Where does the strange but typical ”metallic” smell come from when we touch iron objects such as tools, utensils, railings, or coins?
The mystery of why such objects have no odour themselves, but leave an odour on your hands, has intrigued scientists for centuries. And more intriguing: Why does iron smell like blood?
Now an expert in Germany believes he has found the answer. He says the odour we smell is not that of the iron itself, but is from a chemical reaction with our own skin oils. It is the same reaction that occurs when blood touches our skin as blood contains iron.
”The smell of iron upon contact with skin is ironically a type of human body odour,” states Dietmar Glindemann. ”That we are smelling the metal itself is actually an illusion.”
He believes this ”blood scent” was useful to early humans in tracking down prey or other humans.
Glindemann has done some hunting himself, tracking down the responsible scent molecules, working in conjunction with a team of researchers from the Virginia Polytechnic Institute and State University in the United States, the University of Leipzig and the Leipzig Environmental Research Centre in Germany.
In the scientific study, seven test subjects immediately recognised the ”musty” metallic odour when their hands came into contact with metallic iron or a solution containing iron ions with a twofold positive charge.
In contrast, solutions of iron with a triple negative charge did not cause the odour.
Analysis of gas samples from the skin of the test subjects pointed to a bouquet of different organic compounds that seemed to be characteristic of the metallic smell. The key component is called 1- octen-2-one, which smells fungal-metallic even when highly diluted.
The precursors to the odour molecules are lipid peroxides, which are produced when oils on skin are oxidised by certain enzymes or other processes, for example, under ultraviolet light.
These lipid peroxides are then decomposed by the doubly negative iron ions, which are consequently reduced to triply negative iron ions. When touching objects made of iron, the required doubly negative ions are formed when perspiration on the skin corrodes the iron.
Rubbing blood over skin results in a similar metallic smell based on the same scent molecules. Blood also contains iron atoms. Glindemann says, ”That humans can ‘smell’ iron can be interpreted as a sense for the smell of blood. Early humans were thus probably able to track down wounded prey or tribe members.”
Based on this new knowledge, medical researchers should be able to develop iron tests for skin, blood and tissues to identify specific ”fingerprints” of volatile scent molecules as markers for individual body odour, oxidative stress, and diseases.
The researchers were also able to characterise another iron-type smell: carbon- and phosphorus-containing cast iron and steel develop a metallic-garlic odour when exposed to acids.
Until now, metallurgists ascribed this to the gas phosphine (PH3). However, at breathable concentrations, pure phosphine (also known as a pesticide) is basically odourless.
The true culprits are organophosphines, especially those champions among intensively smelly compounds like methylphosphine and dimethylphosphine. Their structure is like that of a phosphine molecule in which one or two of the hydrogen atoms are replaced with methyl (CH3) groups. – Sapa-DPA