Chicken farmers have been feeding chickens arsenic for decades.

There’s not enough arsenic in the chicken we eat to kill anyone, but legally, stores and restaurants can sell chicken to American consumers with an arsenic concentration as high as 500 parts per billion (that’s 50 times higher than the allowable concentration of arsenic for drinking water). Arsenic has been part of chickens’ diets since the 1940s. It’s an ingredient in a feed additive called roxarsone that helps chickens grow bigger and fight against parasites. By 2000, 70% of all commercial chicken producers used roxarsone.

In response to some of the concerns over health risks, many chicken producers since then have stopped using arsenic, including Tyson, Bell & Evans, Eberly, and the chicken farms used by McDonald’s and KFC (though tests show that some of their chicken still have trace elements of arsenic). To be safe, someone weighing 154 pounds shouldn’t eat more than 2 ounces of chicken every day.
(Sources: 1, 2)