Occurrence of total mercury and methylmercury in rice: Exposure and health implications in Nepal

Emerging studies have indicated that the consumption of rice could be the major methylmercury (MeHg) contributor to human mercury (Hg) exposure. Nonetheless, few studies are available on Hg in rice around the world, especially in countries with heavy rice diet. In this study, total Hg (THg) and MeHg levels in rice samples (n = 172) across Nepal were first investigated. The geometric mean THg was 7.05 +/- 7.71 mu g/kg with a range of 0.622 mu g/kg to 158 mu g/kg, and the maximum THg level was up to 791% of the Chinese National Standard Limit for THg in rice (20 mu g/kg). The geometric mean MeHg was 0.820 +/- 0.660 mu g/kg with a range of 0.189 mu g/kg to 8.59 mu g/kg. Overall, the mean MeHg exposure (0.00445 +/- 0.00477 mu g/kg bw/day) and inorganic Hg (IHg) exposure (0.0360 +/- 0.0739 mu g/kg bw/day) were lower than the reference dose (RfD) of 0.1 mu g/kg bw/day for MeHg and the provisional tolerable weekly intake (PTWI) of 0.571 mu g/kg bw/day for IHg, respectively. Concerning different groups of vulnerable populations, the highest MeHg exposure (0.126 mu g/kg bw/day) and IHg exposure (1.57 mu g/kg bw/day) of preschoolers (37-50 months old) were approximately 126% of the RfD for MeHg and 275% of the PTWI for IHg. When the pregnant mothers eat the rice without awareness of the Hg content in rice, the mean and highest intelligence quotients (IQs) losses were 9554 and 118659 points, respectively, and the corresponding economic costs due to IQ loss could be 15.1 million USD and 188 million USD in Nepal. The results of rice THg and MeHg levels and corresponding exposure in populations highlighted the occurrence of rice THg and MeHg pollution issues in Nepal. More efforts should be made to protect younger groups in Nepal from high rice Hg exposure. Capsule: Owing to the high rice consumption rates relative to body mass, preschoolers (37-50 months) may meet the 126% reference dose (0.1 mu g/kg bw/day) for MeHg and 275% provisional tolerable weekly intake (0.571 mu g/kg bw/day) for IHg exposure in Nepal.