The additional radioactivity in the biosphere caused by human activity due to the releases of man-made radioactivity and of Naturally Occurring Radioactive Materials (NORM) can be divided into several classes.
# Normal licensed releases which occur during the regular operation of a plant or process handling man-made radioactive materials.
#* For instance the release of Tc from a nuclear medicine department of a hospital which occurs when a person given a Tc imaging agent expels the agent.
# Releases of man-made radioactive materials which occur during an industrial or research accident.
#* For instance the Chernobyl accident.
# Releases which occur as a result of military activity.
#* For example, a nuclear weapons test.
# Releases which occur as a result of a crime.
#* For example, the Goiânia accident where thieves, unaware of its radioactive content, stole some medical equipment and as a result a number of people were exposed to radiation.
# Releases of naturally occurring radioactive materials (NORM) as a result of mining etc.
#* For example, the release of the trace quantities of uranium and thorium in coal, when it is burned in power stations.