Does the Metallic Smell After Touching Coins Actually Come from Your Skin?

The metallic smell left on your fingertips after touching coins or keys can seem to come directly from the metal. It may feel rather different to learn that the odor can be generated on the spot from substances in your skin.

A 2006 study showed that the metallic smell produced by touching iron can be explained by reactions between iron ions and substances derived from skin lipids. But the paper did not investigate just one smell of iron. It also traced the chemical origin of a different, garlic-like odor produced when iron is treated with acid.

Two smells of iron with different origins

Dietmar Glindemann and colleagues at Virginia Tech in the United States and research institutions in Germany sought to identify the causes of two odors associated with iron and its alloys in everyday surroundings.

One is the familiar metallic smell noticed immediately after skin touches iron. The other is a garlic- or carbide-like odor when iron containing carbon and phosphorus reacts with acid. The latter had traditionally been attributed to phosphine gas. The team tested this explanation as well, combining chemical analysis with human odor assessment.

Examining the interface between skin and iron

Seven people participated in the skin experiments. The researchers applied aqueous iron-ion solutions to skin or brought iron powder, steel and cast-iron plates into contact with skin moistened with artificial sweat. Metals and equipment were degreased beforehand to avoid effects from oils already present.

A glass funnel was placed over the treated skin to collect volatile compounds. Alongside gas chromatography and mass spectrometry, the team used a method in which people smelled the separated compounds as they emerged. They investigated both what was present and which components had strong odors.

The 7 participants detected a familiar metallic smell on skin exposed to metallic iron or ferrous ions. Ferric-ion solution, in contrast, did not produce the same smell. Ferrous and ferric refer to iron that has lost two or three electrons, respectively. Even for the same element, its state affects its reaction with skin.

The raw materials for the odor came from skin lipids

The analysis detected volatile compounds belonging to the aldehyde and ketone families, collectively called carbonyl compounds. Aldehydes ranging from hexanal to decanal were among those found on skin after contact with iron.

The mechanism proposed by the team was as follows. Contact with sweat produces ferrous ions from the metal, and these react with lipid peroxides on the skin. Lipid peroxides form when skin lipids are oxidized. Their breakdown creates odor molecules that can enter the air.

The metal initiates the reaction, while the materials that become the odor molecules are already present on the skin. Because the reaction happens quickly on contact, it feels as though the iron itself has given off the smell.

Adding more ferrous ions increased the amounts of compounds produced and the metallic odor, but eventually the response leveled off. The authors explain that this is consistent with a limited supply of reactants on the skin. Supplementary experiments found similar compounds and odors with copper and brass, suggesting that the mechanism is also relevant to everyday objects made from metals other than iron.

A mushroom-like ketone that stands out even in small amounts

One especially important odorant was 1-octen-3-one, a ketone described in the paper as having a mushroom-like metallic odor.

Crucially, the compound giving the largest instrumental signal is not necessarily the one that makes the strongest impression on the nose. Although 1-octen-3-one was detected in smaller amounts than other major compounds, its very low odor threshold gave it an important role.

It also smelled particularly strong when the separated compounds were assessed. This does not mean one molecule explains the entire metallic smell. In a reconstruction experiment, mixing the main aldehydes with this ketone produced a fattier metallic odor than the smell from skin and iron. The overall mixture, including trace compounds and their proportions, appears to create that distinctive impression.

Different molecules dominate when iron is treated with acid

The second odor did not require skin. When cast-iron plates contacted artificial sweat or dilute acid, a faint, different garlic-like smell arose even without skin contact.

The researchers went on to dissolve iron samples containing carbon and phosphorus in acid in the laboratory and analyzed the gases produced in detail. Four trained assessors participated in the odor evaluation.

The principal odorants were organophosphines, including methylphosphine and dimethylphosphine. These compounds contain phosphorus and carbon, and both produced garlic-like metallic odors at extremely low concentrations. Phosphine itself, the gas invoked in the traditional explanation, required a much higher concentration to be detected in this comparison.

Here, the carbon and phosphorus originated in the iron alloy. This differs from the first set of experiments, in which skin lipids broke down. Some gases produced during the acid experiments are hazardous: do not try to reproduce the experiment at home by pouring acid on metal and smelling it.

From odor chemistry to corrosion and the smell of blood

The second part of the research went beyond identifying odorants. The authors detected methylphosphinic acid, which can also form by oxidation of methylphosphine, in rusted cast iron. The finding indicates that corroding cast iron can be a source of compounds containing carbon–phosphorus bonds.

The paper discusses how such environmental sources may complicate the interpretation of monitoring and verification for compounds relevant to the Chemical Weapons Convention. Several pathways for forming carbon–phosphorus bonds were considered; the study did not establish a single one.

The first part also examined a possible connection to the metallic smell of blood. When one author applied his own blood to his skin, it produced odors and compounds similar to those generated by iron ions. Adding an iron-binding reagent suppressed the reaction. This experiment indicates that iron in blood can participate in a similar process.

However, the blood experiment was a self-experiment on one person, repeated 4 times. The evolution of human sensitivity to blood odor and the potential use of odor compounds as disease markers are best read as questions the authors raised for future investigation.

What does this mean for an ordinary coin?

This was a small-scale investigation of the chemistry behind familiar metallic odors, not a survey comparing coins from every country or keys with diverse histories of use. The supplementary comparison between metals also used just one person’s skin.

How strongly an object smells depends on the metal and its surface, as well as sweat and skin condition. The supplementary experiments themselves found differences between people before artificial sweat was added. To understand the smell of everyday metal objects, it helps to consider not only their material but also what they contacted and under what conditions.

Conclusion

The metallic smell of skin after touching iron came from volatile compounds formed by reactions between skin lipid peroxides and iron ions, with 1-octen-3-one playing an important part. In contrast, organophosphines dominated the garlic-like odor produced when iron containing carbon and phosphorus was treated with acid. The latter finding also connects to the possibility that corroding iron can release carbon–phosphorus compounds.

The sensations we group together as metallic smells can therefore arise through one pathway involving skin and another involving alloy constituents. The odor remaining on your fingertips is a clue to a small chemical reaction between the object you touched and your own skin.

The first part of the study received funding from the US National Science Foundation (NSF), and the second from Germany’s UFZ Environmental Research Center. A separate conflict-of-interest declaration was not identified in the main paper or supporting information.

References

• Glindemann, D., Dietrich, A., Staerk, H.-J., & Kuschk, P. (2006). The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines. Angewandte Chemie International Edition, 45, 7006–7009. DOI and article details.
• The paper’s Supporting Information. Details of skin-contact experiments, compound identification, comparisons between metals, and chemical and sensory analyses of acid-treated samples.