[AnssiH]

The natural background seems to be around 0.13-0.16 µSv/h in Helsinki: https://www.stuk.fi/web/en/topics/environmental-radiation/ra... (select a station to see history).

Seems there is unit confusion as it only rose to 0.18 µSv/h in Helsinki, not to 0.18 mSv/h which would exceed Chernobyl impact.

[rurban]

Oops, wrong by factor 1000. Thanks! It was only a very small picture, and I mixed up the µ spike with m.