What's the difference between a rad, a rem, and a roentgen?
--Mixed-up in Margaritaville

Dear Mixed-Up:

In the 1960s in the United States, individuals who were deeply involved with the so-called counter-culture were sometimes referred to as radical, or "rad" for short. There were also individuals who didn't want to rock the boat too much, but still liked to be associated with the counter-culture, who were called "rem," short for "regularly enlightened mainstreamers." Everyone else was considered by the rad and rem to be part of the "rank and file," which was later shortened to "roentgen file," after a particularly mainstream leader of the time, Hans Roentgen.

But unbeknownst to these radicals, rad, rem and roentgen had already been used as units to express doses from radiation exposure. Radiation dose is calculated in a number of ways, including measurement of amount of radiation at a particular place, measurement of exposure, measurement of absorption of radioactivity, and of relative effect of that absorption on different organs or types of tissue. Dose calculations take into account the type of radiation involved (alpha, beta, gamma, or neutrons). Radiation doses can also be expressed in standard international units, sieverts or grays (one sievert equals 100 rem, one gray equals 100 rad).

A rad, or "radiation absorbed dose," is a unit of absorbed dose equivalent to the deposition of 100 ergs of energy per gram of tissue. (See Centerfold for more on rad and ergs.) Since a rad measures deposition of energy per gram of tissue, a 2-rad dose to a certain organ, like the thyroid (which weighs roughly 20 grams), reflects a very different total deposition of energy than does a 2-rad dose to the entire body (typical weights range from 40,000 to 100,000 grams).

A rem, or "radiation equivalent man," is a unit of absorbed dose that takes into account the relative biological damage caused by the various ways that ionizing radiation deposits its energy in tissue (known as the Relative Biological Effectiveness, or RBE). In general the larger the amount of energy absorbed, the larger the biological damage. For beta and gamma radiation, rem is equivalent to rad; that is, they have a "quality factor" of 1. (The quality factor is selected to approximate the RBE as it relates to overall risk, and is used in regulatory practice to convert radiation dose measured by energy deposited to radiation dose measured in terms of biological damage.) For alpha radiation, which involves heavy particles, much more damage is done per unit of energy deposited, increasing the ratio between damage to tissue and energy deposited. The currently accepted quality factor for alpha radiation is 20, but may change as more is learned about damage due to ionizing radiation. Neutron radiation has a varying quality factor between 5 and 20, depending on neutron energy. Rem is the unit used to express regulatory limits for radiation exposures.

A roentgen (abbreviated "R" and pronounced "rent-gen") measures gamma and beta radiation dose based on the amount of ionization in the air. It is the common form of calibration of radiation measuring instruments because the beta and gamma radiation ionize air inside the instrument, creating electrically charged particles which can be measured by creating an electric current in the instrument. A roentgen is a unit of radiation dose that causes an ionization charge equal to 2.58 x 10^-4 (0.000258) coulombs per kilogram of air.* Because it measures ionization in air and not in tissue, there is no exact, fixed correspondence between roentgen and rad, but an approximate equivalence is that one roentgen equals 0.93 rad for non-bony biological tissue. Since the uncertainties in dose calculation are typically far larger than 7%, one roentgen is often considered about equal to one rad, for convenience.

* A coulomb is a unit of electrical charge. The charge of an electron is 1.6 x 10^-19 coulombs.