OECD’s Nuclear Energy Agency has a useful overview of the events at the plant. Of particular interest are slides 16 and 17 pasted below which provide the dose rates of the prefectures near the plant on March 23.
Here are some key quotes from slide 17 on public health risks:
Radiation Risks are assumed to exist as a result of ANY exposure, no matter how small.
Exposure to natural radiation is, on average, 2.4 mSv per year.
At the current dose rates offsite, approximately one year of exposure would be needed to reach 1 mSv.
It is not possible to see statistical evidence of public health risk at exposures of less than 100 mSv.