Arsenic and heavy metal contents in white rice samples from rainfed paddy fields in Yangon division, Myanmar-Natural background levels?

Abstract

The presence of potentially toxic metal(loid)s (As, Pb, Cd, Cr, Mn, Fe, Zn, Cu, Ni, Mo and Co) in 120 white (polished) rice grains (Oryza sativa; 2017 or earlier crop) purchased from farmers in the five most agriculturally active townships near Yangon in the eastern edge on Ayeyarwady Delta was determined by triple quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ). Their total-As and Ni concentrations (0.16 mg/kg, 0.39 mg/kg) were around the worldwide average literature values from a heavy metal non-contaminated area of intermediate to acidic (non-mafic) composition. Their Pb, Cd, and Cr mean concentrations (0.010, 0.0056, and 0.056 mg/kg, respectively) were lower than the maximum allowable levels by over one magnitude, reaching the concentration ranges comparable to the lowest level in the literature values. This study's natural background levels were explained by a negligible influence of human, mining and industrial activities in this area, and probably genotype effect, which remains to be examined by the associated paddy soil analysis. Health risks associated with rice consumption (ca. 0.5 kg/day) by the inhabitants were estimated, assuming that inorganic arsenic was 30% of the total. Arsenic was the main contributor (30%) to the total value of the non-cancer risk (HI) of each element, which was 4.5 times the reference value (< 1), followed by Mn, Zn, Cu, Mo, Co and Ni (15-7%) and Pb, Cd, Cr and Fe (below 4%). The total cancer risk (TCR) for each element was around 17 times higher than the upper limit of cancer risk for an environmental carcinogen (< 0.0001): Nickel accounts for two-thirds of the contribution (66%), followed by Cd (16%) and As (13%). This study suggests that consumers of Yangon rice from paddy fields without groundwater irrigation may need to be concerned about the potential risk of Ni intake besides arsenic.