Arsenic is a category one (the highest level) carcinogen. Most of the arsenic in the American diet may gave come from meat; arsenic-containing drugs have been fed to factory-farmed animals to kill intestinal parasites, which can convert to inorganic arsenic during cooking. It accumulates, along with other banned chemicals, as slaughterhouse byproducts can be recycled as animal feed. Chicken probably has the most arsenic, and seems to be the primary source of arsenic found in children. But fish may also not be safe in this regard. In fact, arsenic is used a biomarker for total fish and seafood intake (see also here).
One seaweed species, hijiki (also commonly spelled hiziki), absorbs so much arsenic from the sea water that it is not safe to eat (see here for good seaweed). Rice grown in water logged soil can absorb arsenic found in the environment naturally or from from pesticide use or chicken manure fertilizer. The common Ayurvedic medicine triphala has been found to be contaminated with arsenic, lead, and mercury (see also here, here).
Topic summary contributed by Andrew.