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US rice may carry an arsenic burden

August 2, 2005 By Mark Peplow This article courtesy of Nature News.

Legacy of cotton pesticides might be poisoning crops.

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Rice grown in the United States contains an average of 1.4 to 5 times more arsenic than rice from Europe, India and Bangladesh, according to a survey of grains from around the world.

This means that people eating a 'subsistence' diet of 500 grams of dry American rice a day are probably consuming more than the maximum intake of arsenic provisionally recommended by the World Health Organization (WHO), says Andrew Meharg, a biogeochemist from the University of Aberdeen, UK, and a member of the survey team.

Although no one knows whether this level and type of arsenic is dangerous, Meharg himself has decided to stop eating American grains.

"When we're all paranoid about what's in our food, the question to ask is, 'Would you like your rice to be grown on arsenic contaminated soil?'," he says.

Low doses of arsenic such as these do not cause acute illness. "It's more about long term intake that can elevate levels of cancer," says Meharg. Research in Taiwan has linked arsenic-contaminated rice to an increase in bladder cancer, for example1.

Poison legacy

Would you like your rice to be grown on arsenic contaminated soil?
Andrew Meharg
University of Aberdeen
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.

However, this means that they are more likely to accumulate arsenic in apparently healthy grains, says Meharg, who is now calling for change in farming methods. "I don't think they should be growing rice on old cotton fields," he says.

Of the rice eaten in the United States, the vast majority is home-grown. About half of all US-grown rice is exported.

Gram for gram

Meharg tested rice bought from markets in Aberdeen that had been grown in America, Europe, India, and Bangladesh. He found an average of 0.26 micrograms of arsenic in each gram of US rice. Indian rice hit a low of 0.05 micrograms per gram, whereas Bangladesh, which has had recurring problems with arsenic contamination owing to naturally high levels of the poison in groundwater, and Europe had about 0.15 micrograms per gram. The results are published in Environmental Science and Technology2.

John Duxbury, a soil chemist at Cornell University in Ithaca, New York, points out that there are still many gaps to fill in before we will know whether an overdose of US rice might be bad for you. "The work is fine as far as it goes, but you can't draw broad conclusions from such a limited survey," he says.

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," David Coia, spokeman for the USA Rice Federation in Washington DC.

All tied up

The rice may not be particularly toxic, because of the form that the arsenic takes in the plants. Health effects are diminished if the arsenic atoms are bound up with carbon-based molecules. Inorganic arsenic (the form found in drinking water) is estimated by Duxbury to be five to ten times more toxic.

Meharg found that just 42% of arsenic in US rice was inorganic, compared with 81% of arsenic in Indian rice. But Meharg points out that organic arsenic can still cause problems3, and could convert into the inorganic form in the body4.

The health effects of arsenic in food are hard to verify because the increase in cancer risk is small. Meharg estimates that if 10,000 people were exposed to the WHO limit over their lifetime, this would result in an extra 92 cases of bladder cancer.

Given the uncertainties, regulations are few. Even the WHO has not ratified its provisional guidelines. Australia is the only country that has a safety limit for arsenic in food.

Under review

There are a few different types of arsenic pesticides and herbicides licensed for use in the United States. All are undergoing safety reviews at the moment, says Enesta Jones, a spokeswoman for the Environmental Protection Agency, and decisions are planned for next year.

Duxbury cautions that arsenic in US rice might come from natural, geological sources. It may be that the arsenic in pesticides is in a form that is harder for the plants to take up, he adds, which could lower concerns about the chemicals themselves.

Although Duxbury doesn't share Meharg's health concerns, he says the work serves as a good base from which to work out how crops can be contaminated. This could help to breed plants that tend not to store arsenic in an inorganic form, he suggests. "There's potential for a lot of follow up from this."

References

  1. Chiou H. Y., et al. Am. J. Epidem., 153. 411 - 418 (2001).
  2. Williams P. N., et al. Environ. Sci. & Technol., published online, doi:10.1021/es0502324 (2005).
  3. Cohen S. M., et al. Chem. Res. Toxicol., 15. 1150 - 1157 (2002).
  4. Akter K. F., Owens G., Davey D. E. & Naidu R. Rev. Environ. Contam. Toxicol., 184. 97 - 149 (2005).

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