slightly smaller head size, lower average birth weight, and increased incidence of mental
retardation. Some later showed lower IQ test scores and slower scholastic development,
smaller physical size, and increased incidence of behavioral problems.
1. Sensitivity of the fetus
Embryo/fetal cells are rapidly dividing, which makes them sensitive to many
environmental factors including ionizing radiation. The embryo/fetus is most susceptible
to developing adverse health effects if exposed during the time period of 8 - 15 weeks
2. Factors for potential effects associated with prenatal exposures
Many chemical and physical (environmental) factors are suspected of causing or known
to have caused damage to a fetus, especially early in the pregnancy. Radiation, alcohol
consumption, exposure to lead, and heat, such as from hot tubs, are only a few such
E. Risks in Perspective
Current radiation protection standards and practices are based on the premise that any
radiation dose, no matter how small, can result in health effects such as cancer. Further, it is
assumed that these effects are produced in direct proportion to the dose received (i.e.,
doubling the radiation dose results in a doubling of the risk of the effect). These two
assumptions lead to a dose-response relationship, often referred to as the linear, no-
threshold model, for limiting health effects at very low radiation dose levels.
However, it should be noted that this is a conservative assumption made in the absence of
more conclusive evidence. Health effects (primarily cancer) have been observed in humans
only at doses in excess of 10 rem delivered at high dose rates. Below this dose, estimation
of adverse health effects is speculative. Risk estimates that are used to predict health effects
in exposed individuals or populations are based on epidemiological studies of well-defined
populations (e.g., the Japanese survivors of the atomic bombings in 1945 and medical
patients) exposed to relatively high doses delivered at high dose rates. It is generally
accepted that studies have not demonstrated adverse health effects in individuals exposed to
small doses (less than 10 rem) delivered over a period of many years.
1. Risk from exposures to ionizing radiation
a. No increases in cancer have been observed in individuals who receive a dose of
ionizing radiation at occupational levels. The possibility of cancer induction cannot
be dismissed even though an increase in cancers has not been observed. Risk
estimates have been derived from studies of individuals who have been exposed to
high levels of radiation.