The human body is naturally radioactive, with about 4,400 atoms decaying inside you every second.

The primary source of internal radioactivity in humans is Potassium-40 (40K), a naturally occurring isotope with a half-life of 1.25 billion years. An average adult weighing 70 kilograms (154 pounds) contains about 140 grams of potassium, of which approximately 0.0117% is the radioactive 40K. This results in roughly 4,400 disintegrations per second, also known as Becquerels (Bq), within the body's muscles and organs.Carbon-14 (14C) is the second major contributor, adding about 3,000 to 4,000 decays per second. Unlike potassium, which is regulated by the body's homeostatic mechanisms, carbon-14 levels are maintained through the continuous consumption of organic matter. These isotopes are integrated into our DNA, proteins, and fats during the metabolic process. When these atoms decay, they emit beta particles and gamma rays, which are forms of ionizing radiation.According to the Health Physics Society and the National Council on Radiation Protection and Measurements (NCRP), the average person receives an annual dose of about 0.3 millisieverts (mSv) from internal radionuclides. This is a significant portion of the total background radiation dose of 3.1 mSv that humans receive from all natural sources annually. Other minor contributors include traces of Uranium-238 and Thorium-232 that are naturally present in the soil and water.While the idea of being radioactive might sound dangerous, the human body has evolved robust DNA repair mechanisms to handle this low-level internal exposure. These biological processes are a fundamental part of life on Earth, as radiation has been a constant presence throughout evolutionary history. Scientists use this predictable decay of isotopes like Carbon-14 for radiocarbon dating, though the levels in living tissue remain relatively constant until death.