What is the main source of natural background radiation on Earth?

Radon gas is indeed the main source of natural background radiation on Earth. It is a naturally occurring radioactive gas that is produced from the decay of uranium in the soil and rocks. As uranium decays, it forms radium, which in turn decays into radon. This gaseous element seeps from the ground into the air and can accumulate in buildings, particularly in poorly ventilated areas, contributing to indoor radon levels that can elevate a person's exposure to radiation.

Radon accounts for a significant portion of the average person's annual exposure to ionizing radiation, even more so than cosmic rays or radium. While uranium and radium are part of the decay series that leads to radon production, radon itself is what actually poses the most considerable risk in terms of natural background radiation exposure. Cosmic rays do contribute to background radiation, but their contribution is less than that of radon gas in most locations. Thus, radon gas stands out as the primary natural source of background radiation.