Measuring Radiation Doses

In the USA system of radiological measurements, there are three somewhat confusing units for measuring the exposure to and doses of radiation:

• the roentgen (pronounced rent'-gen),
• the rad, and 
• the rem.

To understand these you might imagine yourself on a sunny beach. The roentgen is analogous to the intensity of the sunlight striking the beach. The rad (radiation absorbed dose) corresponds to the amount of sunlight absorbed by your skin, while the rem (roentgen equivalent man) is comparable to the biological effect of the sunlight exposure. In the case of the rem, however, the difference in its effect is not due to your sunscreen, skin pigment, or hours spent in the tanning salon - but in the type of radiation being absorbed.

You may recall that the different types of radiation were either particles (alpha and beta rays, protons, neutrons) or high-energy photons similar to light (X- and gamma rays). Except for beta rays - which are electrons having some 1/1836 the mass of protons or neutrons - the particles, because of their large masses, have a more catastrophic effect when colliding with a cell in the body. For this reason the quality factor - usually designated as Q - is used to adjust the absorbed dose to its biological counterpart, the rem.

Mathematically, rads x Q = rems.

Fortunately, most of the exposures we will be referring to in the study of hormesis are gamma and X-rays where Q is equal to one, allowing rads and rems to be used interchangeably. (Your radiologist, dental hygienist, and others working with X-rays will usually talk in terms of rads or millirads - but these are the same as rems and millirems, because it is the X-ray source that produces the radiation.) There is one other term with which you should have at least a vague familiarity - Linear Energy Transfer or LET. Beta, famma and X-rays are considered low LET radiation, which means they have a Q of one. High LET particles can have Qs up to 400. A typical alpha particle has a Q of four.

[Wilhelm Roentgen (1845-1923) discovered an unknown emission (X-rays) from cathode ray tubes. It still happens today - that's how X-rays are made today. Incidentally, your TV screen is a cathode ray tube, and it emits many times the radiation we get from nuclear power plants. Somehow this fact escapes notice of the TV doomsayers.]