Researchers Continue to Explore How the Body May Change During Life’s Final Stages

Death is one of the few experiences that eventually touches every human life, yet many aspects of dying and decomposition remain uncomfortable or poorly understood outside scientific settings. For centuries, people have described an instinctive feeling that something is wrong when encountering certain unfamiliar smells. Modern research cannot validate ideas such as a supernatural “sixth sense,” but scientists have investigated whether the human sense of smell can detect chemical information associated with potential threats.

One particularly interesting line of research involves a compound called putrescine. Putrescine is a naturally occurring chemical compound associated with the decomposition process. As biological tissue breaks down after death, chemical changes involving bacteria and organic material produce various compounds responsible for characteristic decomposition odors. Putrescine is one of those compounds, although it would be inaccurate to describe it as the single “smell of death.” Researchers Arnaud Wisman of the University of Kent and Ilan Shrira of Arkansas Tech University investigated whether exposure to putrescine could influence human behavior in measurable ways.

Their study, titled The Smell of Death: Evidence that Putrescine Elicits Threat Management Mechanisms, was published in the peer-reviewed journal Frontiers in Psychology in August 2015. Rather than claiming that humans can literally smell when someone is about to die, the researchers examined something considerably more specific. They wanted to determine whether exposure to putrescine could operate as a chemical warning signal and activate behavioral responses associated with threat management.

The idea was partly inspired by observations elsewhere in the animal kingdom. Many species use chemical information in their environment to identify food, potential mates, predators and other possible dangers. Previous animal research suggested that odors associated with dead members of a species can produce avoidance behavior. From an evolutionary perspective, staying away from decomposing remains could potentially reduce exposure to pathogens or alert an animal to circumstances associated with predators. Wisman and Shrira wanted to investigate whether humans might show comparable responses to one chemical associated with decomposition.

To explore the hypothesis, the researchers conducted four experiments examining different behavioral responses. Importantly, the experiments did not involve exposing participants to dead bodies. Instead, researchers used controlled exposure to putrescine and compared participants’ reactions with those occurring under other scent conditions. Some exposures were noticeable, while another experiment used exposure below participants’ reported conscious awareness. The researchers then measured factors including vigilance, walking speed, threat-related thoughts and defensive responses.

The first experiment examined vigilance. Participants completed a reaction-time task after exposure to different scent conditions. The researchers reported that exposure to putrescine was associated with increased vigilance as measured by participants’ responses to the task. This finding supported their hypothesis that the chemical might activate processes connected with identifying or responding to possible environmental threats. However, this does not mean that participants consciously thought they were detecting death.

The second experiment examined actual movement rather than simply asking people how they believed they would behave. Participants were exposed to putrescine, ammonia or a neutral condition and subsequently left the experimental location. Researchers measured how quickly they walked away over a predetermined distance. Participants exposed to putrescine walked away more quickly than participants in the comparison conditions, providing evidence consistent with an avoidance or “flight” response.

A third experiment produced similar findings while also examining participants’ thoughts. Again, brief exposure to putrescine was associated with faster movement away from the experimental location. The researchers also found increased accessibility of thoughts connected with escape and threat. Taken together, the results suggested that the response went beyond participants simply considering the odor unpleasant. The researchers interpreted the findings as evidence that putrescine could function as a chemosensory warning signal.

The fourth experiment explored what might happen when the chemical exposure was subtle enough that participants did not report consciously noticing it. In that experiment, participants were exposed to a small amount of putrescine while completing questionnaires. According to the study, none reported being aware of the scent during debriefing. Researchers nevertheless observed greater defensiveness toward an out-group member in the putrescine condition, which they interpreted as evidence of increased aggressive readiness under perceived threat.

Taken together, the four experiments led Wisman and Shrira to propose that humans can process putrescine as a warning signal capable of mobilizing protective responses. The University of Kent summarized the results by reporting that brief exposure could increase vigilance and readiness for avoidance or defensive behavior. The research was notable because much previous work on human chemical threat signals had concentrated on substances transmitted through body sweat rather than a specific compound associated with decomposition.

There is an important distinction, however, between these findings and some dramatic claims that have circulated online. The study did not demonstrate that humans can predict another person’s death by smell, nor did it establish that people possess a mysterious sensory ability that tells them someone nearby is dying. It tested responses to putrescine under experimental conditions. Describing the results as proof that humans can “smell death approaching” would therefore go considerably beyond what the research established.

The study also should not be interpreted as evidence of a supernatural sixth sense. Human olfaction is a biological sensory system, even when its influence occurs without deliberate awareness. Scientists have long studied how smells can affect perception, memory, emotion and behavior. Wisman and Shrira noted that olfactory signals may alter psychological or physiological processes even when people do not consciously identify what they are smelling.

Putrescine itself should also be placed in context. Decomposition is a complicated biological and chemical process involving many substances. The odor associated with decomposing tissue does not come from putrescine alone. The 2015 paper describes putrescine as a pungent component associated with putrefying tissue. Calling the compound simply “the smell of death” makes an effective study title, but scientifically it represents only part of a much more complicated chemical environment.

The researchers discussed several possible reasons why sensitivity to such chemical information could be useful. Decomposing remains can be associated with bacteria and other biological hazards, so avoiding them could offer a survival advantage. In the natural environment, a dead animal might also indicate the presence of a predator or another source of danger. The authors therefore considered the possibility that responding to decomposition-related chemical signals could form part of a broader threat-detection system.

Still, the researchers did not claim to have completely identified why humans respond this way. They acknowledged that responses to odors can also develop through learned associations and personal experiences. Whether reactions to putrescine reflect evolutionary adaptations, learning or a combination of mechanisms requires additional research. Scientific findings from controlled experiments are valuable pieces of evidence, but they do not automatically answer every question about how the same processes operate in everyday environments.

The research is particularly interesting because humans often think of vision and hearing as their primary systems for recognizing danger. Smell receives comparatively little attention, yet chemical information constantly surrounds us. We use olfaction when evaluating food, detecting smoke, recognizing spoiled material and responding to countless environmental cues. Some of those responses happen deliberately, while others may influence behavior without becoming the focus of conscious attention.

The experiments involving putrescine provide an intriguing example of this less obvious side of human perception. Under controlled conditions, exposure was associated with increased vigilance, faster movement away from the location and other defensive responses. Those findings support the possibility that human beings retain sensitivity to certain chemical cues associated with potentially dangerous environments. They do not require supernatural explanations, and they become more interesting when described accurately rather than exaggerated.

Nearly eleven years after the study was published, it remains a fascinating demonstration of how much researchers are still learning about the relationship between smell and behavior. Future work may further clarify the biological pathways involved, how specific different responses are to putrescine, and how strongly laboratory findings translate into ordinary life. The original researchers themselves noted that identifying specific chemical warning signals could help scientists better understand the sensory and neural processes involved in detecting environmental threats.

Ultimately, the science is more nuanced than the popular idea that humans possess a hidden ability to “smell death.” What researchers actually found is that a chemical associated with decomposing tissue can influence human behavior in experimental conditions, sometimes without participants reporting conscious awareness of the scent. That distinction matters. Far from making the discovery less fascinating, it shows how sophisticated ordinary human biology can be—and how our senses may influence our actions before we consciously understand what has captured our attention.