Health Risks of Ionizing Radiation: - Clark University

208 Glossary
radiation, one rem equals one rad. For alpha radiation one rad equals 20 rems. One sievert (sv) is
equal to 100 rem.
Roentgen: A unit for describing the exposure dose of x-rays or gamma rays. One unit can liberate enough
electrons and positrons to produce emissions of either charge of one electrostatic unit of electricity
per 0.001293 g of air (the weight of 1 cm 3 of dry air at 0°C and at 760 mm Hg).
Sievert (Sv): The sievert is a unit used to derive a quantity called equivalent dose. This relates the
absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation
has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often
expressed in terms of millionths of a sievert, or micro-sievert. To determine equivalent dose (Sv), you
multiply absorbed dose (Gy) by a quality factor (Q) that is unique to the type of incident radiation.
One sievert is equivalent to 100 rem.
Significance: See statistical significance
Spontaneous abortion: the unaided termination of pregnancy before the fetus reaches a viable age
(between 20 to 24 weeks).
Statistical Significance: “Statistical significance” means statistical analysis has revealed an effect
unlikely to have occurred by chance alone. The level of significance refers to the degree to which the
result could be explained by chance. At the .05 (5%) level the result could have occurred by chance
1 time in 20; at the .01 (1%) level the result could have occurred by chance only 1 time in 100. Any
effect observed in a study or experiment carries with it some degree of uncertainty, or imprecision,
because of randomness and variability in most biological phenomena. Statistical techniques evaluate
an observed effect in view of its precision to determine with what probability it might have arisen
by chance (the level of significance). Values with a low probability of occurring by chance are called
“statistically significant” and are thought to represent a real effect.
Standardized incidence ratio (SIR): a ratio of the measured incidences of a disease to the number of
expected incidences based on the general population’s rates.
Standardized mortality ratio (SMR): The number of deaths observed in the study population to
the number of deaths that would be expected in the study population standardized to the general
population.
Stillbirth: The birth of a dead fetus.
Supramultiplicitive: The combined effect of factors that is more than the independent effect of the factors
being multiplied together
T Cell: A lymphoid cell from the bone marrow that migrates to the thymus gland, where it develops into
a mature differentiated lymphocyte that circulates between blood and lymph, serving as one of the
primary cells of the immune response.
Tailing: The refuse material resulting from washing, concentrating, or treating ground/crushed ore that is
discharged from a mill.
Threshold: A minimum dose that will produce an effect