The human body contains very small quantities of radioactive carbon and potassium. This radioactive material gets incorporated into tissues and organs throughout the body.

The radioactive carbon originates in the atmosphere and results in a dose of approximately 1 mrem/year (0.01 mSv/year) in the soft tissue.

Radioactive potassium is naturally occurring and contributes approximately 20 mrem/year (0.2 mSv/year) to the testes or ovaries.