Radiation can harm you by damaging the DNA in living cells. The amount of damage caused by radiation varies with the dose received. The damage may range from almost no effect (cells repair themselves and continue to function normally) to death (resulting from vascular damage to vital blood providing systems for nervous tissue, such as the brain). For doses received between these two extremes, damaged cells can reproduce to form cancerous ones, tissues can fail to function, and the immune system can be compromised. (More info)
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.
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?
Organ Specific Exposure Limits for Astronauts
Blood Forming Organs
150 - 400 rem [200 + 7.5(age - 30) for men]
100 - 300 rem [200 + 7.5(age - 38) for women]
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.
What are stochastic and non-stochastic effects?
Stochastic effects are random effects for which the probability of occurrence in a population is a function of dose. Cancer, leukemia, and genetic changes are examples of stochastic effects. Non-stochastic, or deterministic effects, are threshold effects the severity of which increases with dose (at a certain threshold, every individual will see these effects). Radiation "sickness" or nausea, skin reddening, sterility, and cataract formation are examples of deterministic effects.