The market survey conducted in this study shows a large variation in heavy metal contaminants in rice grain across the Bahamian market. We detected a total of 18 different metals and metalloids in the rice products; however, we focused this study to As since it was by far the largest contributor to health risk. As shown in Fig. 1a-b, only one of the 42 rice varieties contained estimated iAs concentration above the WHO (200 µg/kg Body Weight) safety limits after accounting the organic As portion (Fig. S1). It is important to note that sample containing iAs above the permissible limit was unpolished. The elevated As in the rice grain may be due to the presence of the bran layer and cereal germ 1, 17. Previous studies have shown that a large portion of trace contaminants especially As is accumulated in the rice husk 11, 17.
Rice is widely consumed in The Bahamas as one of the main staple foods and this may pose a health risk. Since rice consumption is recognized as one of the main routes of human exposure to heavy metals and As. Therefore, increasing rice consumption will result in an increase exposure and risk of heavy metal contamination to the Bahamian consumers. Our preliminary results show that on average, an adult living in The Bahamas consumes 486.5g of rice per day which is comparable to what is consumed in China and East Asia 1, 11–13. The EDI and THQ values for As and Cr are shown in Fig. 2a-b and Fig. 3a-b. Approximately 79% and 57% of the samples had carcinogenic and non-carcinogenic risk, respectively, above WHO/US EPA guidelines (Fig. 2a-b and Fig. 3a-b). This discrepancy between iAs concentration and health risk is due to relative high rice consumption in The Bahamas.
To be clear, in arriving at our values we have assumed a constant iAs percentage of 75% of total As across all rice varieties. While we have performed speciation measurements on multiple samples with iAs ranging from 40–97%, we note the large variation in iAs speciation samples of the same brand varieties bought at different grocery stores. In two of the three cases where the same brand varieties were measured, iAs speciation percentages differed by approximately 20%. Given this we have opted to use the constant 75% until further measurement with larger data sets can be performed. We cannot overemphasize large uncertainties in arriving at our results and therefore the need to proceed with caution when interpreting these results, and also the need for follow up studies.
For example, the RfD which is used to calculate non-carcinogenic risk is an estimate with assumed uncertainties that could span an order of magnitude. Further the cancer slope factor used is subject to large uncertainties since is based on a more than forty year old study of skin cancer in Taiwanese citizens 25–26. While this value is commonly used to calculate broad carcinogenic risk, it is possible to determine better estimates for The Bahamas. However, this would require significant resources and long term epidemiological studies. What is more achievable and useful in the short term is a national total diet study for The Bahamas.
This study is to the best of our knowledge the first of its kind in The Bahamas and served as a preliminary evaluation of selected heavy metals in various rice grains, sold locally, for potential health risks due to consumption. Although potential health risks were found, further studies should be conducted to corroborate these findings. It is also important to note that rice is just one source of As and metal contaminant in staple food. Additional studies of water and cereal are likely to reveal additional risk from those sources. Therefore, continuous monitoring and regular evaluation of the Bahamian marketed rice products, cereal and water is recommended.
In terms of immediate next steps action steps, we recommend the following:
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Replicate parts of this study to focus on those varieties with the highest levels of inorganic arsenic. Similar results may justify the need for urgent action.
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Expand the study to examine metal contaminants in cooked rice on from various restaurants and eateries. This may give a more accurate representation of consumption levels of metal contaminants.
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Conduct survey to get a better understanding on consumption patterns of rice and rice-based products. Rice products consumed by infants and children should be given particular attention since the effects of inorganic arsenic infants and children as been shown to be more pronounced27.
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Expand the study to determine the effects of washing rice as well as cooking rice in high water to rice ratio on levels of arsenic.
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Conduct future studies on rice-based products especially those consumed by infant and children.