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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.

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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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