The body's natural defenses against radiation damage have developed in the naturally radioactive environment we live in. These defenses are overwhelmed by acute exposures.

For example, if a large group of people received an acute exposure of 450 R (0.12 C/kg), approximately half of the group would die within a month without medical care.

However, if this same group were exposed to 450 R over an extended period of time, far fewer would die as a result. If the exposure was protracted over many years, no radiation sickness would be observed, although the delayed effects might be statistically observable.

Therefore, chronic exposures can be tolerated by the body with much less biological effect than acute exposures.

Select this link for more information about the acute effects of high radiation doses.