The US Food and Drug Administration says consumers should not worry too much about levels of arsenic in rice - but should vary their diets just in case.
The agency, which monitors drugs and products for safety, released a study on Friday of arsenic in 1,300 samples of rice and rice products, the largest study to date looking at the carcinogen's presence in that grain. Consumer groups have pressured the FDA to set a standard for the amount of arsenic that can be present in rice products.
The study shows varying levels, with the most arsenic in brown rice and the least in instant rice. Infant cereal and infant rice formulas are also at the low end of the spectrum.
The FDA says the amounts are so small that rice is safe to eat and there isn't any concern of immediate or short-term adverse health effects. But the agency said it was still studying the long-term effects of eating rice.
Rice is thought to have arsenic in higher levels than most other foods because it is grown in water on the ground, optimal conditions for the contaminant to be absorbed.
Arsenic is present in water, air, food and soil in two forms, organic and inorganic. Organic arsenic passes through the body quickly and is essentially harmless. Inorganic arsenic - the type found in some pesticides and insecticides - can be toxic and may pose a cancer risk if consumed at high levels or over a long period.
Average levels of arsenic in the study ranged from 2.6 to 7.2 micrograms of inorganic arsenic per serving. Though the long-term effects are still unknown, that amount is tiny - a microgram is one-millionth of a gram.
Still, it is almost impossible to say how dangerous these levels are without a benchmark from the federal government. The advocacy group Consumer Reports, which is pushing for FDA to create standards, has used the US state of New Jersey's drinking water standard - a maximum of 5 micrograms in a litre of water - as an example of a benchmark because it is one of the strictest in the country.
The FDA study looked at rice from the United States, with some of the highest levels of arsenic found in rice grown in Southern states. It also looked at rice from Asia.
FDA toxicologist Suzanne Fitzpatrick said that because arsenic is naturally occurring it is going to be in food, and because rice is grown in water it will always have higher levels.
"It's not something that we can just pull off the market," she said.