|Little did Thomas know his casual selenium use was adversely affecting his social standing|
Exposure to relatively toxic metals and metalloids can result in a wide range of symptoms, some of which are pretty weird. Body parts can change colour, skin can erupt with lesions, bones can soften, hair can be shed, the nervous system can go haywire, and the smell of one's breath can transition from unnoticeable to downright peculiar.
Long-term exposure to lead (e.g. working at a lead mine or smelter) can cause your breath to acquire a strange sweetish metallic smell, particularly in the morning. Centuries ago this was known as halitus saturninus (saturnism is an old word for lead poisoning). It provides an early indication of being exposed to toxic levels of lead, and is often accompanied by an astringent or sugary taste.
Exposure to inorganic selenium or tellurium compounds can result in a strong garlic-like breath odour as these metalloids are transformed in the body into volatile molecules consisting of a metalloid atom attached to a bit of organic carbon. For example, selenium breath has been attributed to the molecule dimethylselenide (carbon + selenium + carbon). Jöns Jacob Berzelius, a 19th century Swedish chemist who discovered several elements including selenium, was afflicted with intense garlic breath while he focused his investigations on selenium-containing materials. It was sufficiently bad that his housekeeper thought he had been eating too much garlic with his lunch! In other news, apparently tellurium breath usually takes a lot longer (i.e. a couple of months) to disappear compared to selenium breath (resolves in about a week), once a person is removed from contact with the metalloid.
Garlic breath was also thought at one point to result from bismuth exposure, but it was later determined that this was due to the presence of trace amounts of tellurium in the bismuth.
Dummett CO. 1944. Pertinent facts about the tongue and breath as aids in the diagnosis of disease. American Journal of Orthodontics and Oral Surgery 30(2):C57-C66.
Glover JR. 1967. Selenium in human urine: A tentative maximum allowable concentration for industrial and rural populations. Annals of Occupational Hygiene 10(1):3-14.
Nuttall KL. 2006. Evaluating selenium poisoning. Annals of Clinical & Laboratory Science 36(4):409-420. [Full text]
Prinz H, Greenbaum SS. 1939. Diseases of the Mouth and Their Treatment. Lea & Febiger. [Link to snippets]
Riechmann T, Hirner AV, Feldmann J. 1996. Determination of organometallics in intra-oral air by LT-GC/ICP-MS. Fresenius' Journal of Analytical Chemistry 354(5-6):620-623.
Wilber CG. 1980 Toxicology of selenium: A review. Clinical Toxicology 17(2):171-230.