The concept of activity is used as the measure of radioactivity. The activity is the average number of radioactive transformations of definite nuclei per unit time. The measurement unit of the activity is becquerel (Bq); the activity of 1 Bq is correspondent to one transformation of a radionuclide in one second.
People can face terrestrial radiation in the oil and gas industries, where it is known as Naturally Occurring Radioactive Material (NORM). Radon gas is produced by the decay of uranium, has no smell or color and can be accumulated in buildings, thus it can be very harmful for people.