Rice grown in the USA has up to five times higher levels of arsenic than those found in rice grown in Europe, India and Bangladesh, says a report in the August 2005 edition of Environmental Science and Technology.
Scientists from the School of Biological Sciences at the University of Aberdeen
in Scotland found an average concentration of 0.26 micrograms of arsenic per gram of U.S. rice. By contrast, the mean arsenic concentrations found in Indian basmati and Bangladeshi rice were 0.05 µg/g and 0.13 µg/g, respectively.
These results are surprising because Bangladesh struggles with severe arsenic-contaminated ground and irrigation water (see EVISA News, June 22, 2005
). Arsenic levels as high as 0.95 micrograms per gram of rice have been reported there in the past, the researchers wrote.
Consumption of rice contaminated by levels of arsenic such as these does not cause acute illness. "It's more about long term intake that can elevate levels of cancer," says Andrew Meharg
, a biogeochemist from the University of Aberdeen, UK, and a member of the survey team. Research in Taiwan has linked arsenic-contaminated rice to an increase in bladder cancer, for example.
Someone eating a 'subsistence' diet of 500 grams of dry American rice a day - not an uncommon amount in Asia - would probably be taking in more arsenic than the maximum intake of arsenic provisionally recommended by the World Health Organization (WHO), says Meharg.
The health risk imposed by arsenic contaminated rice cannot be assessed without knowing about the present arsenic species. The rice may not be particularly toxic, because of the form that the arsenic takes in the plants. Health effects are diminished for some organoarsenic compounds where the arsenic atoms are bound up with carbon-based molecules. The main arsenic species detected in the rice extract were As(III), DMA(V), and As(V). In European, Bangladeshi, and Indian rice 64 ± 1%, 80 ± 3%, and 81 ± 4%, respectively, of the recovered arsenic was found to be inorganic, most toxic species. In contrast, DMA(V) was the predominant species in rice from the USA, with only 42 ± 5% of the arsenic being inorganic. But Meharg points out that organic arsenic can still cause problems, and could convert into the inorganic form in the body.
The average American only eats about one ounce of uncooked rice a day or a little more than half a cup of cooked rice, says John Duxbury
, a professor of soil chemistry at Cornell University. That puts U.S. arsenic intake far below WHO standards, he says. "So it's pretty clear that we probably don't have a problem."
Others point out that there is no epidemiological evidence that anyone with a high rice diet, such as those of Asian descent, for example, is experiencing ill effects. "There simply are no known negative health issues with US rice," argues David Coia, spokesman for the USA Rice Federation
in Washington DC.
The reported concentrations are also questioned by other scientists. While the Scottish team bought seven samples of rice in supermarkets in Aberdeen, Cornell University scientists tested 25 samples and found higher levels of arsenic in Bangladesh rice (0.35 µg/g) in comparison to U.S. rice (0.18 µg/g).
Indeed, the Aberdeen sample seems not to be large enough to allow for a scientifically sound risk assessment of the U.S. population. It is however an interesting piece of work that may serve as a good base from which to work out how crops can be contaminated.
The survey team thinks that the contamination is a legacy of cotton farming, which relies on arsenic-based chemicals to kill boll weevils and to remove plants' leaves before harvest. Quite a lot of land in Mississippi and Arkansas that previously grew cotton is now used for rice cultivation, says Meharg.
When rice was first grown in these soils, the crop often failed owing to an arsenic-induced disease known as straighthead. So new, straighthead-resistant rice varieties were bred that could withstand the arsenic.
Arsenic is a contaminant often found in rice because it's grown in flooded paddies. That watery, oxygen-free environment may cause naturally occurring arsenic in the soil to be freed and thus more readily taken up by the plant.
Anyhow, the U.S. is the biggest consumer of arsenic in the world (see EVISA News, May 15, 2005
). There are a few different types of arsenic pesticides and herbicides licensed for use in the United States. All are undergoing safety reviews by EPA at the moment.
Arsenic in rice
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USA Rice Federation
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