The smell on your hands

An iron railing leaves a smell on your hand. Calling it the smell of iron seems reasonable. Then you ask which molecule actually reached your nose.

In 2006, Dietmar Glindemann and colleagues tested the familiar metallic smell with seven volunteers. Iron in contact with skin produced it. A solution containing ferrous ions did too. The researchers collected the gases above the skin and analysed them. The odorants were organic compounds, including aldehydes and ketones. Your hand was supplying ingredients.

Sweat corrodes the iron, releasing ferrous ions. Those ions react with peroxides already present in skin lipids, breaking them down into volatile compounds. One important product is 1-octen-3-one, which has a mushroom-like, metallic smell. The reaction happens within seconds, fast enough to feel like you have simply uncovered a property of the railing.

The controls make the explanation more satisfying. Skin with artificial sweat but no iron did not produce a detectable metallic smell. Neither did iron or ferrous solution without skin. Ferric ions, a different oxidation state of iron, also failed to produce it under the tested conditions. "Iron smells like that" leaves out almost the whole experiment.

There is another iron smell in the same paper: certain iron alloys under acid attack produce a garlic-like odor involving organophosphines. So "metal has no smell" would be a bad replacement fact. Different materials and reactions can give you different things to smell.

The interesting correction is smaller. That familiar smell after touching iron belongs to the contact between two things. The railing gets the credit because it is the thing you just touched. Your hand quietly does some of the chemistry.