Humans are radioactive, a natural state stemming from naturally occurring radioactive isotopes within the body. This is a constant aspect of human biology, arising from the environment we inhabit and the substances we consume. The radioactivity found within us is not a cause for concern, but rather a testament to radiation’s pervasive nature. Our bodies continuously incorporate and process these elements.
The Radioactive Elements Within Us
The human body contains several specific radioactive elements, primarily Potassium-40 ($^{40}$K) and Carbon-14 ($^{14}$C). An isotope is a variation of a chemical element where atoms have the same number of protons but a different number of neutrons in their nucleus. Some isotopes are unstable and undergo radioactive decay, releasing energy.
Potassium-40 is a primordial radioisotope, existing since Earth’s formation, and is the largest source of natural radioactivity in humans. It constitutes a small, fixed percentage (about 0.0117%) of all potassium, an element essential for muscle function and nerve signaling. An average adult contains around 140 grams of potassium, leading to approximately 3,000–6,000 becquerels (Bq) of $^{40}$K activity. This means thousands of $^{40}$K atoms decay every second within the body.
Carbon-14 is another significant contributor to internal radiation. This radioactive carbon isotope is present in all organic matter, including human tissues, and is produced naturally in the atmosphere. Other naturally occurring radioactive elements, such as trace amounts of uranium, thorium, and their decay products like radon and radium, are also found in the human body. Tritium, a radioactive isotope of hydrogen, is present as well.
Where Our Internal Radiation Comes From
The radioactive elements within our bodies enter through various environmental pathways, primarily through the food we eat, the water we drink, and the air we breathe. These naturally occurring radionuclides are ubiquitous in the environment, found in soil, rocks, and even the atmosphere. Plants absorb these elements from the soil and water, and subsequently, they are transferred up the food chain to humans.
For example, potassium, including its radioactive isotope $^{40}$K, is present in many foods, with bananas being a well-known example due to their high potassium content. Similarly, Carbon-14 is incorporated into all organic molecules through photosynthesis in plants and then consumed by animals and humans. Our bodies continuously replenish these radioactive substances as part of normal metabolic processes.
Cosmic radiation also plays a role in the formation of some internal radionuclides. Radon gas, a decay product of uranium and thorium naturally present in bedrock and soil, can be inhaled, contributing to internal radiation exposure. The absorption and distribution of these materials throughout the body depend on their chemical properties, with elements like potassium and cesium tending to accumulate in muscles, while strontium, similar to calcium, goes to the skeleton.
Measuring and Understanding Human Radiation
The radiation emitted by humans is very small and is a natural component of the overall background radiation we encounter daily. The body’s natural repair mechanisms are capable of handling the low levels of radiation exposure from these internal sources.
The amount of radioactivity within the human body can be measured using specialized instruments called whole-body counters. These sensitive devices detect gamma rays emitted by radioactive elements inside the body as they undergo decay. A person might sit or lie in a shielded chamber, and detectors positioned around them measure the energy and intensity of these gamma rays, allowing scientists to identify and quantify the specific radionuclides present.
The typical activity from Potassium-40 and Carbon-14 in an adult body results in an internal radiation dose. For an average adult, the effective dose from internal radioactive materials, excluding radon, is approximately 0.3 millisieverts (mSv) per year. This is a small fraction compared to the total natural background radiation dose, which averages around 3.1 mSv per year in the United States, originating from cosmic rays, terrestrial sources, and inhaled radon. This level of internal radiation is significantly lower than doses that could cause adverse health effects, which typically require much higher, acute exposures.