The largest market basket survey of arsenic in U.S. rice, published today on ES&T's Research ASAP website (DOI: 10.1021/es061489k), indicates that rice from California contains, on average, about 40% less arsenic than rice from the south central U.S.-Arkansas, Louisiana, Mississippi, Texas, and Missouri. figure USDA, Photo by Keith Weller U.S. long-grain rice can take up different amounts of arsenic, depending on where it is grown.
Andrew Meharg and co-workers at the University of Aberdeen (U.K.) measured arsenic levels in 134 samples of rice purchased from Arkansas and California supermarkets. Arsenic levels in south central U.S. rice averaged 0.27 micrograms per gram (Âµg/g), whereas arsenic in rice from California averaged 0.16 Âµg/g. The highest arsenic concentration was found in a sample of rice from Louisiana mills (0.66 Âµg/g), and the lowest was found in an organically grown rice from California (0.10 Âµg/g).
The researchers inferred where the rice grew from the location of the processor named on the package label, and they confirmed it with multielemental fingerprinting combined with principal component analysis (PCA). By measuring levels of unusual elements, such as selenium, they could broadly determine the rice's origins because "the plant reflects the environment it's growing in," explains Meharg.
In a previous study (Environ. Sci. Technol. 2005, 39, 5531-5540), Meharg and colleagues measured arsenic levels and speciation in rice from various countries, including several samples purchased in Scotland and labeled as U.S. rice. Subsequent press coverage caused the USA Rice Federation to dispute the significance of the data (Environ. Sci. Technol. 2006, 40, 2077-2078). The team has now followed up on that initial study by analyzing more rice samples.
Meharg speculates that the higher level of arsenic in rice from the south central U.S. can be traced to residual arsenic-containing pesticides still present in old cotton fields now used for growing rice. But not everyone is convinced. The higher amounts of arsenic in south central U.S. rice could be due solely to natural background levels in soils, notes Mark Barnett, an arsenic expert at Auburn University. Meharg mentions that rice growers are developing strains of rice especially for these high-arsenic soils, and he asserts in the paper that regardless of the original arsenic source, "the consequences for human health are identical."
Although U.S. rice consumption averages about 25 g per day per capita, some ethnic groups and people with gluten intolerance eat much more, Meharg says. On average, half the arsenic in rice is composed of the more dangerous inorganic form, although this varies widely, he continues. A person eating 100 g of rice that contains arsenic at a concentration of 0.3 Âµg/g would consume 30 Âµg of arsenic, probably half of which is inorganic. That person's dietary exposure from rice alone would be higher than that from water, for which the U.S. EPA has set a maximum contaminant level of 10 Âµg of inorganic arsenic per liter.
The levels of arsenic in rice from the south central U.S. make the report interesting, Barnett says, "however, I don't think that we should overreact." He points out that "none of the levels of arsenic in rice exceeded the 1.0 milligram per kilogram threshold recommended by some countries." Barnett also notes that arsenic is a normal part of the human diet and that arsenic intake from food often exceeds that from drinking water. More studies may be necessary to confirm the regional loads of arsenic in rice, says Barnett, who points out that the amount of arsenic that moves from rice in the digestive tract into the rest of the body-its bioavailability-is important as well.
Richard Loeppert, a soil chemist at Texas A&M University, says that the arsenic levels Meharg and colleagues report are about what he would expect. However, he stresses the limited usefulness of market-basket surveys, indicating that they do not help to identify the soil, crop management, and varietal factors that might affect arsenic concentrations. Loeppert also questions the value of PCA for sourcing rice, because soil metal concentrations can vary considerably over very short distances. He notes that south central U.S. samples may contain rice grown at several locations within a broad area, but Loeppert agrees that the California samples were probably grown in that state.
Meharg hopes that U.S. researchers will be able to monitor and survey individual farms to "establish the relationships between the soil contamination and the rice contamination, which we have been doing elsewhere in the world."
Rice's genetic variability might be exploited to yield both straighthead disease resistance and lower arsenic levels, notes Loeppert. That, he says, is both "the challenge to the industry" and "a window of opportunity".
Study: High Arsenic Levels in Rice from Southern U.S. States
Arsenic in U.S. rice varies by region
A market-basket study provides more evidence that some U.S. rice, particularly from southern regions, has elevated arsenic levels.
By BARBARA BOOTH
Environmental Science & Technology Online, February 21, 2007
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