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Radiation Units, Dose Limits and Monitoring
Effective dose in sieverts sets the limits: 50 mSv a year for workers, 1 mSv a year for the public.
Air Kerma
Kerma stands for kinetic energy released per unit mass. Air kerma is the energy that x-ray photons transfer to electrons in a kilogram of air, measured in gray and sometimes written Gya to show the medium is air. It describes the beam itself: tube output, entrance skin air kerma and dose-area product readings are all based on it. Air kerma replaced the older quantity exposure, measured in coulombs per kilogram or the traditional roentgen (R); 1 R equals about 8.7 mGy of air kerma.