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Interpreting Readings
Health physics, the field that pertains to radiation and its effects on
man, is very complex, and theories and conclusions are constantly
being updated as information becomes available. Data from
occupational exposure, animal studies, and events like Hiroshima
and Nagasaki have fairly well established the maximum safe
exposure limits for man. Whether low level radiation causes cancer
and birth defects is still being debated. Delayed effect, which
could take years to develop, is difficult to study, and therefore,
there are no well-defined lower limits on ionizing radiation. Two
publications entitled "Hormesis with Ionizing Radiation," 1980 and
"Radiation Hormesis," 1991 (CRC Press, Boca Raton) present over
one thousand examples of statistically valid data showing no
physiological harm in vertebrates from the whole body exposures
to low dose radiation (<60mR per year).
As previously mentioned in the section on operation, the units
mR/hr (milli-Rem per hour, or 1/1000th of a Roentgen per hour)
pertain only to gamma radiation. Often other units of measurement
similar to mR/hr are used. The term REM (Roentgen Equivalent
Man) includes the affects of beta, alpha and neutron radiation.
Measurement in REMS is more complete as radiation affects man,
but such measurements are a complicated combination of many
measurements each made with specialized detector.
It is important to note that the field intensity from a radioactive
object decreases very qulckly with distance. If the object is very
small, increasing the distance from the object by a factor of two
decreases the radiation level by a factor of four. This is called a
square law situation, which demonstrates the dependence of
proximity on dose for small radioactive sources. Larger sources,
such as a large deposit of radioactive minerals, will show much
less of this effect. In trying to estimate the danger of radioactive
materials, it is important to take into account many aspects of the
situation. For instance, the radiation level at the face of a
radium-dial watch (common in the 1930's) may be 3mR/hr, but the
measurement taken from the back of the watch may be 0.3mR/hr.
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