Mercury contamination status of rice cropping system in Pakistan and associated health risks

Rice is a known bioaccumulator of methylmercury (MeHg). Rice consumption may be the primary pathway of MeHg exposure in certain mercury (Hg)-contaminated areas of the world. Pakistan is the 4th-largest rice exporter in the world after India, Thailand, and Vietnam. This study aimed to evaluate the Hg contamination status of rice from Pakistan and the health risks associated with Hg exposure through its consumption. 500 rice grain samples were collected from two major rice-growing provinces, Punjab and Sindh, which contain 92% of Pakistan's rice cultivation area. Analysis of polished rice showed mean total Hg (THg) concentration of 4.51 ng.g(-1), while MeHg concentrations of selected samples averaged 3.71 ng.g(-1). Only 2% of the samples exceeded the permissible limit of 20 ng.g(-1). Samples collected from Punjab showed higher Hg contents than those from Sindh, possibly due to higher rates of urbanization and industrialization. Rice samples collected from areas near brick-making kilns had the highest Hg concentrations due to emissions from the low-quality coal burned. THg and MeHg contents varied by up to five and fourfold, respectively, between point and non-point Hg pollution sites. Moreover, the %Hg as MeHg in rice did not differ significantly between point and non-point Hg sources. Health risk was assessed by calculating a mean probable daily intake, revealing that Hg intake through rice consumption is within the safe limits recommended by the World Health Organization. However, rice intake may be a substantive pathway of MeHg exposure because fish, which are another major source of Hg, are consumed in Pakistan at some of the world's lowest rates. This study provides fundamental data for further understanding of the global issue of Hg contamination of rice and its related health risks. Furthermore, the current study suggests there is a need to conduct further research in rice-growing areas at the regional level. (C) 2020 Elsevier Ltd. All rights reserved.