We all have millions of isotope atoms in our bodies just by eating and breathing. Some are man made and some are natural. But we do NOT have millicuries of activity from these.
Isotopes can emitt electrons, photons, protons, neutrons, alpha particles.
Breathing in an isotope, can cause significant damage to a small region of the lung, if it remains there. This is especially true of alpha emitters. None of these are used in medical procedures. Depends on the region of the body where they collect. Typical effective exposure rates are of the order of a year of background radiation.