years after the ingestion of thorium for symptoms to manifest themselves. Thorium has no known biological role. Chemical Powdered thorium metal is pyrophoric: it ignites spontaneously in air. In 1964, the United States Department of the Interior listed thorium as "severe" on a table entitled "Ignition and explosibility of metal powders". Its ignition temperature was given as 270 °C (520 °F) for dust clouds and 280 °C (535 °F) for layers. Its minimum explosive concentration was listed as 0.075 oz/cu ft (0.075 kg/m3); the minimum igniting energy for (non-submicron) dust was listed as 5 mJ. In 1956, the Sylvania Electric Products explosion occurred during reprocessing and burning of thorium sludge in New York City, United States. Nine people were injured; one died of complications caused by third-degree burns. Exposure routes Thorium exists in very small quantities everywhere on Earth although larger amounts exist in certain parts: the average human contains about 40 micrograms of thorium and typically consumes three micrograms per day. Most thorium exposure occurs through dust inhalation; some thorium comes with food and water, but because of its low solubility, this exposure is negligible. Exposure is raised for people who live near thorium deposits or radioactive waste disposal sites, those who live near or work in uranium, phosphate, or tin processing factories, and for those who work in gas mantle production. Thorium is especially common in the Tamil Nadu coastal areas of India, where residents may be exposed to a naturally occurring radiation dose ten times higher than the worldwide average. It is also common in northern Brazilian coastal areas, from south Bahia to Guarapari, a city with radioactive monazite sand beaches, with radiation levels up to 50 times higher than world average background radiation. Another possible source of exposure is thorium dust produced at weapons testing ranges, as thorium is used in the guidance systems of some missiles. This has been blamed for a high incidence of birth defects and cancer at Salto di Quirra on the Italian island of Sardinia. See also Thorium Energy Alliance Explanatory notes Citations General bibliography Further reading International Atomic Energy Agency (2005). Thorium fuel cycle – Potential benefits and challenges External links Chemical elements Chemical elements with face-centered cubic structure Actinides Carcinogens Nuclear fuels Nuclear materials