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