The composition of pennies changed in late 1982, when zinc became the primary metal instead of copper. Before 1982, pennies were forged of 95 percent copper and 5 percent zinc. However, as copper became more expensive, it was abandoned as the primary metal. Pennies are now made of a zinc core and a thin copper coating. Coins made of zinc can cause a corrosive reaction when they are exposed to gastric acid. O'Hara and associates conducted a study to determine the radiographic appearance and features of corrosion in coins exposed to gastric acid.

The authors became interested in the effects of corrosion of a coin in the stomach when they encountered a two-year-old boy who presented to the emergency department with abdominal pain and vomiting four days after he had swallowed a coin. Initial radiographs of the child showed an intact metallic disk, which was presumed to be a coin, in the stomach. However, radiographs four days later showed a metallic object with a moth-eaten appearance. Endoscopy yielded a blackened, corroded 1989 penny and revealed a bleeding gastric ulcer.

The authors analyzed corrosion after exposure to hydrochloric acid in four types of coins: three silver-colored coins (a quarter, a nickel and a dime), six pre-1982 pennies that were worn but not intentionally damaged, six post-1982 pennies in good condition and six post-1982 pennies that had been scratched by scraping the edges until the zinc was exposed. Each group of coins was immersed in hydrochloric acid in a concentration that approximated gastric acid. The coins were rinsed and radiographed daily for seven days. Each group of coins was weighed at the beginning and at the end of the study. In addition, the penny retrieved from the two-year-old patient was weighed.

Immersion of the damaged post-1982 pennies in the hydrochloric acid was immediately followed by the development of bubbles of gas, indicating an immediate chemical reaction. With the undamaged post-1982 pennies, bubbles were seen on the surface by the third day of immersion in hydrochloric acid. Bubbles were not seen at any time on the silver-colored coins or the pre-1982 pennies. The surfaces of all of the coins grew increasingly dull as the duration of exposure to hydrochloric acid increased.

Radiolucent erosions were radiographically visible on the post-1982 pennies within 24 hours of immersion in the hydrochloric acid. After two days of exposure to the acid, full-thickness holes developed through the flat surface of these pennies. No radiolucent erosions or holes developed on the silver-colored coins or the pre-1982 pennies after seven days of exposure to hydrochloric acid.

All of the coins lost weight over the seven days of testing. The quarter, nickel and dime lost 12 percent of their initial weight. The post-1982 pennies lost between 5 and 8 percent of their weight. The penny retrieved from the child weighed 26 percent less than would be expected. The amount of weight lost in this penny far exceeded the amount of weight lost in the post-1982 pennies exposed in the hydrochloric acid experiment.

According to the authors, the chemical reaction between gastric acid and post-1982 pennies yields a highly absorbable form of zinc, zinc chloride. The zinc could have toxic effects. The two-year-old patient had absorbed a quantity of zinc that was equivalent to 22 cold-prevention lozenges. Reported toxic effects of excessive zinc absorption include local corrosion and ulceration of the esophagus and stomach, and gastrointestinal symptoms such as nausea and abdominal cramping.

The authors conclude that recognition of coin corrosion may become increasingly important as the number of zinc-based coins in circulation increases. They state that the radiographic appearance of a scalloped border and holes in an ingested coin should prompt endoscopic removal, because such features indicate that the coin has been retained for longer than one or two days. In such a situation, the potential for zinc-related morbidity is increased.

editor's note: This basic science study demonstrates the value of asking and answering a simple question that has clinical relevance. There are reported cases of multisystem organ failure from zinc toxicity after ingestion of many coins. Coins should not be considered innocuous foreign bodies in the gastrointestinal tract.—b.a.