They are often billed as the healthy alternative.

But dark chocolate bars contain toxic metals linked to a host of health problems, a report suggests.

Tests on 28 bars made by popular brands, including Hershey's, Lindt and Tony's Chocolonely, revealed all were positive for lead and cadmium.

The heavy metals have been linked to lung issues, memory problems, cancer and even early death. But experts say you would have to eat more than an entire sharing-sized chocolate bar per day to notice any effect.

Cadmium is a natural element found in soil that is sometimes absorbed by the roots of the plant and ends up in cocoa beans. Lead contaminates the beans though the environment, possibly when it is blown by wind in the surrounding areas as the beans dry in the open.

Lead is well-known to be dangerous to humans, causing memory loss, abdominal pain and low mood in adults after long-term exposure.

For children, the heavy metal can damage their brains and central nervous systems in high concentrations leading to learning and behavior problems.

On the other hand, even low levels of exposure to cadmium have been linked to cancer in the kidneys and fragile bones.