This isolated reading is baffling scientists here.
"We have not had this situation before," says Jan Johansson, expert at the Swedish Radiation Safety Authority, Strålsäkerhetsmyndigheten.
Although harmless, the amount of cesium-137 detected in Helsinki on March 3-4 was the highest detected since the Chernobyl disaster in 1986. But despite such a large concentration of radioactive material reaching one sensor, no other Finnish stations are reporting detecting it.
The wind was blowing from an easterly and south-easterly direction when the radiation was detected, but Sweden has not found anything.
"We just got the results from the six Swedish measurement stations. There is no indication, at all, of any increased cesium levels."
"This is something local," says Jan Johansson at Strålsäkerhetsmyndigheten.
He hopes that further investigations by Finland's STUK will shed light on the mystery. You can read their report in English here.
Could it just be an error in the instrument? "We have never had a reading that turned out to be false, in Sweden."
Cesium-137 is a radioactive material that does not occur naturally.
The concentration detected in Finland is not anywhere close to a dangerous level, which would need to be a million times more.
Update: The outbreak of radiation has now been localised to the garage and basement used by the Finnish radiation body STUK itself, which it shares with a company that handles radioactive material.