Footnote

“Soil deposition of 137Cs equal to 37 kBq m-2 (1 Ci km-2) was chosen as a provisional minimum contamination level, because

(1) this level was about ten times higher than the 137Cs deposition in Europe from global fallout, and

(2) at this level, the human dose during the 1st year after the accident was about 1 mSv and was considered to be radiologically important."

Source & © Chernobyl Forum - Environmental Consequences of the Chernobyl Accident and Their Remediation,
 2.1.5, page 32