Dark chocolate has long been touted as having health benefits. We've been told it can improve our moods, decrease inflammation and even increase blood flow.

But some researchers are now warning of heavy metals in some of our favorite dark chocolate bars.

Consumer Reports tested 28 dark chocolate bars, including Dove, Ghirardelli, Lindt, and Hershey's, for lead and cadmium. For 23 of those bars, just an ounce of chocolate violates California's maximum allowable dose levels (MADL) for lead or cadmium, which are 0.5 micrograms and 4.1 micrograms per day, respectively, the publication said.

The typical chocolate bar loosely ranges from 1.5 ounces to 3.5 ounces.

California's limitations set by Proposition 65 are some of the most protective in the country, according to Consumer Reports. The U.S. Food and Drug Administration offers more flexible recommendations for daily lead intake at 2.2 micrograms for children and 8.8 micrograms for women of childbearing age.

As examples cited by Consumer Reports, an ounce of Lindt's Excellence Dark Chocolate 70% Cocoa or Dove's Promises Deeper Dark Chocolate 70% Cacao exceeds the acceptable cadmium levels, while an ounce of Godiva's Signature Dark Chocolate 72% Cacao or Hershey's Special Dark Mildly Sweet Chocolate exceeds the acceptable lead levels.

Trader Joe's The Dark Chocolate Lover's Chocolate 85% Cacao had higher levels in both lead and cadmium than California's limitations. Trader Joe's did not immediately respond to a request for comment.

The National Confectioners Association, which represents chocolate manufactures including Hershey's, Lindt and Godiva, reached a settlement in 2018 with As You Sow, a group that advocates enforcement of Proposition 65. The settlement established concentration levels for both lead and cadmium that require warning labels if surpassed. The association maintains that the industry has adhered to the levels established by the settlement.

"The products cited in this study are in compliance with strict quality and safety requirements, and the levels provided to us by Consumer Reports testing are well under the limits established by our settlement," association spokesperson Christopher Gindlesperger said. "Food safety and product quality remain our highest priorities and we remain dedicated to being transparent and socially responsible."

Johns Hopkins Medicine toxicologist Andrew Stolbach told NPR that MADLs are set to be "very conservative" to account for people with higher risk due to their age and other medical conditions. When the chocolate is consumed in moderate amounts, the lead and cadmium levels are nothing to worry about, he says.

"The safety levels for lead and cadmium are set to be very protective, and going above them by a modest amount isn't something to be concerned about," he said. "If you make sure that the rest of your diet is good and sufficient in calcium and iron, you protect yourself even more by preventing absorption of some lead and cadmium in your diet."

Significant exposure to cadmium can cause lung cancer, birth defects, and other reproductive harm, while significant exposure to lead can slow children's growth and development and damage the brain and nervous system.

But the Consumer Reports tests proved it is also possible for dark chocolate bars to maintain low levels of heavy metals, as five of the 28 bars had levels of lead and cadmium agreeable with California's limitations.

The settlement between confectioners association and As You Sow, an organization that promotes corporate social responsibility, required both parties to undertake a multiyear study to understand the root causes of heavy metals in chocolate and strategies to reduce these levels. The report discussing findings from a three-year study was released in August.

The researchers found that cadmium in cocoa beans naturally comes from the soil and is directly transported to the beans by the cocoa tree. Lead contamination occurs post-harvest, when wet cocoa beans are exposed to soil and dust during the drying, fermenting and transport phases.

"The industry should communicate to farmers the value of implementing Better Agricultural Practices related to reducing wet cocoa bean contact with soil during fermentation and drying," wrote Timothy Ahn, co-author of the report who manages food safety at Lloyd's Register. "Drying wet beans in direct contact with the ground, road surfaces, and concrete patios should be discontinued as a farmer controllable Pb (lead) reduction activity."

Reducing wet cocoa bean contact with soil and dust can lower lead in chocolate by 10% to more than 25%, according to co-author and toxicologist Michael DiBartolomeis.

Other ways to reduce heavy metal levels include blending high cadmium content cocoa beans with those with lower levels, identifying areas of contamination, and conducting more robust testing, according to the report from As You Sow.