What happens if a human is exposed to a lot of radiation all at once?

Depending upon the level of exposure, that person will suffer from what is known as Acute Radiation Syndrome. The following is a description of effects versus dose:

• Less than 25,000 millirem, there are no directly observable effects. There are changes in some human cells that can be observed with a microscope at exposures above 10,000 mrem.
• 25,000 to 50,000 millirem, there will be no symptoms, but there might be some changes in the chemistry of the individual's blood.
• 100,000 to 300,000 millirem, some physical changes (such as skin reddening and temporary hair loss) are seen, particularly at the high end of the range.
• 300,000 to 1,000,000 millirem, vomiting is the first symptom, and the human loses his/her ability to produce blood. At the upper end of this range, bone marrow transplants are generally needed and, if medical care is not available, the condition can be fatal within one month of exposure.
• 1,000,000 to 5,000,000 millirem, there will be vomiting, loss of blood production, and failure of the gastrointestinal system. In general, an acute dose of this magnitude is fatal within two weeks.
• Greater than 5,000,000 millirem, central nervous system failure is likely, and death will occur within a period of days.

For comparison purposes, the maximum an individual is allowed to receive from occupational exposure to ionizing radiation is 5,000 millirem in a year.