Handling coins or other metal objects often leaves a distinctive odor. This “metallic” smell is familiar. People often assume the smell emanates directly from the metal itself. Its true origin sparks curiosity.
The Surprising Source of the Odor
The metal itself is largely odorless. The characteristic scent is actually a type of body odor, emerging from a chemical reaction occurring on human skin. This reaction happens when certain compounds on the skin come into contact with the metal. Research indicates that the smell is a result of oils in the skin breaking down after contact with metallic objects. Therefore, the sensation is not due to metal particles transferring to the skin, but rather a transformation of skin components.
The Chemistry of Metallic Smell
The distinctive metallic smell arises from a specific chemical process involving iron ions and skin lipids. When the iron in a penny touches the skin, perspiration on the skin can corrode the iron, forming positively charged iron ions (Fe2+). These Fe2+ ions then react with and break down lipids present on the skin’s surface. Specifically, these iron ions catalyze the decomposition of lipid peroxides, which are products of oxidized skin lipids. This decomposition generates volatile organic compounds, primarily a molecule called 1-octen-3-one.
This compound is largely responsible for the characteristic “metallic” or “mushroom-like” odor. Other aldehydes and ketones may also contribute to the overall scent profile. The human nose is remarkably sensitive to 1-octen-3-one, able to detect it even in very minute concentrations, as low as 0.03–1.12 micrograms per cubic meter. This high sensitivity explains why the odor is easily noticed after brief contact with a penny.
Beyond Pennies: Other Metals and Similar Scents
The phenomenon of metallic odor is not limited to pennies, but often involves iron. Other metals, particularly those containing iron, can produce a similar smell upon contact with skin. For instance, handling steel, which is an iron alloy, can also lead to the same metallic scent. Even blood, due to its iron content, produces a similar metallic odor when it comes into contact with skin, triggering the same chemical reactions that generate 1-octen-3-one. The smell is a result of the interaction between iron and skin, rather than a unique property of a specific metallic object.