Overview
This study measured 16 metals (Na, Mg, K, Ca, Fe, Se, Mn, Cu, Zn, Co, Hg, Cr, Ni, As, Cd, Pb) in 82 samples of 13 vegetable species from a local farmer’s market in Grand Forks, North Dakota, and assessed non-carcinogenic and carcinogenic health risks. Spinach showed the highest total metal burden and the highest concentrations of Cd, Cr, Pb, and Hg among the 13 species. Cadmium levels exceeded FAO/WHO maximum allowable concentrations in 100% of samples of tomato, green chili, white eggplant, capsicum, and spinach, and in 86% of onion, sugar beet, and potato samples. Non-carcinogenic target hazard quotients (THQ) exceeded 1 for Cd, Co, and As in most vegetable species; mean carcinogenic target cancer risk (TCR) values for Cr, Ni, As, and Cd all exceeded 1.0 × 10−4, indicating carcinogenic risk by USEPA criteria.
Key numbers
All concentrations reported as µg/g dry weight (DW). n values per vegetable shown in Table 2 of the source (total n = 82).
Mean concentrations across all 13 vegetables (µg/g DW):
- Pb: 0.019 (range 0.007–0.081, max in spinach = 0.081)
- Cd: 0.298 (range 0.006–0.985, max in spinach = 0.985)
- tAs (total arsenic): 0.083 (range 0.009–0.436, max in cucumber = 0.436)
- tHg (total mercury): 0.0030 (range 0.0008–0.0113, max in spinach = 0.0113)
- Ni: 0.736 (range 0.342–2.139, max in green bean = 2.139)
- Cr (total): 0.049 (range 0.014–0.179, max in kale = 0.114)
Selected per-vegetable concentrations (µg/g DW):
Spinach (n=5):
- Cd: 0.985 ± 0.223 (highest of all 13 species)
- Pb: 0.081 ± 0.013 (highest)
- Cr: 0.179 ± 0.063 (highest)
- Hg: 0.0113 ± 0.0035 (highest)
- Ni: 0.342 ± 0.070
Cucumber (n=6):
- tAs: 0.436 ± 0.145 (highest)
- Ni: 0.713 ± 0.274
Green bean (n=7):
- Ni: 2.139 ± 0.774 (highest)
Kale (n=5):
- Cr: 0.114 ± 0.088
- Cd: 0.105 ± 0.021
Corn (n=7):
- Cd: 0.006 ± 0.004 (lowest; below FAO/WHO limit of 0.05 µg/g for fruit vegetables)
FAO/WHO Cd exceedances (limits in µg/g FW; dry weight concentrations must be interpreted with caution for direct comparison):
- Fruit vegetables (tomato, chili, eggplant, capsicum, cucumber): limit 0.05 µg/g; exceeded in 100% of tomato, green chili, white eggplant, capsicum samples
- Leafy vegetables (kale, spinach, dill): limit 0.20 µg/g; exceeded in spinach (0.985 µg/g DW)
- Root/tuber and bulb vegetables (potato, onion, sugar beet): limits 0.05–0.10 µg/g; exceeded in 86% of onion, sugar beet, potato samples
- Note: source reports concentrations on DW basis; FAO/WHO limits are typically expressed on FW basis; direct comparison requires moisture correction not performed in this study
Non-carcinogenic risk (THQ > 1 threshold):
- Cd THQ range: 0.03 (corn) to 4.79 (spinach); above 1 in most species
- Co THQ range: 0.25 (onion) to 3.54 (green chili); above 1 in most species
- As THQ range: 0.14 (green bean) to 7.05 (cucumber); above 1 in most species
- HI (all metals combined): range 1.59 (corn) to 18.3 (spinach); all species above 1
Carcinogenic risk (TCR; acceptable threshold 1.0 × 10−4):
- Pb TCR: all vegetables below 1.0 × 10−6 (negligible for all species)
- Cr TCR range: 3.4 × 10−5 (potato) to 4.3 × 10−4 (spinach); mean above 1.0 × 10−4
- Ni TCR: all 13 vegetables above 1.0 × 10−4 (unacceptable for all species by USEPA criteria)
- As TCR: above 1.0 × 10−4 in 9 of 13 species
- Cd TCR: above 1.0 × 10−4 in 9 of 13 species
Methods (brief)
Samples collected from Town Square Farmer’s Market, Grand Forks, ND (mid-June through September); 2–3 sub-samples (~0.2–0.5 kg each) per species from different vendors combined. Washed with deionized water, air-dried 24 h, homogenized, oven-dried at 70–80°C to constant weight, stored in desiccators. Digestion: 0.5 g dried-ground sample in 5 mL concentrated HNO3, microwave-assisted (Milestone UltraWAVE), EPA method 3051A (Revision 1); ramp to 175°C over 5.5 min, hold 4.5 min, N2 pressure 35 bar; diluted to 50 mL with deionized water. ICP-MS: Thermo Scientific iCAP Qc in KED mode, EPA method 200.8 (Revision 5.4); internal standards Sc, Rh, Bi; all measurements in triplicate. Hg by direct solid-sample analysis: Milestone DMA-80 Tri Cell, EPA method 7473. QC: NIST SRM 1567b (wheat flour); recovery >85% for most elements; RPD of duplicates <20%. Detailed LOD, LOQ, blank spike recovery in supplementary Table S1.
Health risk parameters: EDI used body weight 70 kg, daily vegetable intake 0.34 kg/day. THQ reference doses (RfD) from ATSDR/USEPA-IRIS. TCR oral cancer slope factors from USEPA. Arsenic reported as total arsenic (tAs); speciation into inorganic arsenic (iAs) not performed; tAs values cannot be equated to iAs for regulatory comparison. Chromium reported as total Cr; no Cr-VI speciation.
Implications
Certification: The elevated Cd levels in spinach (0.985 µg/g DW) and multiple fruit-vegetable species in this North Dakota local-market survey suggest Cd is a material concern even in low-industrial agricultural areas of the US. The Ni TCR result (all 13 species above 1.0 × 10−4) and As TCR results highlight that Ni and As in common vegetables may collectively drive carcinogenic risk calculations in this population at standard consumption rates; however, the absence of iAs speciation limits direct regulatory comparison for arsenic, and total Cr should not be interpreted as Cr-VI for carcinogenicity purposes.
Courses: Illustrates the pattern that leafy vegetables (spinach, kale, dill) tend to accumulate more metals than fruit vegetables (corn, capsicum) due to larger surface area and higher transpiration; spinach consistently represents the high end of the distribution across metals.
App: Spinach should carry elevated-Cd and elevated-Pb flags. Cucumber should carry elevated-tAs flag (though iAs fraction unknown). Green bean should carry elevated-Ni flag. Corn presents the lowest total metal burden and is a low-risk ingredient for these analytes in this dataset.
Related evidence
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