Lead and cadmium are metals found in soils, so cacao beans naturally take up these toxins. But most of the lead issue seems to occur after harvesting while the beans are drying in the open air or during the manufacturing process.
A quick check of medical journals found high cadmium and lead exposure and levels increase the risk of lung, kidney, breast, prostate, colon, pancreas cancers.
A low zinc level increases the risk of dying from higher cadmium levels. [i] It seems zinc helps balance the cancer risks for several toxic metals including cadmium. If you can’t get a blood test for zinc, ask your local compounding pharmacy for a zinc taste test. An example is https://flourishrx.com/test-kit-products/zinc-taste-test?rq=zinc
Consumer Reports used California's maximum allowable dose level (MADL) for lead (0.5 micrograms) and cadmium (4.1mcg). Shown are the percentages of the MADL supplied in an ounce of each chocolate.
According to Consumer Reports
The 3 safest dark chocolate bars:
Mast 80% Organic Cocoa
Lead 14% Cadmium 40%
Taza Chocolate 70% Organic
Lead: 33% Cadmium 74%
Ghirardelli 86% Intense Dark Cacao
Lead 36% Cadmium 39%
Trader Joe’s 895% Cacao
Lead 127% Cadmium: 229%
Special Dark Mildly Sweet Chocolate
Lead: 265% Cadmium: 30%
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[i] Lin YS, Caffrey JL, Lin JW, et al. Increased risk of cancer mortality associated with cadmium exposures in older Americans with low zinc intake. J Toxicol Environ Health A. 2013;76(1):1-15. doi:10.1080/15287394.2012.722185