Consumer Reports recently tested a bunch of dark chocolate for lead and cadmium, and found that:

For 23 of the bars, eating just an ounce a day would put an adult over a level that public health authorities and CR's experts say may be harmful for at least one of those heavy metals. Five of the bars were above those levels for both cadmium and lead.

If you regularly eat chocolate, how worried about this should you be?

Consumer Reports used the limits from California's Prop 65, which you may recall from the "this thing contains chemicals known to the state of California to cause cancer, birth defects, or other reproductive harm" warnings that are all over everything in CA. For both lead and cadmium Consumer Reports used the reproductive harm "maximum allowable daily level" (MADL), which are 0.5 µg/d and 4.1µg/d respectively.

CA calculates these limits by looking at studies that try to find the highest dose that has no observable effect (NOEL, "No Observable Effect Level") or the lowest dose that does have one (LOEL, "Lowest Observable Effect Level"), and estimating an effect threshold. Then they divide by 1,000 (as required by the original proposition text) to get the MADL limit. For example, here's the process they used for cadmium (pdf).

A safety factor of 1,000 is large (though not nuts for converting from a single dose to an ongoing exposure), so it's not surprising that CA's limits are tighter than you see elsewhere. For example, the FDA's limits for lead are 3µg/d for kids and 12.5µg/d for women who might become pregnant (pdf), 6x and 25x the Prop 65 lead MADL. The FDA translates this into a limit of 0.1 ppm for candy likely to be consumed by small children, 5.7x the level Consumer Reports used. [1]

Similarly, for cadmium the CDC reports that the EPA limit (which I can't find on epa.gov) is for food is 1µg/kg/d. CA used a pregnant adult weight of 58 kg in their cadmium calculation, so the equivalent from the EPA limit would be 58µg/d, 14x the CA Prop 65 level. Using 25kg for a child weight would give 25µg/d, 6.1x the Prop 65 cadmium MADL.

Putting these all on the same footing, assuming you're eating 1oz of chocolate daily:

µg/d ppm (µg/g)
CA Prop 65 lead MADL 0.5 0.018
FDA kid lead limit 3 0.1
FDA pregnant adult lead limit 12.5 0.44

and:

µg/d ppm (µg/g)
CA Prop 65 cadmium MADL 4.1 0.15
EPA child cadmium limit 25 0.88
EPA adult cadmium limit 58 2.0


(sheet)

My overall interpretation is that while it's better to eat chocolate with lower heavy metal content, all things being equal, these levels are basically fine and it's not worth avoiding chocolate over. The Consumer Reports findings seem alarming only because dividing by 1,000 in determining CA Prop 65 levels gives a super conservative safety factor.


[1] Consumer Reports used a limit of 0.5µg/oz, which is 0.018µg/g (the same as ppm).

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I mostly agree, but I'm particularly surprised at the results for the Hershey's 45%. That's not all that dark (i.e. children might want to eat it), and 2 oz is not all that much chocolate for a child to eat, and it looks like 2 oz would be enough to rise above the less stringent FDA limit for children.

Technically it wouldn't be above the FDA limit for lead for kids if they're eating 2oz, because that's measured in ppm not ug/d. But yeah, that might be the most worrying one on there.

I have cocoa powder for breakfast every morning (mixed with yoghurt, fruit, nuts). Since my understanding is that cocoa powder will have the highest concentration of heavy metals (relative to other chocolate products), I wanted to do an extremely quick investigation to see if I reached a similar conclusion to you.

From consumerlabs.com (I haven't paid for their full report yet, just viewing the free summary): "most cocoa powders to have high concentrations of cadmium -- around 1 to 1.5 mcg per gram -- which is much higher than the World Health Organization limit of 0.3 mcg per gram. The dark chocolate bars we tested had concentrations which were about 1/10th the amount found in cocoa."

The serving size for the cocoa powder I have currently is 1 Tbsp=5 g. I would estimate I use less than 1 Tbsp in my daily scoop (maybe 3 g), but I'll stick w 5 g as an upper limit.

Assuming that my brand has an above average cadmium concentration (in reality, I have no idea), let's say 2 mcg/g.

So perhaps I'm getting (5 g)*(2 mcg/g)=10 mcg of cadmium per day from cocoa powder.

You cite the EPA limit as 1µg/kg/d. I found a source that says "The EU has set a TWI (tolerable weekly intake) limit for cadmium of 2.5 µg/kg body weight"--this is 0.35 µg/kg/d.

I weigh about 55 kg, so my limits would be 55 mcg/d (EPA) or 19 mcg/d (EU).

So my cadmium consumption from cocoa powder alone could be about half the EU limit for total cadmium consumption. This is not scarily high, but doesn't seem great either, since there are significant sources of cadmium in the diet besides chocolate. This study of Americans says: "The average dietary Cd consumption in the population was 4.63 μg/day, or 0.54 μg/kg body weight/week, which is 22% of the tolerable weekly intake (TWI) of 2.5 μg/kg body weight/week." This study of Belgians says: "The population mean, median and 95th percentile of the dietary intake values were 0.98, 0.85 and 2.02 µg kg⁻¹ body weight per week respectively. Two percent of the Belgian adult population has a dietary Cd intake above the recent TWI of 2.5 µg kg⁻¹ body weight established by EFSA in 2009. Cereal products and potatoes contribute for more than 60% to Cd intake."

So my personal conclusions:

  1. I would definitely buy low-cadmium cocoa powder if it were easy to buy and easy to identify (and have confidence in the measurements, etc)
  2. Assuming I can't find a good source of low-cadmium cocoa powder, I haven't really decided whether I will reduce my consumption of cocoa powder. Probably I will just because the cost of not eating cocoa powder seems low to me. But I won't really obsess over it.

I didn't look into lead.

Any suspicion about where these metals may be coming from? Cocoa, milk, or sth else?

Thanks for the analysis, and I mostly agree with your interpretation (having done no further research into this myself), but I'm confused how dividing by 1000 is the problem here. The levels are "basically fine" because 9*-they are well below the FDA/EPA limits, but the CA levels are only about 1 order of magnitude lower, not 3. If they had divided by 100, would we be interrogating their divisor choice? (The current implication is that that arbitrary approach would have been fine since it would correspond to FDA/EPA levels). It makes me think that maybe the impetus to question the arbitrary approach mainly stems from the conclusions not fitting our palate.

I think if the proposition had required the state to set a level for ongoing exposure, that would be fine. But instead it tells the state that the way they have to do that is by determining the level of single dose that just barely doesn't give birth defects and dividing by 1,000. This is a really rough proxy, and ignores things like how quickly different materials are shed from the body. Learning that they were using such a rough measurement mostly pushes me in the direction of ignoring their level and paying attention instead to the levels that the federal government sets.

(I also initially misunderstood the role of the 1000x adjustment and edited the post based on Facebook discussion)

To answer your question, if they instead had a divisor of 100 or 10 I would still be advocating to put very little weight on their limit.