Hands et al. 2024 — Multi-year heavy metal survey of 72 US dark chocolate and cocoa products
This multi-year study analyzed 72 popular consumer cocoa-containing products purchased in the United States across four cohorts (2014, 2016, 2019, 2022) for lead (Pb), cadmium (Cd), and total arsenic (tAs) using ICP-MS following acid digestion. The study is notable for documenting temporal trends (declining Pb and Cd across cohorts) and the counterintuitive finding that organic-certified products carried significantly higher Cd concentrations than non-organic counterparts. Across all products, 43% exceeded California Proposition 65 MADLs for Pb per serving and 35% exceeded Prop 65 MADLs for Cd per serving, but median concentrations fell below the Prop 65 thresholds, indicating a strong influence of high-contamination outliers on mean values. Arsenic concentrations were universally below Prop 65 limits.
Key numbers
All concentrations reported as means/medians in µg/g (mcg/g); per-serving exceedances based on Prop 65 MADLs (Pb: 0.5 µg/day, Cd: 4.1 µg/day, As: 10 µg/day).
Lead (Pb), all cohorts, n=72:
- Mean: 0.062 µg/g; Median: 0.050 µg/g; Min: 0.009 µg/g; Max: 0.269 µg/g
- Mean per serving: 0.615 µg/day; Median: 0.375 µg/day; Max: 3.136 µg/day
- 43% of products (31/72) exceeded Prop 65 MADL for Pb per serving
- 97.2% (70/72) fell below FDA IRL for Pb
Cadmium (Cd), all cohorts, n=72 for µg/g:
- Mean: 0.396 µg/g; Median: 0.239 µg/g; Min: 0.016 µg/g; Max: 2.2 µg/g
- Mean per serving (n=37): 4.358 µg/day; Median: 2.725 µg/day; Max: 14.12 µg/day
- 35% of products exceeded Prop 65 MADL for Cd per serving
Total arsenic (tAs), all cohorts, n=55 for µg/g (not tested in 2014 cohort):
- Mean: 0.094 µg/g; Median: 0.050 µg/g; Min: 0.017 µg/g; Max: 0.200 µg/g
- 0% exceeded Prop 65 MADL for As per serving
Temporal trends by year (µg/g):
| Year | Pb mean | Pb median | Cd mean | Cd median | As mean | As median |
|---|---|---|---|---|---|---|
| 2014 | 0.098 | 0.100 | 0.512 | 0.224 | NA | NA |
| 2016 | 0.060 | 0.050 | 0.593 | 0.288 | 0.196 | 0.200 |
| 2019 | 0.044 | 0.030 | 0.295 | 0.225 | 0.058 | 0.050 |
| 2022 | 0.050 | 0.055 | 0.208 | 0.240 | 0.038 | 0.037 |
Organic certification finding: Organic-certified products had significantly higher Cd (approximately +0.4 µg/g Pb, p<0.001) and Pb (+0.020 µg/g, p<0.001) concentrations than non-organic. Per-serving Cd was 6.09 µg/day (organic) vs. 3.18 µg/day (non-organic), with Welch’s T-test significant (p=0.02).
Method: ICP-MS after acid digestion (concentrated HNO3 with HCl and H2O2 for Pb; HNO3 hotplate for As and Cd). Reporting limits: 0.01 µg/g (Pb), 0.02 µg/g (As and Cd). AOAC 2015.01 methods. Two independent US commercial laboratories.
Methods (brief)
Products purchased from retail and online sources in the US by ConsumerLab.com across four cohorts spanning 2014–2022. Acid or microwave digestion followed by ICP-MS (PerkinElmer NexIon 350X). Pb LOQ 0.01 µg/g; As and Cd LOQ 0.02 µg/g. Spike recovery QC 75–125%. Samples exceeding Prop 65 thresholds were retested at a second independent laboratory. Total arsenic reported (no HPLC speciation; arsenic in cocoa products is predominantly inorganic due to soil uptake by cacao, distinct from marine matrices where organoarsenicals dominate).
Implications
Certification: Key reference for HMT&C threshold-setting for dark chocolate and cocoa products. The Prop 65 exceedance rates (43% for Pb, 35% for Cd) and the organic premium finding are directly relevant to certification criteria and consumer advisories. The 2022 cohort shows meaningful improvement, consistent with industry reformulation pressure. Courses: Illustrates temporal trends in market contamination, the organic paradox for Cd, and the divergence between median and mean in skewed distributions — all valuable teaching points. App: Cocoa/dark chocolate: Pb typical range 0.03–0.10 µg/g (ppb as µg/g = mg/kg × 1000; expressed in ppb: median ~50 ppb, mean ~62 ppb); Cd typical range 0.02–2.2 µg/g (median ~239 ppb, mean ~396 ppb); tAs median ~50 ppb. High Cd is the dominant heavy metal concern. Microbiome: Not addressed in this study.