OAR 333-100-0070: Units of Exposure and Dose

The Metric Conversion Act of 1975 (PL 94-168) urged the increasing awareness and use of the International System of Units (SI). The generally accepted regulatory values in the narrative portions of this document are followed by the SI equivalents in parentheses. Where appropriate, schedules and appendices are provided with notes concerning conversion factors. The inclusion of the SI equivalent is for informational purposes only. (1) The unit of exposure is the coulomb per kilogram (C/kg). One roentgen is equal to 2.58x10-4 coulomb per kilogram of air. (2) The units of radiation dose are: (a) Gray (Gy) is the SI unit of absorbed dose. One gray is equal to an absorbed dose of one joule per kilogram (100 rad); (b) Rad is the special unit of absorbed dose. One rad is equal to an absorbed dose of 100 erg per gram or 0.01 joule per kilogram (0.01 Gy); (c) Rem is the special unit of any of the quantities expressed as dose equivalent. The dose equivalent in rem is equal to the absorbed dose in rad multiplied by the quality factor (1 rem = 0.01 Sv). (d) Sievert is the SI unit of any of the quantities expressed as dose equivalent. The dose equivalent in sievert is equal to the absorbed dose in gray multiplied by the quality factor (1 Sv = 100 rem). (e) As used in these regulations, the quality factors for converting absorbed dose to dose equivalent are shown in 10 CFR 20 Part 20.1004 Table 1004(b).1. (3) If it is more convenient to measure the neutron fluence rate than to determine the neutron dose equivalent rate in rem per hour or sieverts per hour, as provided in (2)(c) of this rule, one rem (0.01 Sv) of neutron radiation of unknown energies may, for purposes of the regulations in this part, be assumed to result from a total fluence of 25 million neutrons per square centimeter incident upon the body. If sufficient information exists to estimate the approximate energy distribution of the neutrons, the licensee may use the fluence rate per unit dose equivalent or the appropriate Q value from 10 CFR 20 part 20.1004 Table 1004(b).2 (at the end of this division) to convert a measured tissue dose in gray or rad to dose equivalent in sievert rem. [ED. NOTE: Tables referenced are available from the agency.]