Some chocolates are contaminated with cadmium or lead. Depending on the details, the amounts may be too low to matter or so high they pose a significant toxic burden. Fortunately there is an organization that has measured this contamination and presented it at this website. They tested 309 different chocolate products. The database is searchable and sortable. The chocolates highest in cadmium concentration are displayed in the image at top.
If your eyes glaze over at the medical literature cited below just skip on down to the "how to calculate" section:
There are many sources of cadmium in the diet:
Staple foods constitute 40–60% of total dietary Cd intake by average consumers. Diets high in shellfish, crustaceans, mollusks, spinach, and offal add to dietary Cd sources.
The current tolerable exposure was set at 25μg/kg body weight per month (62μg/day for a 70-kg person) ... (FAO/WHO 2010)
However that number is excessive and needs to be revised downward because
simulation models predict that dietary intake of Cd at current tolerable monthly intake rate for 50 years will result in ... kidney damage and CKD [chronic kidney disease] ..., concurrent with death from cancer ... liver-related disease .., cardiovascular disease (CVD), ischemic heart disease, coronary heart disease ..., and Alzheimer’s disease [References omitted for readability]
The observed numbers are troubling:
In the NHANES [National Health and Nutrition Examination Survey] 1999–2006, overall (female) prevalence of urinary Cd > 1, > 0.7 and > 0.5 μg/g creatinine* among ≥ 20-year nonsmokers without CKD was 1.7 (2.5)%, 4.8 (7.1)%, and 10.8 (16)%, respectively … These data are a cause for concern because urinary Cd levels ≥ 0.37 to ≥ 0.65 μg/g creatinine have been associated with female breast cancer ... death from heart disease ..., death from cancer ..., and liver-related diseases .... Further, the prevalence of diminished kidney function among the NHANES 2011–2012 participants of 7.4% exceeds the 5% acceptable disease prevalence ... Thus, restrictive dietary intake guidelines are required to safeguard against a further increase in dietary Cd intake. [References omitted for readability]
*Cadmium content of urine in proportion to excreted creatinine. This kind of measurement is necessary because absolute concentrations are highly variable based on the amount of liquid a person has recently consumed.
In other words the current tolerable exposure is probably set too high.
How to calculate
The levels stated as ppm in the chart at top - parts per million – are the same as micrograms, abbreviated ug, per gram. So multiply this ppm number by the number of grams of your daily intake of your favorite chocolate, to get total daily micrograms of cadmium or lead intake. One teaspoon of cocoa is about 1.8 grams. One ounce of ready-to-eat chocolate is 28 grams
For example, one teaspoon of the the worst-case cocoa powder shown at top would have 7.1 x 1.8 = 13 ug cadmium. Five teaspoons would add up to 65 ug cadmium which hits the maximum tolerable intake figure. One ounce of the worst-case ready-to-eat chocolate, the cocoa nibs, would contain about 1.2 x 28 = 34 ug of cadmium, half the maximum tolerable daily intake, which is probably set at too high a number per the above. Understand that this is not the only source of cadmium in the diet. So if you are a regular consumer of chocolate it is worth checking your chocolate’s cadmium content to make sure it is not burdening your body with excessive cadmium.
The chocolate products highest in lead concentration:
Click to embiggen.
Once again, if your eyes glaze over at citations to the medical literature just skip on down to the "lead calculations" below.
Regarding lead, we have the U.S. Food and Drug Administration's interim reference levels for dietary lead exposure in children and women of childbearing age. There is a major omission in this work:
to confirm through a literature review that with the exception of neurodevelopment, which was not evaluated here, no adverse effects of lead consistently occur at the blood lead level (BLL) associated with the IRL.[interim reference level]
Emphases added. Neurodevelopment is kind of important, so this omission is not exactly comforting. With that understood, let us see what these reference levels are:
Because no safe level of lead exposure has yet been identified for children's health, the IRLs [interim reference levels] of 3 μg/day for children and 12.5 μg/day for WOCBA [women of childbearing age] were derived from the Centers for Disease Control and Prevention reference value of 5 μg/dL BLL [blood lead level], the level at which public health actions should be initiated.
Emphases added. So these are not exactly proven safe levels but at least it is a maximum level to refer to for now.
The worst-case cocoa powder had .35 ppm lead content, which is about .6 ug of lead per teaspoon, 20% of a child's maximum 3ug daily exposure, 5% for an adult. The worst-case chocolate bar had .14 ppm lead content. One ounce of that chocolate bar would contain about 4 ug of lead, already over a child's maximum daily exposure of 3 ug and one third of an adult's 12 ug, with the understanding that these maximum levels may be reduced with more complete knowledge. So once again, for regular chocolate consumers it is worthwhile to check on your chocolate's lead content and evaluate your exposure.
All health matters fair game for comment.