How many RADs are actually lethal?

How many RADs (Radiation Absorbed Dose) per hour are lethal? Google says Doses of 200 to 1,000 rad delivered in a few hours will cause serious illness with poor outlook at the upper end of the range. however that's not very specific.


Lethality of radiation varies with the type of radiation, area of the body exposed, the intensity of exposure to a particular location, and access to medical care

However, the US Nuclear Regulatory Commission provides an estimate of the LD50/30, defined as:

The dose of radiation expected to cause death to 50 percent of an exposed population within 30 days

The LD50/30 is:

Typically... in the range from 400 to 450 rem

For X-rays and gamma rays, rad and rem are approximately equivalent. For other types of radiation, use the following approximations:

Type Rad Rem
X-rays 1 1
Gamma rays 1 1
Neutrons 1 5-20
Alpha 1 20

In a review of individuals exposed accidentally to ionizing radiation and who received modern medical care, the vast majority of individuals exposed to more than 600 rad expired within 1 year.

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