Small traces of many radioactive materials are present in the human body. These come mainly from radioactivity present in minute quantities in the food we eat.
The only radionuclide which gives a significant dose is
K. A
person weighing
contains
of potassium, most of which is located
in the muscle. About 0.01% of this is K and
this delivers an equivalent dose of about 
a year.
C which is produced through
nuclear transmutation by cosmic
rays in the upper atmosphere, delivers about
a year.