In the United States, radiation absorbed dose, dose equivalent, and exposure are often measured and implied in the units called rad, rem, or roentgen (R). This exposure can be from an external source irradiating the whole body, an extremity, or organ resulting in an external radiation dose. Alternately, internally deposited radioactive material may cause an internal radiation dose to the whole body, organs, or tissue.

Smaller fractions of these measured quantities often have a prefix such as  milli (or m) which means 1/1,000. For example, 1 rad = 1,000 mrad.

The International System of Units (SI) for radiation measurement is now the official system of measurement and uses the “gray” (Gy) and “sievert” (Sv) for absorbed dose and equivalent dose respectively. Conversions are as follows:

• 1 Gy = 100 rad
• 1 mGy = 100 mrad
• 1 Sv = 100 rem
• 1 mSv = 100 mrem

With radiation counting instruments (e.g., Geiger counters, liquid scintillation counters), radiation can be measured in units of “disintegrations per minute” (dpm) or, “counts per minute” (cpm). Natural background radiation levels are typically less than 0.02 mrem per hour (0.2 microSv), but due to differences in detector size and efficiency, the cpm reading on various survey meters will vary considerably.