Deposition refers to radioactive materials that have fallen to the ground or water from air. Deposition may come with rain (wet deposition) or as dry deposition.

Radioactive materials originating from nuclear tests performed decades ago are still detected in Finland.

A nuclear power plant accident happened in 1986 in Chernobyl, Ukraine. Radioactive materials spread into many countries over many days. Even today, the Chernobyl fallout increases the radiation dose received by Finns. However, this represents only less than one hundredth of our average annual dose of 5.9 millisieverts.