The U.S. Food and Drug Administration allows Roxarsone to be used in chicken feed, although arsenic-based food additives are banned in Europe. Tyson, the largest U.S. chicken producer, announced last year it was abandoning arsenic additives.

Perdue still uses Roxarsone to keep chickens from getting parasitic diseases and says it is safe because the arsenic is the organic variety. But a researcher at Duquesne University has found evidence that the arsenic converts to the inorganic kind when it combines with bacteria in chicken manure, The Baltimore Sun reports.

That manure is used for fertilizer and can leak into waterways.

"It's very disturbing to me that people are being exposed to this arsenic, but we don't have a choice -- we have to feed the chickens what the company gives us," said Carole Morison, a contract grower for Perdue on Maryland's Eastern Shore. "Farmers spread manure with arsenic in it all over their land as fertilizer, and we don't know what the risks are."