Some naturally occurring radioactive isotopes include Sodium-24 (Na-24), Carbon-
14 (C-14), Argon-41 (Ar-41), and Potassium-40 (K-40). Most of our internal
exposure comes from K-40.
Radon (total average dose ~ 200 mrem/yr)
Radon comes from the radioactive decay of uranium, which is naturally present in
Radon is a gas. It can travel through the soil and enter through building foundation
cracks. The greatest concentrations of indoor radon are found in basements.
Radon emits alpha radiation. It presents a hazard only when taken into the body
(e.g., when inhaled).
The difference between manmade sources of radiation and naturally occurring sources is the
origin of the source, i.e., where the radiation is either produced or enhanced by human
The four top sources of manmade radiation exposures are:
Tobacco products (average dose ~1300 mrem/yr)
Medical radiation sources (total average dose ~ 54 mrem/yr)
X rays (total average dose ~ 40mrem/yr)
X rays are similar to gamma rays; however, they originate outside the nucleus.
A typical radiation dose from a chest X ray is about 10 mrem.
Diagnosis and therapy (total average dose ~14 mrem/yr)
In addition to X rays, radioactive materials and radioactive sources are used in
medicine for diagnosis and therapy.
Building materials (total average dose ~7 mrem/yr)
Domestic water supply (total average dose ~5 mrem/yr)