Another example of the United States hanging on like grim death to a ancient deprecated non-decimal system of units, instead of adopting the metric system like the rest of the scientific / civilized world :wink: For acute exposure scenarios (i.e. the types that give you radiation sickness rather than cancer) the correct unit of measurement is the Gray (Gy) - which is analogous to the traditional obsolete unit of the Rad (1 Rad == 0.01 Gray.) i.e. Grays are used to measure the Absorbed Dose of ionising radiation. The traditional Rem is an obsolete unit representing the Equivalent Dose measure (amount of radiation absorbed by a living being) - this unit has been replaced by the metric unit the Sievert(Sv). (1 Rem equals 0.01 Sievert). Also this means the Rem is used for chronic exposure dosage (i.e. the kind that nuclear powerplant employees hope is the only sort they need to worry about on a daily basis). Sieverts are determined from Grays, and are usually measured in milliSieverts (mSv) with respect to human exposure levels. Remember: acute exposure is measured in Grays, chronic exposure is measured in Sieverts. Also, for measuring actual radioactivity levels, e.g on a Geiger counter, the correct unit is the Becquerel (which replaces the Curie). It's also important to note that different types of radiation have different 'qualities' regarding their exposure effect on living tissue. Thanks for coming to my TED talk.