, enhancing their ability to locate targets under pressure.