Final answer:
A US citizen living at sea level generally receives an annual radiation exposure of around 620 mrem, predominantly from radon, medical sources, and cosmic radiation.
Step-by-step explanation:
The estimated yearly exposure of a US citizen living at sea level to radioactivity and radiation is approximately 620 millirem (mrem). The three primary sources contributing to this exposure include radon, medical sources, and cosmic radiation. Radon accounts for more than half of the total exposure, while medical sources (such as X-rays and other medical imaging) and background radiation, which includes cosmic rays, each account for about 15%. It is noted that the majority, about 82%, of the radiation exposure comes from natural sources, with cosmic rays being partially shielded by the Earth's atmosphere.