Researchers at nonprofit organization Consumer Reports discovered that some popular brands of dark chocolate contain high levels of either lead, cadmium or both, according to new data released Thursday.

According to their study, 28 popular brands of dark chocolate bars contained Cadmium and lead, and the contamination is believed to have happened during cocoa farming.

Some of the brands tested included Hershey's Special Dark Mildly Sweet Chocolate, Trader Joe's Dark Chocolate 72% Cacao, Dove Promises Deeper Dark Chocolate 70% Cacao, and Godiva Signature Dark Chocolate 72% Cacao.

CR used California's maximum allowable dose level of 0.5 micrograms for lead and 4.1 micrograms for cadmium.

The study concluded that for 23 of the bars, eating just an ounce a day would put an adult over a level that may be harmful with at least one of those heavy metals.

There were also five chocolate bars that were above those levels for both cadmium and lead.

There is concern over the findings due to some people believing that turning to dark chocolate over other types of chocolate is a healthier option.

Dark chocolate can be a rich supply of antioxidants, may improve heart health, and has relatively low levels of sugar, but it must be sourced from a responsible grower.

Consumer Reports did list some dark chocolate brands that are considered to be "safer choices," which included Mast Oragnic Dark Chocolate 80% Cacao and Ghirardelli Intense Dark Chocolate 86% Cacao.