High Radiation Doses
Because radiation from nuclear material is strictly regulated, humans seldom experience large doses (~50 rem) of radiation. Nonetheless, lower doses can still damage or alter the genetic code (DNA) of irradiated cells. Moreover, high radiation doses (particularly over a short period of time) have a tendency to kill cells. In fact, high doses can sometimes kill so many cells that tissues and organs are damaged immediately. This, in turn, may cause a rapid whole-body response, which is often called "acute radiation syndrome."
In general, the higher the radiation dose, the sooner the effects will appear, and the higher the probability of death. (The time between radiation exposure and cancer occurrence, for example, is known as the "latent period.") This syndrome was observed in many atomic bomb survivors in 1945, as well as emergency workers who responded to the Chernobyl nuclear power plant accident in 1986. Approximately 134 plant workers and firefighters battling the fire at the Chernobyl power plant received high radiation doses of 70,000 to 1,340,000 mrem (700 to 13,400 mSv) and suffered acute radiation sickness. Of those 134, 28 died from the radiation injuries that they sustained.
Although radiation affects different people in different ways, it is generally believed that humans exposed to about 500 rem of radiation all at once will likely die without medical treatment. Similarly, a single dose of 100 rem may cause a person to experience nausea or skin reddening (although recovery is likely), and about 25 rem can cause temporary sterility in men. However, if these doses are spread out over time, instead of being delivered all at once, their effects tend to be less severe.