How Do Pennies Get Dirty?

Dirty pennies have a variety of causes.

There are currently over 130 billion pennies circulating in the United States. With the average penny lasting over 25 years, this humble coin has many opportunities to accumulate dirt and grime.

Environmental Damage

A walk down any busy street reveals loose change that has fallen from pockets and purses As over half the coins produced in the United States are pennies, it stands to reason that a higher percentage of the coins on America's streets are pennies. Like all coins, there are millions opportunities for pennies to be affected by rain, dirt, refuse exposure and other environmental hazards.

Coin Composition

Pennies are 97.5 percent zinc and 2.5 percent copper. This copper, found in the plating on the exterior of the coin, absorbs oxygen, which in turn forms copper oxide, which leads to the tarnish common on many pennies. A simple buffing with a soft rag dampened with vinegar will remove tarnish stains.

Human Contact

Coins of all kinds are constantly exchanging hands, and pennies are no different. Much of the dirt on pennies is due to repeated exposure to the oils and dirt on human hands.

References

• The Naked Scientists: Why Does Vinegar Make Pennies Shiny? (SFW)
• Bankrate.com: Loose Change. Penny Facts.
• Americans for Common Cents: Penny Details