220Rn

220Rn, commonly known as thoron, is the isotope radon-220. It is a radioactive noble gas produced in the thorium-232 decay chain and originates from soils, rocks, and thorium-containing building materials as 224Ra decays to 220Rn.

With a half-life of 55.6 seconds, 220Rn decays by alpha emission to 216Po, which rapidly decays further along the thorium-232 decay chain to stable lead-208. The short half-life means thoron tends to decay close to its source, creating strong spatial gradients in indoor air and limiting its reach from source to living spaces.

Occurrence and indoor relevance: Thoron is present wherever thorium-bearing materials are nearby, including some natural stones, concrete, clinker, and certain ceramics. Indoor levels are typically lower than those of radon-222, but can be locally significant when materials rich in thorium are in contact with air and ventilation is limited.

Measurement and detection: Because of its brief existence, detecting thoron requires specialized equipment capable of discriminating 220Rn from 222Rn and capturing its short-lived progeny. Some passive detectors and active monitors are designed to measure thoron separately, but standard radon tests may underestimate exposure if they do not distinguish the isotopes.

Health considerations and regulation: Inhalation of thoron and its decay products emits alpha particles that contribute to lung dose. The health risk from thoron is less well characterized than that of radon-222 and depends on proximity to sources and ventilation. Some countries include thoron in indoor air quality guidelines and radon risk assessments, though practices vary.