Which radioactive element is primarily responsible for indoor radon exposure?

Uranium is primarily responsible for indoor radon exposure due to its natural decay process. When uranium found in soil and rock decays, it produces radium, which subsequently decays into radon. Radon is a colorless, odorless gas that can accumulate in homes, particularly in basements and lower levels of buildings. This accumulation occurs because radon can seep through cracks in floors and walls, construction joints, and around service pipes.

Understanding the geochemical context is important; uranium is abundant in certain geological formations, particularly in areas with granite or sedimentary rocks like phosphate deposits. Once uranium is present in these environments, its decay series leads to radon production, which can pose health risks, including lung cancer upon prolonged exposure. The other elements listed, while radioactive, do not produce radon as part of their decay chains and are not significant contributors to indoor radon levels.