I heard about this on KCRW's "Good Food" this morning and found this
among many articles about the use of arsenic in chicken feed to make
them eat more so they can be sent to market more quickly:


Arsenic In Chicken Feed May Pose Health Risks To Humans, C&EN Reports
[excerpt]

Pets may not be the only organisms endangered by some food additives. An
arsenic-based additive used in chicken feed may pose health risks to
humans who eat meat from chickens that are raised on the feed, according
to an article in the April 9 issue of Chemical & Engineering News, the
weekly news magazine of the American Chemical Society.

Roxarsone, the most common arsenic-based additive used in chicken feed,
is used to promote growth, kill parasites and improve pigmentation of
chicken meat. In its original form, roxarsone is relatively benign. But
under certain anaerobic conditions, within live chickens and on farm
land, the compound is converted into more toxic forms of inorganic
arsenic. Arsenic has been linked to bladder, lung, skin, kidney and
colon cancer, while low-level exposures can lead to partial paralysis
and diabetes, the article notes......

<http://www.medicalnewstoday.com/medicalnews.php?newsid=67500&nfid=crss>