The International Standard (SI) Units

In the International Standard measuring system, there is no equivalent for the roentgen - which is just as well as far as we're concerned because it is seldom used in relation to human exposure. The following relationships exist between the USA and SI units:
100 rad = 1 gray or Gy
and
100 rem = 1 sievert or Sv

One gray or sievert represents and enormous amount of radiation - about four times as much as a U.S. resident would normally receive in a 76-year lifetime. Smaller units, the centigray, cGy, and the centisievert, cSv, are more commonly used. These conveniently convert to USA units -
1 cGy = 1 rad = 1000 millirad (mrad)
and
1 cSv = 1 rem = 1000 millirem (mrem).

If learning these measuring systems seems too complicated, try learning a few reference exposures and compare the value in question to these. Here are the ones I use, which then give me a feel for other values. After a while, they start all becoming second nature.

• Sleeping with your spouse for a year - 1 mrem or 0.001 cSv (for the ambitious learner, 0.01 mSv). Since your spouse emits gamma rays; the rads, rems, cSv and cGy are all the same. In almost all of the cases (except internal radium and plutonium) that we're going to be examining, this will be the case.
• Background radiation in the United States - 300 mrem or 0.3 cSv. In the International System, the millisievert - one-tenth of the centisievert - is often used in this range. Our normal background dose in this unit is 3 mSv. Since a good portion of this radiation is from radon sources (an alpha emitter); rads, rems, Gy, and Sv are not interchangeable.
• Radiation sickness - ensues at about 100,000 mrem, or 100 rem, or 100 cSv, or 1 Sv. Because doses of this magnitude are usually low LET radiation, units of 100,000 mrad, 100 rads, 100 cGy, or 1 Gy may be used interchangeably. By the way, sickness results from an acute exposure of 1 Sv over a period of a couple of days or less. The same radiation over a longer exposure time gives no symptoms.

If you'll commit these three points to memory (or place a bookmark here), it will give you some frame of reference with which to compare other doses. Your bookmark will also give you easy access to Table 7, which gives some typical millirem and cSv values for other exposure situations.

Table 7 – Selected Radiation Doses Per Year
Source of Exposure	mrem	cSv
Nuclear plant within 50 miles	0.01	0.00001
Average Three Mile Island dose	0.1	0.0001
Color television	1	0.001
One coast-to-coast jet flight/trip	5	0.005
Border of nuclear power plant	5	0.005
From food	25	0.025
Cosmic radiation	27	0.027
Building materials	34	0.034
Your own blood (Potassium 40)	45	0.045
On-site for duration, TMI accident	80	0.080
One shoe X-ray (SXR)	175	0.175
Grand Central Station	525	0.525
Living on Colorado plateau	600	0.6
Barium enema	800	0.8
Max permissible for nuclear worker	5,000	5
Radiation sickness (50% people) [acute exposure over a day or two]	100,000	100
Death (50% of people) [acute exposure over a day or two]	400,000	400

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.]