There are several units used for expressing the level of ionizing radiation. Some of these units simply express the activity of the radiation source; others relate the effect of a specific type of radiation to its effect on living tissue.
The curie, abbreviated Ci, is the unit used to describe the activity of the radiating source. One curie is equal to 3.7 x 1010 disintegrations per second, which happens to be the disintegration rate of 1 g of radium.
The roentgen, abbreviated r, measures the ionizing ability of x rays and gamma rays. One roentgen is the amount of radioactivity that produces 2 x 109 ion pairs in 1 cubic centimeter of air.
The rad (radiation absorbed dose) measures the amount of energy absorbed by living tissue, regardless of the type of radiation. (The energy absorbed is measured in units called ergs. When 1 g of irradiated tissue absorbs 100 ergs of energy, that is 1 rad). In general, the exposure to radiation in roentgen units is numerically equal to the absorbed dose in rads.
The toxicity of radiation is sometimes expressed in terms of a lethal dose, which is the amount of radiation that will kill 50% of those organisms exposed to it within 30 days. The symbol for this 30-day, 50% kill lethal dose is LD30/50. The units of radiation are usually expressed in roentgens.
Different types of radiation have different effects on body tissue.
In other words, 1 rad of alpha radiation will have a different effect on the body than will 1 rad of beta radiation. To account for this difference, scientists have developed a unit called the rem (roentgen equivalent man).
The rem is calculated by multiplying the number of rads by a weighting factor. The weighting factor for alpha radiation is 10, whereas that for beta and gamma radiation is 1.
rem = rad x weighting factor
When a dose of radiation is expressed in rems, it doesn't matter what type of radiation it is since it has already been weighted. The rem is a useful unit for expressing amounts of radiation that individuals receive each year.