The largest market basket survey of the arsenic content of rice grown in the United States has found elevated levels of arsenic in rice produced in the South Central part of the country, scientists report in an article scheduled for the April 1 issue of ACS’ journal Environmental Science & Technology.
The University of Aberdeen’s A. A. Meharg and colleagues did the study, which involved analyses of rice purchased at U. S. supermarkets.
A previous study found that U. S. rice purchased in the United States had higher arsenic levels than rice grown in Europe, India or Bangladesh. Subsequent press coverage caused the USA Rice Federation to dispute the significance of the data because of the limited number of samples. The team has now followed up on that initial study by analyzing more rice samples.
In the study, the research group compared total arsenic levels in 134 samples of rice from the two main rice-producing areas of the country — the South Central States (Arkansas, Louisiana, Mississippi, Texas, and Missouri) and California.
The researchers inferred where the rice grew from the location of the processor named on the package label, and they confirmed its origin with multielemental fingerprinting combined with principal component analysis (PCA). By measuring levels of trace elements, such as cobalt, copper, selenium, zinc and manganese , they could broadly determine the rice’s origins because “the plant reflects the environment it’s growing in,” explains Meharg.
Rice grown in the South Central States had significantly more arsenic than California rice. Arsenic levels in south central U.S. rice averaged 0.27 µg/g, whereas arsenic in rice from California averaged 0.16 µg/g. The lowest arsenic contamination was found in an organically grown rice from California (0.10 µg/g), the highest arsenic concentration was found in a sample of rice from Louisiana mills (0.66 µg/g). Rice, high in arsenic often is grown in old cotton fields that previously were treated with arsenic pesticides, the study states, adding that arsenic-tolerant strains of rice often are grown in those fields.
While none of the rice samples exceeded the 1.0 milligram per kilogram threshold recommended by some countries, the researchers concluded from intake models that certain population groups could get dietary exposure to arsenic that exceeds California’s state exposure limits. Those groups include low-income individuals who consume large amounts or rice as an inexpensive food; people with celiac disease (who eat rice as part of a gluten-free diet); Asian-Americans who consume a high-rice diet; and Hispanic infants and toddlers, who also have a diet high in rice, the study notes. Even for the average American, rice would still contribute significantly to total dietary exposure.
While it may be disputed, wether the arsenic in the rice originates from the historic use of pesticides or from geological sources, the consequences for human health are identical. Further studies should therefore more closely investigate the relationship between arsenic in the soil and the rice, the speciation of arsenic in soil and rice and the factors influencing the transfer from soil to the plant. Since inorganic arsenic is classified a group A human carcinogen, the target should be to reduce the arsenic contamination of rice by growing rice on soils not contaminated by arsenic, avoiding further arsenic contamination through arsenic containing fertilizers (e.g. chicken litter or sewage sludge) and by using rice varieties with low arsenic uptake. The original study
P.N. Williams, A. Raab, Jörg Feldmann
, A.A. Meharg, Market Basket Survey Shows Elevated Levels of As in South Central U.S. Processed Rice Compared to California: Consequences for Human Dietary Exposure
, Environ. Sci. Technol., 2007. DOI: 10.1021/es061489k
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