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Origin of the metallic smell

The metallic smell of iron or copper objects does not come from the metal itself. Copper has a similar effect, accounting for the metallic smell created by handling coins made of copper alloys. Oct-1-en-3-one is also known as 1-octen-3-one, octeneone or amyl vinyl ketone. It is the degradative reduction product of the reaction of skin lipid peroxides and Fe²⁺. Skin lipid peroxides form from skin lipid by oxidation, either enzymatically by lipoxygenases or by air oxygen. When iron is touched, the iron ions needed for the reaction form as perspiration on the skin corrodes the iron. Oct-1-en-3-one has an odor detection threshold of 0.03-1.12 μg/m³.

Dietmar Glindemann of the University of Leipzig and co-workers reported the results in Angewandte Chemie in 2006. Seven test subjects immediately recognized a musty metallic odor when their hands touched metallic iron or a solution of iron ions with a twofold positive charge. The researchers captured the vapours emitted from the skin of people who had handled iron objects, and the gases contained several aldehydes and ketones. Oct-1-en-3-one contributes about a third of the total odour concentration, along with several C6 to C10 n-alkanals and other unsaturated aldehydes and ketones. Rubbing blood over skin results in a similar metallic smell based on the same scent molecules.

Other metallic odours

Cast iron and steel containing carbon and phosphorus develop a metallic-garlic odor when exposed to acids. The cause is organophosphines, especially methylphosphine and dimethylphosphine, and not pure phosphine, which is basically odorless at breathable concentrations. Acids in sweat react with carbon and phosphorus impurities that are typically present in iron, which generates volatile, smelly organophosphines.

An industrial source of the odorant

People were surprised by a mushroom smell near a Canadian car painting plant. It came from oct-1-en-3-one that formed when methanal in paint resins reacted with the solvent heptan-2-one.

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