The researchers documented 2.3 ppb — that's parts per BILLION -- of inorganic arsenic (the more toxic type of arsenic) in the meat of chicken that had measurable levels of Roxarsone.

By comparison, the meat from chicken that had no detectable levels of Roxarsone had .8 ppb of inorganic arsenic. That's three times less.

But it's important to point out that these low levels are far below the 500 ppb tolerance levels set by the FDA.

The researchers found no measurable trace of the arsenic-based drug in the 25 organic samples they tested (Roxarsone is not allow in organic chicken). By comparison, 20 of the 40 samples of meat from chickens raised conventionally did contain the drug.

The National Chicken Council released a statement calling the study's conclusions misleading. Chicken producers, the council says, are no longer using any arsenic-based drugs.

In lieu of Roxarsone, which had been used to prevent intestinal parasites, chicken producers have switched to drugs known as ionophores.

"Today, folks (chicken producers) are just doing the best they can without" Roxarsone, says Tom Super of the National Chicken Council. He says the ionophores are not as effective against the parasites.

The FDA, in this Q & A, says another arsenic-based drug known as Nitarsone is still being marketed. It's approved for use in chickens and turkeys. Though FDA does not disclose animal drug sales data, the drug is used to prevent outbreaks of Blackhead, an infection caused by parasites, in turkeys.

The National Turkey Federation says Nitarsone is used primarily in the turkeys' first few weeks of life, and used more heavily during the summer months, when Blackhead is more likely to occur. The industry depends on the drug as a preventive, since there's no effective treatment once an outbreak occurs.

The authors of the new study say they hope the FDA considers their conclusions in making decisions about the approvals for these drugs.

"Roxarsone still continues to be sold by (drug company) Zoetis in Latin America" and is still approved for use here, despite the fact that it was voluntarily pulled from the market in 2011," Johns Hopkins' Nachman says.

The FDA says it continues to investigate all uses of arsenic-based drugs in food-producing animals, and agency spokesperson Jalil Isa says the agency "will take the appropriate action to protect public health."

Copyright 2013 NPR.

NitarsoneNitarsone

How Trace Amounts of Arsenic End Up In Grocery Store Meat