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Dietary risk analysis of heavy metal and metalloid pollution in rice: case study of Nanning City in China

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Overview

Li and colleagues evaluated heavy metal and metalloid contamination in locally produced rice from Nanning City, Guangxi Province, southern China. The study analysed 1,844 rice samples collected between 2014 and 2020 for probabilistic risk assessment, with a subset of 538 recent samples (2019-2020) used for deterministic point-estimation. Among four analytes (cadmium, lead, mercury, and inorganic arsenic), cadmium was the dominant contaminant, with a maximum detected concentration of 1.28 mg/kg, an exceedance rate of 16.36% against the Chinese national standard, and a P75 cadmium concentration across the study townships of 0.18 mg/kg. Monte Carlo probabilistic simulation showed that 15.8% of the simulated population exceeded THQ = 1 for cadmium despite deterministic mean THQ remaining below 1, underscoring the utility of probabilistic over deterministic approaches. Cadmium sensitivity analysis identified rice cadmium concentration as the dominant determinant of exposure risk, with rice consumption rate having a smaller but notable effect.

Key numbers

  • Sample set: 538 total samples from Nanning 2019-2020 (180 paddy, 358 milled rice); 1,844 milled rice samples 2014-2020 used for probabilistic modelling.
  • Detection rates (2019-2020 samples, n = 538): cadmium 99.07%, lead 53.90%, inorganic arsenic 24.91%, mercury 99.81%.
  • Concentration (2019-2020, mean +/- SD, mg/kg): cadmium 0.124 +/- 0.137 (maximum 1.28); lead 0.073 +/- 0.187; inorganic arsenic 0.168 +/- 0.087; mercury 0.008 +/- 0.006.
  • Exceedance rates against Chinese national standard (GB 2762-2017): cadmium 16.36% (88 exceedances of n = 538), lead 2.04%, inorganic arsenic 6.32%, mercury 1.30%.
  • Cadmium P75 pooled across townships: 0.18 mg/kg (Table 4). Range of township-level P25 values 0.02-0.08 mg/kg, P50 values 0.05-0.19 mg/kg, P75 values 0.10-0.38 mg/kg.
  • LOD (mg/kg): arsenic 0.0040, cadmium 0.0030, inorganic arsenic 0.0500, mercury 0.0005. LOQ was 10 times LOD.
  • Reference doses (RfD) used: arsenic 0.0003, cadmium 0.0005, lead 0.0014, mercury 0.0003 mg/kg body weight/day (Table 1). Cadmium slope factor (SF) 6.1 kg day/mg, arsenic SF 1.5 kg day/mg (Table 1).
  • Dietary survey: 300 households surveyed, 524 individuals; mean cooked rice intake 227.8 +/- 18.24 g/person/day, median 231.56 g/person/day.
  • Deterministic THQ (median intake, 2019-2020): cadmium 0.898 (< 1); lead 0.104 (< 1); mercury 0.091 (< 1); inorganic arsenic 0.036 (< 1). Combined HI = 1.13 (> 1), indicating a potential non-carcinogenic concern at the aggregate level.
  • Probabilistic Monte Carlo (1,844 samples, 5,000 iterations): mean THQ for cadmium 0.610; 15.8% of simulated population with THQ >= 1; at P75 exposure, Wuming County male children 6-17 years exhibited THQ 2.76 and females THQ 2.57.
  • Carcinogenic risk (CCR, combined cadmium and inorganic arsenic): 1.75 x 10^-5, interpreted as a moderate carcinogenic health concern. Inorganic arsenic CR = 1.63 x 10^-5; lead CR = 1.24 x 10^-6.
  • Estimated Monthly Intake (EMI) for cadmium at P75: ranged 46.97 to 68.94 micrograms/kg body weight across age-sex-district groups in high-exposure counties (Table 6), with the JECFA Provisional Tolerable Monthly Intake (PTMI) for cadmium at 25 micrograms/kg body weight.
  • Analytical method: microwave acid digestion followed by ICP-MS (cadmium, lead, inorganic arsenic); mercury via direct mercury analyzer (DMA-80, Milestone). Inorganic arsenic by HPLC-AFS. Certified reference material GBW(E)100359 analysed simultaneously for QC. Recoveries not separately reported but reference material values consistent with certified values.

Methods (brief)

A total of 300 farmer households were surveyed in Nanning City rice-growing areas, yielding 524 individuals with mean age 55.14 +/- 8.67 years. Dietary intake was assessed by 24-hour dietary recall combined with a food frequency questionnaire. Rice samples were collected from 15 administrative districts and counties across Nanning. Sample preparation: 0.5 g homogenised sample microwave-digested with 8 mL concentrated nitric acid (HNO3), digested at 100 degrees Celsius for 1 h then secondarily digested using a microwave digestion system (MARS6, CEM, USA). Elemental concentrations were determined by ICP-MS (iCAP Q, Thermo Fisher Scientific, USA). Mercury was analysed by direct mercury analyzer DMA-80. Inorganic arsenic was determined by HPLC-AFS (HPLC-AFS, SA-20, Beijing Jitian). LOD values: arsenic 0.0040, cadmium 0.0030, inorganic arsenic 0.0500, mercury 0.0005 mg/kg. Probabilistic risk assessment used lognormal distribution for cadmium content in rice and triangular distribution for rice consumption (Triangular 121.63, 231.56, 308.42 g/person/day). Monte Carlo simulation performed with 5,000 iterations using Risk 7.6 software and Latin Hypercube sampling.

Implications

  • Certification (HMTc): Cadmium exceedance rates of 16.36% in locally produced rice from Nanning and high-exposure-subgroup THQ values above 1 in children support the rationale for tight cadmium limits in grain-based products, particularly rice. The P75 cadmium value of 0.18 mg/kg (dry weight, Nanning) exceeds the EU maximum level of 0.10-0.20 mg/kg (wet weight, depending on cereal type) under Regulation (EU) 2023/915.
  • Courses: Demonstrates a situation where aggregate HI > 1 from rice alone even though individual metal THQ values are each below 1, illustrating the importance of cumulative exposure assessment. Also an effective case study for why deterministic point estimation underestimates population-level risk compared with Monte Carlo simulation.
  • App: Rice cadmium concentration data from this source are specific to Nanning City, Guangxi, an area with elevated cadmium from karst geology and lead-zinc mining. These values should not be used to characterise global or US rice market baselines without adjustment.
  • Microbiome: No microbiome data.

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