Measuring radiation

There are a number of units for measuring radioactivity.

Curie is a unit of radioactivity. One curie refers to the amount of any radionuclide that undergoes 37 billion atomic transformations a second. A nanocurie is one one-billionth of a curie ( 37 Becquerel, = 1 nanocurie).   The curie just measure the amount of activity.  It is proportional to the number of disintegrations per second.

Roentgen is a measure of exposure; it describes the amount of radiation energy, in the form of gamma or x-rays, in the air.

Rad (radiation absorbed dose) measures the amount of energy actually absorbed by a material, such as human tissue (Gray=100 rads).   The rad takes into account both the amount of radiation and the matter that is absorbing it.  Some compounds such as lead or barium absorb more radioactivity than others, such as water. 

Rem (roentgen equivalent man) measures the biological damage of radiation. It takes into account both the amount of radiation and the biological effect of the type of radiation in question. As was stated earlier, an alpha particle does more damage than a beta particle which does more damage then a gamma ray.  A millirem is one one-thousandth of a rem (Sievert=100 rems).

For more information on radiation, read this statement from the EPA

http://www.epa.gov/radiation/rert/radfacts.html

As you read in your book, radon is the largest source of radiation exposure for an average person.  The EPA publishes a wealth of information on radon.  You may visit their site if you wish: