Blood has a distinct smell, a common observation. This characteristic odor results from complex chemical interactions. Understanding why blood has a particular smell involves exploring the specific compounds and reactions that occur when it is exposed to the environment.
The Metallic Scent of Fresh Blood
The familiar “metallic” or “coppery” scent of fresh blood arises primarily from the iron within hemoglobin, a protein found in red blood cells. This smell is not from elemental iron itself, as metals do not typically release enough volatile molecules at room temperature for direct olfaction. Instead, the metallic odor is a product of specific chemical reactions that occur when blood is exposed to the environment.
When iron-containing compounds in blood interact with oxygen in the air, particularly in contact with lipids on the skin, a chemical process unfolds. This interaction triggers the decomposition of lipid peroxides, leading to the release of certain volatile organic compounds (VOCs). One key compound identified as a significant contributor to this metallic scent is 1-octen-3-one. This molecule can be detected at very low concentrations and is described as having a fungal-metallic or mushroom-like aroma.
Another potent compound implicated in the characteristic metallic smell of mammalian blood is trans-4,5-epoxy-(E)-2-decenal (E2D). This aldehyde is highly sensitive to our olfactory system and can be perceived at extremely low levels. The generation of these specific volatile compounds stimulates our olfactory receptors, resulting in the distinct metallic sensation.
How Blood’s Odor Changes Over Time
As blood leaves the body and is exposed to the air, its chemical composition begins to change, leading to an evolution in its scent. The initial metallic smell of fresh blood gradually gives way to different odors as it dries and undergoes decomposition. This transformation is influenced by various environmental factors, including oxygen exposure and the activity of microorganisms.
Microorganisms, primarily bacteria, play a significant role in altering blood’s smell over time. These microbes break down the complex organic compounds found in blood, such as proteins and lipids, through processes like putrefaction. This breakdown releases a new array of volatile organic compounds (VOCs) that differ from those responsible for the fresh metallic scent.
The decomposition of blood by bacteria generates distinct and often unpleasant odors. For instance, the breakdown of proteins produces biogenic amines like cadaverine and putrescine, which contribute a characteristic rotting fish or meat-like smell. Other sulfur-containing compounds, such as hydrogen sulfide (smelling like rotten eggs) and methanethiol (reminiscent of rotting cabbage), also arise from the degradation process.
Further decomposition can yield dimethyl disulfide and trisulfide, which have a foul or rancid garlic-like odor and attract insects. Indole and skatole, byproducts of amino acid breakdown, can contribute fecal or mothball-like scents. The changing pH of decomposing blood also influences the types of bacteria that thrive and the specific VOCs they produce, creating a complex and evolving odor profile.
Dried blood can develop a more intense and unpleasant pungency compared to its fresh state, reflecting advanced chemical and microbial alteration. This progression is from a metallic tang to more putrid or musky notes.