Two types of measurements are used to describe the effects of radiation - the absorbed dose and the dose equivalent. The absorbed dose relates to the amount of energy actually absorbed by some material. It is used for any type of radiation and for any type of material. The dose equivalent relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. The differences in biological effects caused by different types of radiation are described as radiobiological effectiveness (RBE). Mathematically, RBE is the ratio of the absorbed dose of low-LET radiation (X-Rays, g rays) necessary to cause the same level of the same biological effect as that of high-LET radiation (neutrons, a particles). LET is an acronym for Linear Energy Transfer, and describes the average energy released per unit length of track. The RBE for a particular type of radiation is used to determine the Q factor. The dose equivalent is calculated by multiplying the Q factor by the absorbed dose.
Just as is the case for measuring mass, length, and temperature, one set of units is used to measure radiation doses in the United States Federal Regulations and another set is used in other countries. The following table summarizes these units.
International Standard Units (SI)
rad - defined as 100 ergs per gram of material
Gray (Gy); defined as one joule of energy deposited in one kg of material 1 Gy = 100 rad
rem - determined by multiplying the absorbed dose (rad) by a quality factor (Q) that is unique to the type of incident radiation
Sievert (Sv); determined by multiplying the absorbed dose (Gy) by a quality factor (Q) that is unique to the type of incident radiation 1 Sv = 100 rem
What are the dose limits for astronauts and for terrestrial workers?
Astronaut Permissible Exposure Limits are maintained in NASA Standard 3001 Volume 1.
The dose limit for terrestrial radiation workers is 5 rem per year.
How does the radiation that astronauts receive in space compare to radiation exposure that one might receive otherwise?
Type of Exposure
Shuttle (Average Skin Dose)
Apollo 14 (Highest Skin Dose)
Skylab 4 (Highest Skin Dose)
Shuttle (Highest Skin Dose)
Airline Flight Crew
Gas Cooking Range
CT Scan (Chest)
* 1 mrem = .001 rem
What is radiobiological effectiveness (RBE)?
RBE is the ratio of the absorbed dose of low-LET radiation (X-Rays, g rays) to cause the same level of the same biological effect as that of high-LET radiation (neutrons, a particles). LET stands for Linear Energy Transfer, and describes the average energy released per unit length of track. The RBE is used to determine the Q factor used in calculating equivalent dose from absorbed dose.