The nose knows: Where does the smell of rust come from? (2024)

Rust doesn’t appear to release molecules into the air. So why does it have a smell?

Stefan Badham, Portsmouth, Hampshire, UK

Rust is another name for iron oxide. It occurs when iron is oxidised by being exposed to oxygen in the presence of moisture.

Although not obvious in dry air, rust’s odour can be detected in damp conditions, when oxidisation is most likely to occur. Rust’s odour is carried in airborne water droplets, and this is how it is transmitted to our senses. Human blood, which also contains water and iron, has a smell similar to rust.

David Muir, Edinburgh, UK

This is an olfactory illusion. Smell a dry metal paper clip. Lick your fingers and give it a good rub, then smell it again.

Ferrous ions, Fe²⁺, form when skin touches the iron in rust, especially if skin is moist. These ions react with fatty compounds called lipid peroxides, present on your skin, to produce carbonyl compounds such as 1-octen-3-one. This has a smell we associate with metal. It is skin chemicals that produce 1-octen-3-one, not the iron or rust.

Researchers at the Virginia Polytechnic Institute and State University have shown that the amount of 1-octen-3-one increases as skin is rubbed with increasing concentrations of Fe²⁺ solutions.

Blood, containing Fe²⁺, also gives a metallic smell when smeared on our skin. This gives rise to the “coppery” odour of blood, but the smell is actually derived from the skin’s fatty chemicals.

Luce Gilmore, Cambridge, UK

While metals and metallic oxides are unlikely to have any odour, the same isn’t true of organic-acid salts of metals. This is what is responsible for the bitter smell of bronze coinage, for instance, on which copper forms salts with lactic acid from human sweat.

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