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Frequently Asked Questions

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What is "significant" radiation exposure?
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In the discussion of cancer risks presented here, attention is focused on survivors with estimated exposure doses greater than 0.005 Gy (5 mGy). No excess risks of cancer or other diseases have been seen among survivors with doses below 0.005 Gy.

A dose of 0.005 Gy is somewhat greater than the typical annual background radiation level to which people are exposed in normal daily life (0.001 to 0.003 Sv per year) and about one-fourth the currently accepted maximum annual dose allowed for radiation workers (0.02 Gy).

Survivors with doses of 0.005 Gy or more were typically within about 2,500 meters of the hypocenter in Hiroshima and 2,700 meters in Nagasaki. The average dose received by such survivors is about 0.2 Gy. The radiation dose decreased by half for every 200-meter increase in distance from the hypocenters.


  Figure. Relationship between distance from hypocenters and radiation dose in air.
If inside a typical house, the dose is reduced by 50% or more.
Shown at the right are general biological symptoms and
radiation doses from other sources.