Ionizing radiation has several different units, with a somewhat confusing relationship between them, because it is absolutely critically important to measure human exposure and get it right.

Radiation absorbed dose is how much radiation that reaches you (exposure) gets into your tissues. It is measured in grays: a gray (symbol Gy) is equal to one joule per kilogram. The old unit was the rad, with 100 rad = 1 Gy.

Absorbed dose is different from absorbed dose equivalent (measured in sieverts or formerly rems): different human tissues take in radiation differently, so the dose equivalent is an averaged figure. See sievert for a better explanation.

  • Male fertility can be affected by a dose of 0.15 Gy, with sperm production being depleted for a year.
  • Survivors of the atomic bomb averaged an absorbed dose of 0.4 Gy.
  • A dose between 1 Gy and 2 Gy causes temporary sterility in men.
  • 3.5 Gy causes permanent sterility in men.
  • Somewhere between 2.5 Gy and 6 Gy causes permanent sterility in women.
  • Skin burns and hair loss occur at about 3 Gy.
  • At 5 Gy visual impairment occurs.
  • 10 Gy causes blisters.