HALF-LIFE, BIOLOGICAL	The time required for the body to eliminate 50 percent of a dose of any substance by the regular processes of elimination. This time is approximately the same for both stable isotopes and radionuclides of a particular element.
HALF-LIFE, EFFECTIVE	The time required for a radioactive nuclide in a system to be diminished by 50 percent as a result of the combined action of radioactive decay and biological elimination.
HALF-LIFE, RADIOACTIVE	The time required for a radioactive substance to lose 50 percent of its activity by decay. Each radionuclide has its own unique half-life.
HALF VALUE LAYER	The thickness of any specified material necessary to reduce the intensity of a gamma or x-ray beam to one-half its original value.
HEALTH PHYSICS	A science and profession devoted to the protection of man and his environment from unnecessary radiation exposure.
HIGH RADIATION AREA	An area in which radiation levels could result in an individual receiving a dose equivalent in excess of 0.1 rem (1 mSv) in one hour. These areas must be posted as "high radiation area" and access is maintained under strict control.
HOT SPOT	The region in a radiation/contaminated area in which the level of radiation/contamination is noticeably greater than in neighboring regions in the area.