The Japanese government is releasing hourly radiation observations from each prefecture in Japan (where available). The official site for this information is in Japanese. Online translation services can be used to view directly in English.
Japanese site: (http://www.mext.go.jp/a_menu/saigaijohou/syousai/1303723.htm)
There are two main concerns that come from a potential exposure to radiation. The first is the short term effects of exposure to very high levels of radiation. The second is the long-term effects of exposure.
Acute Radiation Sickness usually develops from immediate exposure to high amounts of radiation and could cause a variety of health effects, ranging from nausea to death. Radiation levels consistent with Acute Radiation Sickness are usually observed at or near ground zero of a major radiation release. The effects of radiation in the short term can be found in the chart below.
Long-term, repeated exposure to radioactive materials may eventually lead to some disease, approximately 5-20 years after exposure. According to the United States Nuclear Regulatory Commission (NRC)