Relationship between energy deposited and dose in rad

A rad is defined as the deposition of 100 ergs of energy per gram of tissue. Since the measurement is per gram, then the same dose in rad can reflect very different radiation exposures, depending on the mass into which the energy is deposited:
If the dose is:	...and the total mass into which the energy is deposited is:	...then the total energy deposited is:
2 rad	2 grams (weight of an infant's thyroid)	400 ergs*
2 rad	20 grams (weight of an adult's thyroid)	4,000 ergs
2 rad	70,000 grams (the official definition of 'standard man" in radiation protection practice; equal to about 154 lbs.)	14million ergs
Conversely, the same amount of deposted energy can result in veyr different doses, depending on the mass into which the energy is deposited:
If the total energy deposited is:	...and the total mass into which the energy is deposited is:	...then the total dose is:
4,000 ergs	2 grams (weight of an infant's thyroid)	20 rad
4,000 ergs	20 grams (weight of an adult's thyroid)	2 rad
4,000 ergs	70,000 grams (the official definition of 'standard man" in radiation protection practice; equal to about 154 lbs.)	0.0006 rad
*In everyday terms, an erg is a very small amount of energy. It takes almost 1.5 trillion ergs to light a 40-watt bulb for one hour. But in the context of the tiny cells that make up living beings, 100 ergs is a large amount of energy. One erg is enough energy to disrupt tens of billions of the chemical bonds that hold molecules together.

For more on rad, rem and roentgen, see "Dear Arjun."