Myat Soe et al. 2023 — Arsenic and heavy metals in Myanmar rainfed rice
Myat Soe et al. measured eleven metal(loid)s (As, Pb, Cd, Cr, Mn, Fe, Zn, Cu, Ni, Mo, Co) in 120 white polished rice grains from five agriculturally active townships near Yangon in the eastern Ayeyarwady Delta using triple quadrupole ICP-MS (ICP-QQQ). The study found total arsenic (mean 0.16 mg/kg) and nickel (mean 0.39 mg/kg) concentrations consistent with worldwide background levels for non-mining, non-contaminated areas, while lead (0.010 mg/kg), cadmium (0.0056 mg/kg), and chromium (0.056 mg/kg) were well below maximum allowable limits. The paper argues that the absence of groundwater irrigation and proximity to industrial or mining activity explains the unusually low contamination, and flags nickel alongside arsenic as the main contributors to cancer risk (Ni: 66%, Cd: 16%, As: 13% of total cancer risk from rice consumption at ~500 g/day).
Key numbers
All values are dry weight (mg/kg = ppm) unless noted. The study assumes iAs = 30% of tAs for health risk calculations.
- tAs: mean 0.16 mg/kg; range approximately 0.04–0.30 mg/kg across townships (Table 2 in source)
- Pb: mean 0.010 mg/kg; below LOQ at many sites
- Cd: mean 0.0056 mg/kg; Codex maximum allowable concentration for polished rice is 0.2 mg/kg
- Cr: mean 0.056 mg/kg
- Ni: mean 0.39 mg/kg
- Mn: mean 7.21 mg/kg; Fe: mean 2.27 mg/kg (essential elements, not regulated)
- Non-cancer hazard index (HI) = 4.5 (>1 threshold) driven by Mn, Zn, Cu at high Myanmar rice consumption rates (~500 g/day)
- Total cancer risk (TCR) approximately 17× USEPA upper limit (10^-4); Ni contributes 66%, Cd 16%, As 13%
- LOQ calculated from blank SD × 10; no values below LOQ are reported as detects
Sample collection: 120 rice samples from 25 sites across five townships (Dala, Taik Kyi, Htan Ta Pin, Kaw Mhu, Kon Chan Kone), April 2018. Rice was polished (white) grain only. Source was farmer houses, not retail markets.
Methods (brief)
Agilent 8800 triple quadrupole ICP-MS (ICP-QQQ). Arsenic measured by mass-shift mode (m/z 75 → 91 as AsO+) to eliminate isobaric interferences; other elements by helium collision mode. Sample prep: 30 mg powdered dry rice digested in PFA vial with HNO3/H2O2 at 80°C, diluted to 30 mL. Four certified reference materials used (NMIJ CRM 7501-a, 7502-a; NIES CRM No. 9, 10); recovery within 5–15% at LOQ, 2–4% at concentrations one order of magnitude above LOQ. Method measures total arsenic, not inorganic arsenic. The study estimates iAs as 30% of tAs for risk calculations; this is a modeled assumption, not a measured speciation.
Limitation: tAs only, no HPLC speciation for iAs. The 30% iAs/tAs estimate is a literature proxy, not measured. This paper therefore contributes to the tAs sub-block on the rice ingredient profile, not to iAs directly.
Implications
Certification: Background reference data for Myanmar rainfed (non-groundwater-irrigated) rice. The low tAs, Pb, and Cd values confirm that paddy origin and irrigation regime are primary drivers of rice metal contamination — relevant to supply-chain sourcing for HMT&C-certified products using rice ingredients.
Courses: Useful illustration of how geography (delta, no mining, rainfed) determines background metal concentrations in rice. Nickel as a risk driver alongside arsenic is an underappreciated finding worth including in rice modules.
App: Contributes Myanmar rainfed rice data point to tAs and Ni contamination profile ranges; flagged as low end of global distribution. Cannot be used for iAs estimates without speciation.