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Radiation and the Human Body
In terms of pure physics, radiation is the process of transport of energy across space.
Radioactivity is the process of decay of a physical element and involves the emitting of "bundles of energy", which may have a mass or not and may have an electric charge or not.
Relatively few natural elements undergo this process and they are called "radioactive" elements.
Alpha and beta particles, gamma-rays are emitted when radioactive decay takes place.
When particles reach the human body they interact with its physical components.
This interaction results in the deposit of part or all of the energy carried by the “intruder” particle.
The particles are so tiny that their effect is not immediately sensed by the body.
It is the consequences of this interaction that is felt inside the body - by disruption of the bonds that keep molecules together and by creating ions that further interact with our system.
Each particle emitted has a certain amount of energy.
The energy multiplied by the total number of particles gives the total amount of "uninvited" energy released in the body.
To illustrate this point, consider the number of alpha particles emitted by a single spherical pellet of uranium oxide (UO2) 0.0001 inch or 2.5 microns in diameter (equivalent to 1/40th the width of a human hair) and the dose rate it produces.
Tiny as it is, the 2.5 micron depleted uranium oxide pellet contains 210 billion atoms (2.1 x 10 to the power of 11) of U238.
Each year, the pellet will emit an average 32.3 alpha particles.
It also contains U234, 235, 236 which together yield an additional 5.3 alpha particles per year.
Thus a single pellet of depleted UO2 will produce a total of 37.6 alpha particles per year.
The 37.6 alpha particles will deliver a radiation dose of 17 rads/year.
With an RBE (Relative Biological Effectiveness) factor of 10, the dose rate is 170 rem/year for the surrounding body tissue.
In the US, the Code of Federal Regulations regarding energy specifies an annual limit of 0.17 rem/year and a specific limit of 0.5 rem/year for an individual in the general population.
A quick calculation shows one single pellet delivers 1,000 times the annual limit.
This number is multiplied by the total number of pellets present in the body.
For example, if a single or series of exposures resulted in the presence of 10 pellets then the annual limit is exceeded by 10,000.
Another factor to consider is "permanence".
Objects or particles less than 5 micron in diameter are considered respirable, meaning that it is small enough to enter into the lungs and become permanently trapped.
If the body does not manage to somehow release it then the radiation is internalized and the dosage is permanent during the individual's lifetime and even remains in their physical remains after death.
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