The environment in which we live is filled with radioactive materials. For example, the rocks and soil of the earth contain small quantities of the natural radioactive elements uranium and thorium. The concentration of these and other radioactive elements varies considerably depending on the type of rock. Thus, the dose rate from this source depends on the geographical location—though, on average, terrestrial radiation accounts for three percent of an individual's total radiation exposure.

The main sources of radiation in rocks are potassium-40 and the two series of radioactive elements that come from the decay of uranium-238 and thorium-232. Uranium-238 is dispersed throughout the soil at various low levels of concentration. Where the concentration exceeds 1,000 ppm, it may be economic to mine the ore to make fuel for nuclear reactors. Thorium-232 is similarly dispersed in soil. Potassium-40 constitutes a significant part of elemental potassium which makes up 2.4 percent of the earth's crust. Phosphate, when used in fertilizer or in supplements for livestock feed, can also contribute a small amount to an individual's dose from natural sources.

In the U.S., the dose rate to the body may vary between approximately 15 and 140 mrem/year (0.15 and 1.4 mSv/year). The average is approximately 30 mrem/year (0.3 mSv/year).