How long radiation remains in the human body is complex, lacking a single, simple answer because “radiation” is a general term for many different phenomena. The duration radiation persists depends entirely on the source’s nature and how it interacts with the body’s systems. Most commonly encountered forms of radiation do not linger at all. Retention time is only a concern when radioactive material is physically introduced internally, governed by specific biological and physical rules.
Distinguishing Radiation Exposure from Internal Contamination
External radiation exposure occurs when the body passes through a field of energy, such as during a medical X-ray or when standing near a radioactive source. This energy, whether X-rays, gamma rays, or neutrons, penetrates the body and may deposit some dose, but it does not cause the person to become radioactive. As soon as the source is turned off or the person leaves the radiation field, the exposure ceases entirely.
The concern about radiation “staying” in the body applies only to internal contamination, which involves the physical presence of radioactive material inside tissues. Internal contamination happens when radioactive atoms—called radionuclides—are swallowed, inhaled, absorbed through the skin, or enter through a wound. Once inside, these materials continue to emit radiation until they decay or are naturally eliminated. The length of time these radionuclides remain active inside the body determines the total dose received.
The Science of Clearance: Physical and Biological Half-Life
The time it takes for a radionuclide to clear from the body is governed by two independent rates, each measured by a “half-life.” The physical half-life (\(T_p\)) is a fixed, characteristic constant for every radionuclide, defining the time required for half of the radioactive atoms to decay naturally into a more stable form. This decay rate is unaffected by external factors.
The second rate is the biological half-life (\(T_b\)), which is the time needed for the body to eliminate half of the substance through natural processes like excretion or metabolism. This biological clearance rate can vary significantly between individuals based on their metabolism, age, and health.
The true measure of how long a radionuclide remains active inside a person is the effective half-life (\(T_e\)), which combines both the physical decay and the biological excretion. The effective half-life is always shorter than either the physical or the biological half-life alone, reflecting that the material is disappearing due to two mechanisms simultaneously. This combined rate determines the total duration the internal contamination poses a risk.
How Long Radiation Stays After Medical Procedures
The most common way people encounter internal radiation is through nuclear medicine procedures like PET scans or diagnostic imaging, which use radiopharmaceuticals. These isotopes are specifically chosen because they have a very short physical half-life, ensuring rapid clearance. For instance, Technetium-99m, the most widely used medical isotope, has a physical half-life of just six hours.
The short physical half-life means the material quickly disappears through decay, even if the body’s natural processes were slow. The biological half-life for Technetium-99m is about one day, resulting in a very short effective half-life. Due to this fast decay, over 90% of the radioactivity from the procedure is gone within 24 hours.
Patients receiving these short-lived radiopharmaceuticals are sometimes given simple safety instructions to minimize exposure to others. These precautions might include flushing the toilet twice after use or maintaining a limited distance from infants and pregnant people for a short period. These protocols are temporary, typically lasting only a day or two.
Factors Affecting Long-Term Retention
While medical isotopes clear rapidly, a few radionuclides can be retained in the body for years or even decades due to unique chemical properties. This long-term retention occurs when an isotope chemically mimics an element the body needs, causing it to be incorporated into specific tissues. The body’s metabolic processes dictate which organs retain the material, significantly extending the biological half-life.
Strontium-90
Strontium-90, a fission product from nuclear events, is retained long-term because it is chemically similar to calcium. When ingested, the body mistakes Strontium-90 for calcium and deposits it directly into the bones and bone marrow. Since bone turnover is a slow process, the biological half-life of Strontium-90 can be estimated to be around 18 years, ensuring its physical half-life of 28.91 years continues to influence the dose over a long period.
Cesium-137
Another example is Cesium-137, which chemically mimics potassium, an element that is distributed throughout the body’s soft tissues. Cesium-137 has a physical half-life of about 30 years. While its biological half-life is much shorter, often measured in months, the isotope’s presence throughout the muscle and organs means it continuously irradiates a large portion of the body until it is excreted.