Why does iron smell like metal when we touch it?

Have you ever noticed how your hands smell of metal after touching iron? This phenomenon is common and fascinating, and has deep scientific roots. In this article, we will explore why iron gives off a metallic smell when touched.

When we touch iron objects, such as keys or railings, our hands tend to develop a characteristic smell that we often describe as an ‘iron smell‘. However, surprisingly, this smell does not come directly from the metal itself, but from a chemical reaction between the iron and our skin.

The Chemical Reaction

When iron comes into contact with sweat and the natural oils on the skin, it undergoes a series of chemical reactions. Human sweat contains a variety of compounds, including fatty acids and amino acids. When iron comes into contact with these compounds, it catalyses the decomposition of some of them, producing a series of volatile organic compounds (VOCs). It is these VOCs that we perceive as metallic odour.

Volatile Organic Compounds

One of the main compounds responsible for this odour is 1-octen-3-one, an organic molecule that has a strong metallic odour. This compound is formed when iron catalyses the breakdown of fatty acids in our skin. Other compounds, such as aldehyde and ketones, can contribute to this characteristic aroma.

Oxidation and Iron

Another factor contributing to the metallic smell is oxidation. When iron is exposed to air and moisture, it can oxidise, forming iron oxide, commonly known as rust. Although rust itself has a weak odour, the oxidation process can affect the production of VOCs on the skin.

Why Is This Smell So Strong?

Our perception of this smell is accentuated by the sensitivity of our sense of smell to certain compounds. 1-octen-3-one, in particular, can be perceived in very low concentrations, making the smell easily detectable even with brief contact with iron. The same odour is also generated when blood comes into contact with the skin: the iron contained in haemoglobin triggers a similar reaction. According to some studies, humans and other predators are able to detect this odour even at very low concentrations, an evolutionary trait that made chasing and flushing out wounded prey more effective.

1-octen-3-one: the smell of metal

Among the volatile compounds produced by the reactions described above, 1-octen-3-one plays a leading role in the metallic odour of iron. Its formation occurs through a series of steps:

1. Oxidation of skin lipids: Lipids (fats) on our skin are oxidised by oxygen in the air, a process catalysed by enzymes called lipoxygenases. This process produces lipid peroxides.
2. Reaction with metal ions: Lipid peroxides react with metal ions, in particular iron ions (Fe2+), which are found on the metal surface or come from sweat. This reaction, called the Fenton reaction, produces unstable free radicals.
3. Free radical degradation: The unstable free radicals produced by the Fenton reaction degrade spontaneously or with the help of other molecules, leading to the formation of various volatile compounds, including 1-octen-3-one.

Curiosity: Not Only Iron

Interestingly, this phenomenon is not exclusive to iron. Other metals, such as copper, can also produce similar odours when they come into contact with the skin. However, the volatile compounds produced may vary slightly depending on the metal involved.