The millisievert and milligray as measures of radiation dose and exposure
    In the SI system, a millisievert (mSv) is defined as "the average accumulated background radiation dose to an individual for 1 year, exclusive of radon, in the United States." 1 mSv is the dose produced by exposure to 1 milligray (mG) of radiation. In the historical system of dosimetry, exposure to 1 roentgen (R) of X-rays results in absorption of 1 rad [radiation-absorbed dose], which had the effect of 1 rem [roentgen-equivalent (in) man]. The unit equivalences between the systems are 1 Sv = 100 rem, or 1 rem = 10 mSv. Thus, 1 mSv = 100 mrem.

    The whole-body exposure threshold for acute hematopoietic syndrome or "radiation sickness" is 500 mGy. A whole-body dose of ~3,000 mGy produces an acute gastrointestinal syndrome that can be fatal. A whole-body dose of ~ 5,000 mGy is considered the human LD 50 / 30, that is, the lethal dose for 50% of the population in 30 days.
    "Radiation doses that exceed a minimum (threshold) level can cause undesirable effects such as depression of the blood cell-forming process (threshold dose = 500 mSv, 50 rem) or cataracts (threshold dose = 5,000 mSv, 500 rem). The scope and severity of these effects increases as the dose increases above the corresponding threshold. Radiation also can cause an increase in the incidence, but not the severity, of malignant disease (e.g., cancer). For this type of effect, it is the probability of occurrence that increases with dose rather than the severity. For radiation protection purposes it is assumed that any dose above zero can increase the risk of radiation-induced cancer (i.e., that there is no threshold). Epidemiologic studies have found that the estimated lifetime risk of dying from cancer is greater by about 0.004% per mSv (0.04% per rem) of radiation dose to the whole body (NRC, 1990)."
Quoted text from An Evaluation of Radiation Exposure Guidance for Military Operations: Interim Report (1997). J. Christopher Johnson and Susan Thaul, Editors. National Academy of Sciences. ISBN 0-309-05895-3. Additional text  ©2007 by Steven M Carr;