Can't say historical, but 1,000 μsv/year is the annual limit / point at which increase cancer risk exists.
Fukushima disaster levels were 400,000 μSv/h at peak. Short term dose of 10,000
μSv causes radiation sickness but probably not death. Beyond that it gets worse.
According to the Linear-no-threshold model [1], any level of radiation, no matter how small, causes cancer. According to this model, the 1,000 μsv/year of normal background radiation already cause cancer for 1% of people. And any additional dose increase the risk further.
However this model is contested. And the analysis on the cancers incidence in Hiroshima and Nagasaki and Chernobyl seems to indicate that this model is flawed. The exact model is difficult to estimate because of the low number of extra cancer is lost in the noise.
Fukushima disaster levels were 400,000 μSv/h at peak. Short term dose of 10,000 μSv causes radiation sickness but probably not death. Beyond that it gets worse.