Internal radiation sources arise from food that is eaten. According to the NRC, the average person receives an average internal dose of approximately 30 millirem per year.

Radionuclides in food include other daughter products from the decay of uranium and thorium. Lead-210 and polonium-210, for example, are concentrated in fish and shellfish. Persons who eat a large amount of seafood are getting a higher dose from this source of natural radiation than those who don't.

Like many of the other situations discussed in this section, some areas of the world have certain foods higher in some radionuclides due to the particular rocks and minerals prevalent in the soil where the crops or livestock are grown.