  • ### How is radiation measured?

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.

Measurement USA Units International
Standard Units (SI)
Absorbed Dose rad - defined as 100 ergs per gram of material Gray (Gy); defined as one joule of energy deposited in one kg of material

Dose Equivalent 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.

Type of Exposure Dose Equivalent
Shuttle (Average Skin Dose) ~433 mrem*/mission
Apollo 14 (Highest Skin Dose) 1,400 mrem*/mission
Skylab 4 (Highest Skin Dose) 17,800 mrem*/mission
Shuttle (Highest Skin Dose) 7,864 mrem*/mission
Airline Flight Crew 200 mrem*/year
Gas Cooking Range 0.02 mrem*/year
Dental Prosthesis 0.02 mrem*/year
CT Scan (Chest) 700 mrem*/event
Barium Enema 400 mrem*/event
Houston Background 100 mrem*/year
* 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. 