The question of how long radiation stays in the body depends on understanding what “radiation” means in this context. Radiation is energy—either particles or electromagnetic waves—that travels through the body, and this energy does not persist. The actual concern is the retention of radioactive material, which is the source of the radiation. The persistence of radiation is dependent on whether a person has been exposed to an external source or has become contaminated internally with radioactive atoms.
External Exposure Versus Internal Contamination
External exposure occurs when a person is near a source of radiation, such as having an X-ray or standing near a radioactive object. Energy passes through the body, and some may be absorbed, but the body does not become radioactive itself. Once the source is shielded or the person leaves the radiation field, the exposure stops immediately.
Internal contamination is fundamentally different and determines how long radiation remains a factor. This occurs when radioactive material (radionuclides) enters the body through inhalation, ingestion, skin absorption, or an open wound. Once inside, these atoms incorporate into the body’s tissues and continue to emit radiation until they decay or are physically eliminated. The radioactive material acts as a continuous, internal source of radiation, exposing the body as long as the material is present.
The Biological Mechanisms of Clearance
The body possesses mechanisms to remove foreign or excess substances, and these pathways also clear internal radioactive contaminants. The main route of elimination is through excretion, primarily via urine and feces, though smaller amounts are removed through sweat and breath. How quickly a radionuclide is cleared depends heavily on its chemical form, as the body processes it based on its elemental properties.
If a radioactive atom mimics a naturally occurring nutrient, the body treats it similarly, often leading to rapid uptake but efficient clearance. For instance, radioactive cesium acts much like potassium, distributing throughout muscle tissue before being cleared relatively quickly. Conversely, radioactive iodine is specifically concentrated by the thyroid gland, like stable iodine needed for thyroid hormones, where it can be slowly released or decay.
Some radionuclides are chemically similar to elements that the body uses to build durable structures, leading to longer retention times. For example, radioactive strontium behaves like calcium and is readily incorporated into bone tissue. Once deposited in the mineral matrix of bone, these materials are released only through the slow process of bone remodeling and turnover, which can take years or even decades. The body’s rate of metabolic turnover dictates the speed of biological clearance for these chemically integrated contaminants.
Determining the Time Radiation Remains
The time a radioactive substance remains in the body is determined by two independent processes: radioactive decay and biological elimination. This persistence is quantified using three distinct types of half-lives. The first is the Physical Half-Life, which is the time it takes for half of the radioactive atoms to naturally decay into a more stable form, regardless of their location.
The second factor is the Biological Half-Life, which is the time required for the body to eliminate half of the substance through natural excretion and metabolic processes. This value is independent of the atom’s radioactivity and depends entirely on the body’s physiology, chemical form, and the specific organ the material targets. For example, radioactive Cesium-137 has a physical half-life of about 30 years, but its biological half-life in the human body is only about 70 to 100 days because it is quickly flushed out like potassium.
The true measure of a contaminant’s duration is the Effective Half-Life. This value represents the combined effect of physical decay and biological clearance, determining the total time it takes for the radioactive material in the body to be reduced by half. The effective half-life is always shorter than the shortest of the two individual half-lives, as both elimination processes work simultaneously.
The effective half-life can range from hours for certain medical isotopes used in imaging, such as Technetium-99m, to many decades for elements that integrate into bone. For instance, Strontium-90 has a physical half-life of 28.8 years. Because it is chemically similar to calcium, its biological half-life in bone can be 50 years or more, resulting in a long-term contamination concern.