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Long before human beings decoded the human genome or split the atom, they discovered that arsenic is very good at killing things. The ancient Romans prized it as a murder weapon because it could be mixed into food or drink without altering its color, taste, or smell. Plus, a tiny dose kills without fail.

What the Romans didn’t know about arsenic, and what scientists didn’t discover until the 20th century, is that a form of it – inorganic arsenic – causes cancer. And in 1999, the National Academy of Sciences found that the amount of arsenic legally allowed in U.S. drinking water posed serious cancer risks.

Since then, the U.S. government slashed the amount allowed in drinking water from 50 micrograms per liter to just 10. The potent carcinogenicity of arsenic was what Donald Rumsfeld might call an “unknown unknown” for most of human history. So was the fact that Americans can consume dangerous amounts of inorganic arsenic in one of our most common foods: rice.

Consumer Reports dropped that bombshell on the nation in September, recommending that adults limit rice consumption to just two servings per week if they wish to reduce their cancer risk from inorganic arsenic. The U.S. Food and Drug Administration didn’t go so far as to recommend Americans limit their rice intake. But the agency did say that “consumers should continue to eat a balanced diet that includes a wide variety of grains.” They also published their own data on arsenic in rice. It was consistent with the findings of Consumer Reports.