What is one thousandth of a REM called?

One thousandth of a REM is referred to as a millirem. The term "REM" stands for roentgen equivalent man, a unit used to measure the biological effect of ionizing radiation. The prefix "milli-" denotes one-thousandth, so a millirem is equal to 0.001 REM. This unit is important in radiation safety and monitoring, as it provides a more manageable figure for measuring small doses of radiation exposure that can accumulate over time. Understanding this unit is vital for professionals involved in HAZMAT situations, where exposure to radioactive materials may occur.

The other choices represent either incorrect definitions or mixed prefixes that do not accurately describe the relationship to REM. For example, a microrem would refer to one millionth of a REM, while femtorem and nanorem do not correspond to standard radiation measurement prefixes.