An article in Nature.com says:
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.
http://www.nature.com/news/2005/050801/full/050801-5.html