Williams et al. 2007 — Market basket survey: elevated arsenic in South Central U.S. rice vs. California
This paper reports the largest U.S. market basket survey of arsenic in domestic rice at the time of publication, comparing total arsenic levels in rice sourced from California (n=27) and from the South Central U.S. (TX, LA, AR, MS, MO; n=107). The central finding is that South Central U.S. rice had mean total As of 0.30 µg/g (300 µg/kg) and a median of 0.27 µg/g (270 µg/kg), while California rice averaged 0.17 µg/g (170 µg/kg) mean and 0.16 µg/g (160 µg/kg) median — a 41% difference that was statistically significant (GLM p < 0.001). The authors attribute South Central elevation to arsenic pesticide legacy from historical cotton cultivation in Arkansas and Mississippi Delta soils where rice varieties were introduced.
Key numbers
Total As in U.S. rice grain (µg/g dry weight, Table 1):
California (n=27):
- Range: 0.10–0.30; Mean ± SE: 0.17 ± 0.01; Median: 0.16
South Central (n=107):
- Range: 0.15–0.66; Mean ± SE: 0.30 ± 0.01; Median: 0.27
- 66% of samples between 0.20 and 0.30 µg/g; 29% exceeded 0.30 µg/g
Lowest sample: California organic long-grain brown basmati (Baby Basmati Kalijari), range 0.06–0.10 µg/g Highest sample: Louisiana long-grain aromatic specialty white rice, 0.66 µg/g
Inorganic As fraction: approximately 42% of total As (based on two separate speciation studies cited); used for dietary exposure modelling
- Modelled iAs at mean South Central level: ~0.28 µg/g × 42% ≈ 0.118 µg/g (118 µg/kg iAs)
- Modelled iAs at mean California level: ~0.17 µg/g × 42% ≈ 0.071 µg/g (71 µg/kg iAs)
No statistically significant difference between polished and unpolished rice within U.S. (GLM p=0.213)
Selenium (Se) grain levels: South Central mean 0.19 µg/g, California mean 0.08 µg/g
Methods (brief)
Agilent 7500c ICP-MS with octopole reaction system (ORS). LOD As: equivalent to 0.019 µg/g; LOD Se: 0.079 µg/g. NIST CRM 1568a Arkansas rice flour CRM; overall recovery 94 ± 5%. Samples purchased September 2005 from large supermarkets in AR, CA, LA, MS, MO, TX. Processing classification by region (CA vs. South Central). PCA of six elements (As, Se, Cu, Zn, Mn, Co) used to assess origin fingerprinting.
Limitations
Total arsenic reported, not iAs directly measured — inorganic fraction estimated at 42% from external studies, introducing uncertainty. Samples from a single season (September 2005), limiting temporal representativeness. The 42% iAs fraction estimate has since been superseded by Meharg et al. 2009 and other speciation studies showing higher variability (27–61% reported). Bangladeshi imported rice (n=5) included for context; not part of the US regional comparison.
Implications
- Certification: Establishes the US regional baseline that remains a foundational reference. South Central U.S. rice at mean 0.30 µg/g total As would exceed EU ML for rice (0.20 mg/kg tAs equivalent) if measured on comparable basis. Supports origin-based sourcing specification for rice purchasing.
- Courses: The canonical US-specific rice arsenic paper. The cotton-soil legacy explanation for South Central elevation is the key teaching point distinguishing US from other high-producing regions.
- App: California-origin white rice is meaningfully lower risk than South Central U.S. origin; both are dry-weight figures that need conversion for as-consumed comparison.