Toxicity to humans from heavy metals is largely the consequence of industrial activity that results in accidental environmental contamination and subsequent human exposure through air, water, and food. Less commonly, heavy metal exposure results from intentional poisoning (such as arsenic or thallium). There is no uniformly accepted definition of heavy metals, but the most medically relevant elements are usually cited as arsenic, cadmium, lead, mercury, and thallium. Otoneurological effects of heavy metal toxicity are nonspecific (with the possible exception of impaired vertical optokinetic nystagmus in mercury toxicity), so these diagnoses are more likely to be suspected from the history and laboratory tests (such as serum markers). If heavy metal toxicity is suspected, a medical toxicology consultation is appropriate.