background radiation

Radiation that is always present in the environment. By far the greater proportion (87%) of it is emitted from natural sources. Alpha particles, beta particles, and gamma radiation are emitted by the traces of radioactive minerals that occur naturally in the environment and even in the human body (for example, by breathing in ¹⁴C). Radioactive gases such as radon and thoron are found in soil and may seep upwards into buildings. Radiation from space (cosmic radiation) also contributes to the background level.

The background count is the count registered on a Geiger counter when no other radioactive source is nearby.