Scientists use three different ways of measuring the energy in radiation. Here they are:

CURIE	Unit of Activity (A)	A= # of decays per second	1 curie = 3.7 × 1010 Decays per second
RAD	Unit of Absorbed Dose (D)	D = the energy absorbed per gram of tissue in the body	1 rad = 100 ergs per gram
REM	Unit of Dose Equivalent (H)	H = the absorbed dose multiplied by a biological effectiveness factor (Q)	1 rem = Q × D

The best tool to measure an individual's risk from radiation exposure is the REM. In this formula, Q is really a weighting factor that represents the density of the ionization from various radiation types: alpha, beta, gamma particles, protons, neutrons that are emitted by a substance. Low level exposures are measured in millirems, or MREM, a thousandth of a rem.

EXAMPLE:
One chest X-Ray gives you about 10 mrem of radiation, or .01 rem.
A dental X-Ray emits 1 mrem, or .001 rem.